Organic weed management
This section covers a wide variety of topics. Some are directly oriented toward management, whereas others are more general explanations of the biology of weeds or of particular types of weeds. Understanding these more general topics will assist you in managing weeds and developing strategies for controlling several species of weeds at once. While written primarily as advice to gardeners, the same principals apply to larger plantings. Detailed documentation of most of this section can be found the Weed profiles section.
Weeds are commonly defined as plants that are growing in a place where they are not desired. Although that definition has some practical utility, it does little to further understanding of how weeds operate as species or how to use an understanding of weed biology to improve weed management.
From an ecological point of view "weeds are plants that are especially successful at colonizing disturbed, but potentially productive, sites, and at maintaining their abundance under conditions of repeated disturbance" (Liebman, Mohler and Staver 2001, Resources). Recognition that weeds are plants that specialize on disturbed habitats makes sense of a great many aspects of their biology. For example, the generally small seed size of annual weed species follows from the fact that they must produce many seeds in order to have some seedlings survive repeated disturbance. Similarly, persistence in the soil seedbank (Seed longevity) and ability to recognize environmental cues associated with tillage (Tillage & germination) allows weeds to pop up after many years of absence when sod is turned to prepare soil for planting.
In general, weeds differ from the crops with which they compete in a variety of ways. Their seed weight is usually low whereas many crops have large seeds (Seed size). The size of the two types of plants at establishment is further exaggerated since small seeded crops are often transplanted. Although their initial size is often less than that of the crop at the beginning of the season, common weeds have some of the highest relative growth rates recorded for any plants. Weeds usually also have very high rates of nutrient uptake. They are adapted to take advantage of the brief pulse of nutrient release that accompanies the breakdown of organic matter when soil is disturbed. Consequently, weeds often have 1.5 to 3 times higher concentrations of N, P, K, Ca, and Mg in their tissues than the crops with which they compete. This not only tends to deprive the crops, but in combination with the weed's higher relative growth rate, allows the weeds to eventually overtake the crops in size.
There are many ways to be a weed: different combinations of characteristics allow a plant species to specialize on disturbed habitats. Some species are annuals. They complete their lifespan within less than a year and survive between years as seeds in the soil. Lambsquarters and barnyardgrass are good examples of these.
Other species are perennials that are fixed in place and unable to spread vegetatively except by human intervention. Dandelion and perennial populations of annual bluegrass are examples. Most of these species are primarily weeds of grasslands (lawns, pastures, hayfields) rather than annual crops, because turning the soil kills them. Stationary perennials are partial exceptions to that rule (e.g., dandelion).
Finally, some of the worst weeds are the wandering perennials. These spread by horizontal roots or underground shoots (rhizomes) from which daughter plants emerge above-ground. They are notoriously hard to kill, and because they renew themselves by vegetative reproduction. Individual clones are essentially immortal if left alone. Some of the contrasting characteristics of these three types of weeds are summarized in the following table.
|< 1 year
|2 to a few years
|Years to decades
|Years to decades
|A few years
|Energy allocated to seed production
|Usual means of dispersal
|In soil, manure
|Soil, wind, feces, crop seed
|Common Lambsquarters, Barnyardgrass
|Quackgrass, Hedge Bindweed
(Modified from Liebman, Mohler and Staver 2001)
Seedbank: The seeds in the soil, but especially those that persist for one or more years
Rhizome: An underground stem
Cole crops: Crops in the mustard family (Brassicaceae) such as cabbage and broccoli
Perennial: A plant that lives for more than one year
Annual: A plant that completes its lifespan in less than one year
Stirrup hoe: A hoe made of a narrow band of steel bent in the shape of a stirrup and attached to a long handle. It is used for shallow hoeing of small weeds.
After ripening: Transition of a seed from a dormant to a non-dormant state which takes place after the seed is physiologically independent of the parent plant
Asparagus knife: A narrow blade, slightly forked and sharpened at the tip, that is used for cutting asparagus stalks belowground. It is useful for digging out taprooted weeds with minimal disturbance to adjacent crop plants.
Tilth: A desirable quality of the soil in which it is porous and easily broken into small stable crumbs.
Most annual weed seeds are tiny - often about the size of the head of a pin (Seed size). Because their food stores are small, they cannot emerge from deep in the soil, and they are very thin and fragile shortly after germination. Hence very shallow (~ 1") disturbance of the soil can be very effective for eliminating a large percentage of these weeds. Deeper soil disturbance brings additional seeds to the surface where they will germinate.
If planting has been delayed since the seedbed was prepared, scratch the soil surface thoroughly with a garden rake or stirrup hoe before planting. Repeat when the first tiny seedlings appear. Note that you may not be able to see newly emerged seedlings unless you get down on hands and knees. For large seeded crops like sweet corn, snap beans etc. that are planted >1" deep, the soil can be raked right over the rows before the crop emerges, and if you are careful, after the shoots are fully out of the ground. Since you can't see the weeds, these actions often seem pointless. They are easy to perform, however, and they are probably the single most effective way to reduce difficult pulling and hoeing later in the season. Note, however, that this sort of 'blind cultivation' as it is sometimes called is pointless if the surface soil is too dry for seed germination.
Pulling weeds is a last resort when other methods of management have failed, or when a few escapes need to be removed to prevent seed production. For some small seeded, slow establishing crops like carrots or parsnips that do not transplant well, hand pulling is sometimes necessary to remove small weeds from around the young crop. In the latter case, the amount of hand weeding can be reduced by sowing slow emerging crops in relatively weed free areas of the garden and preceding planting with a short period of clean fallow to reduce weed density.
The best technique for pulling weeds depends on the type of weed and the situation. Small weeds are easiest to pull when the soil is wet (i.e., too wet for tillage). Keeping your weight off of the soil at such times is critical, however, to avoid destroying soil structure (Stay of soil). To pull small weeds from among small, fragile crop plants like young carrots, place a finger on the ground on both sides of the weed and pull with the other hand. This holds the soil in place, and prevents uprooting the crop along with the weed.
Species with strong taproots are also easiest to pull when the soil is wet, but again, care should be taken to avoid trampling the soil. Also, the crop should be dry to avoid spreading disease. Grasp the weed by the top of the taproot rather than by the stem or foliage. Then slowly pull straight up with a slight twisting motion. This will break the feeder roots free from the taproot and allow the taproot to be pulled up whole. A jerking pull will tend to break the root. Removing most of the root is critical since the plant will resprout from dormant buds in any large pieces that remain in the soil. The resulting complex root system will be impossible to pull and you will have to dig to remove it. Maintaining a high state of tilth is critical for hand pulling weeds with taproots (Tilth & weeding). If the soil is moist, loose and has a good crumb structure, even large dandelions can be pulled whole. If the soil is not in good condition or is not wet enough or the weed is really large, an asparagus knife, long trowel or narrow spading fork may be needed to get the whole root. If the plant is so large that you have to hand pull it, it may reroot if the soil is moist or rain is expected. Also, if the plant is flowering, it may make seeds even after you uproot it. Consequently, carrying along a couple of 5 gallon buckets to use for removing the weeds from the garden may help reduce subsequent weeding. Weeds that are unlikely to set seeds can be left on a hard surface like concrete or boards until thoroughly dead and then composted.
Fibrous rooted species like annual grasses and plantains are easiest to pull when the soil is starting to dry. If the soil is dry and hard, the shoot will tend to break off, leaving the root system to resprout, whereas if the soil is moist, a lot of soil will cling to the roots. If the soil is moderately dry, hitting the root crown against any hard object will knock most of the soil off the roots. This will decrease likelihood of the weed rerooting if it is left on the ground and avoid exporting your precious topsoil if it is removed from the garden.
If a few weeds with spreading rhizomes or root systems are encroaching from the edge of the garden, pulling the shoots is more effective than hoeing. Hoeing cuts the shoots near the surface whereas pulling the shoot usually brings up a long white underground shoot. This depletes the underground root-rhizome system more quickly than hoeing. Canada thistle is one such species. Since the base and underground portion of the shoot is free of thorns, the plant can be pulled from this point without heavy gloves.
Hoeing is best done when the weeds are very small seedlings or newly emerged shoots of perennial weeds. This allows shallow hoeing to kill the weeds without bringing new seeds to the soil surface. Shallow hoeing also reduces root damage to the crop. Stirrup hoes (shuffle hoes) are ideal for shallow weeding. A garden rake moved in an oval motion covers large areas quickly (e.g., the areas between winter squash before they begin to run). Traditional chopping type hoes are sometimes useful for hacking back weeds in untilled corners of the garden, but in loose soil they tend to dig too deep, damage crop roots and bring up more weed seeds. If the weeds are so large that a traditional hoe is needed, hand pulling or digging them out may be more efficient in the long run.
One objective of hoeing should be the creation of a dust mulch. This is a layer of very loose soil crumbs, typically 0.5 to 1.5 inches thick. It can be achieved with most tools that work the soil shallowly including a rake, garden claw or stirrup hoe. Weeds seeds need good contact with the soil for germination just like crop seeds. Since most individuals of most annual weed species emerge from the top inch of soil, maintenance of a dust mulch greatly decreases weed density. Obviously, a dust mulch is impossible to maintain during wet weather, but when it is feasible, a dust mulch is a highly effective weed management technique.
Crops that you expect to hoe should be planted with hoeing in mind. For example, sweet corn plants should be spaced sufficiently to allow your hoe to slide between the plants after the prop roots form. Planting cole crops using three rows to the bed and with the middle row staggered relative to the outer rows allows you to hoe in a cross hatched pattern that leaves few weeds left standing. Having hoes of several widths and types helps you match the hoe's action to the crop and situation.
Hoeing is best done when the soil is slightly dry and the weather is warm and sunny. First, such conditions are ideal for drying out uprooted weeds and producing a dust mulch. Second, the hoeing will do less damage to soil structure under such conditions than when the soil is wet. Third, hoeing in rainy, or foggy conditions is likely to spread disease, both on your clothing and by bringing soil into contact with crop foliage.
Flame weeding is the use of an open flame, usually propane fueled, to kill or damage weeds. The objective of flame weeding is not to burn up the weeds, but rather to vaporize the water in the surface cells. This causes the weeds to dry up within a few hours to a day. Flame treatment does not kill perennial weeds, and usually grasses and larger broadleaf weeds will recover as well. Nevertheless, flame weeding is quick and relatively easy, and it is a highly effective way to remove small broadleaf weeds.
Most crops are damaged by flaming, but it can be used to remove weeds just prior to emergence of a crop, and to control weeds in the cracks between pavers and along fences. Crops like corn and onions in which the bud is protected inside a whorl of leaves can tolerate a light touch by the flame after they have established. Thus, flame weeding can be used to remove small weeds from within the rows for these crops.
Flame weeding works best when the weeds are dry. To kill small weeds, the flame should pass over the weeds at about a normal walking speed. Hand held flame weeders are available for about the price of a string trimmer.
Many perennial weeds have deep storage roots or rhizomes that resprout after the tops are cut or pulled (e.g., hedge bindweed, Canada thistle. etc.). Since the storage roots or rhizomes are too deep in the soil to damage with normal spading or rototilling, your best hope for organically controlling these weeds is to exhaust the storage organs by repeated removal of top growth. Generally, the net flow of nutrients is from the root until formation of the third or fourth leaf, so time your removals accordingly. Typically, eradication of deep-rooted perennials requires about 6-8 well-timed weedings the first year followed 3-5 the second.
Perennial weeds depend on resources stored in roots or rhizomes to establish new shoots the following year. An effective, though laborious, method of controlling many perennial weeds is to physically pick the storage organs out of the soil when an infested area is being prepared for planting. Because the labor involved is great, this approach is only recommended for combating small patches of otherwise difficult to manage species.
Turn the soil by hand with a spade. Save the first shovel-fulls to one side so that you are always turning into a trench. Place the shovel-full of soil on its side in the trench and strike it with the back of the shovel blade to break up the soil. The soil will tend to fracture along the roots and rhizomes. These will generally be 1/8 to 1/4 inch thick and white or brown. Pick them out into a bucket. You do not need to remove every tiny fragment to get very substantial control, but you will need to consistently pull any shoots that emerge from the remaining fragments. Although picking rhizomes is hard work, if your follow up is good you can eradicate the weed and thereby prevent many hours of weeding in future years.
Several studies have shown that weeds are often better equipped for taking up mineral nutrients like nitrogen, phosphorus and potassium than are the crops with which they compete. Not only do the weeds produce root surface area at a faster rate (seed size), but also weeds typically attain 1.5 to 3 times higher concentrations of nutrients in their tissues. Consequently, highly available forms of nutrients like chemical fertilizers and rapidly decomposing organic fertilizers (e.g., fish emulsion) tend to favor weeds relative to crops. In contrast, the slow release of nutrients from green manures and compost tends to favor crops relative to the weeds. Particularly for long season crops, the slow release from organic materials may slow early top growth slightly but encourage a stronger root system and an overall healthier, more productive plant at harvest.
Although most of the mineral nutrition of the crop should come from soil organic matter built up by feeding the soil with green manure and compost, some heavy feeding crops like sweet corn and broccoli will yield better if given an additional nitrogen source (e.g., blood meal, composted chicken manure etc.) after they are well established. When applying such supplements, attempt to get the fertilizer close to the crop and work it in shallowly to avoid volatilization of ammonia. Avoid broadcasting the material widely where it will feed inter-row weeds before the crop can reach it.
Similarly, drip irrigation, which applies water next to the crop plants rather than sprinkling it over the whole garden, will favor the crop plants relative to the weeds. This is especially useful for managing weeds along the edges of beds where they usually receive less shade or root competition from the crop.
Except for a few very weakly competitive crops like carrot and onion, most weed control comes from competition with the crop rather than from weeding by the gardener. You can easily demonstrate this for yourself by leaving two patches of ground unweeded for a few weeks in the middle of the season - one in the crop and one that is left unplanted. Because crop competition is an important element of weed management, competition from the crop should be encouraged.
High crop density provides early canopy closure and more effective competition against weeds than a sparse planting. Any crop that makes multiple units of produce on a single plant (e.g., squash, beans, tomatoes) and most leafy greens (e.g., chard, collards, kale) can be planted at higher than recommended rates without yield loss. Each plant will yield less, but the yield of the overall planting will change little provided the high density does not encourage the spread of disease (evaluate where your problems lie!). Root crops will tend to make small roots if planted too closely, although surprisingly dense plantings are possible if soil tilth and fertility are high.
For many vegetables with small plants like carrots, parsnips, leaf lettuce etc. seeds can be sown in continuous beds of a convenient width for your arm rather than in rows. This fills the space up with crop plants allowing less room for weeds. Carrots, for example, are usually considered to be a relatively noncompetitive crop, but once the foliage is well established on a continuously sown bed of properly spaced carrots, emergence and growth of new weeds becomes difficult.
Solid seeding generally precludes hoeing. Vegetable crops that make small plants usually have to be hand weeded within the row, however, even if they are hoed between the rows, and depending on the spacing between the plants, solid seeding may not require extra hand labor. Solid seedings should always be reserved for relatively weed free parts of the garden.
Use competitive varieties
Many factors enter into variety selection. Most gardeners consider produce quality, yield, and disease resistance more important than ability to suppress weeds. Nevertheless, as you evaluate varieties note whether the variety has features likely to suppress weeds. Among these are vigorous early growth, speed of canopy closure, height, and foliage density. All crop species vary in capacity for weed suppression among varieties.
Most annual weeds have very small seeds and consequently they establish relatively slowly. If the crop is also relatively small seeded (e.g., cole crops, lettuce, tomato, leek etc.) then growing the crop in weed free soil and transplanting after the plants are well established gives the crop a substantial head start over the weeds. Few professional organic growers direct seed small seeded crops that tolerate transplanting. If the bed was prepared before the transplants are ready, the surface should be stirred vigorously to a depth of 1-2 inches to kill any weed seedlings before transplanting. This should be done even if seedlings have not yet emerged so as to give the transplants the maximum head start over the weeds.
Many of our vegetable crops have their origins in the tropics or subtropics (e.g., corn, beans, squash, tomato, peppers etc.). Consequently, they grow most vigorously when the soil and air are warm. In contrast, every season has weeds that are well adapted to the prevailing weather conditions at that time of year. Consequently, pushing warm season crop species to get an exceptionally early or late harvest puts the crop at a disadvantage relative to the weeds. You may find harvesting warm weather crops outside of their usual season worth the additional effort, but expect to do extra weeding.
Another way to increase the competitiveness of crops is to plant them in mixtures. Some mixtures may be less competitive against weeds if the crops compete more with each other than they do with the weeds, so the mixtures must be chosen carefully.
- Radishes grow rapidly whereas carrots establish slowly. Consequently, the early crop of radishes in a mixed sowing helps suppress weeds but leaves space for subsequent carrot growth once the radishes are pulled.
- Lettuce is harvested much sooner than tomatoes. Consequently, a row of lettuce next to a row of tomato plants competes with the weeds but is harvested before it can compete with the tomatoes. Note that many other crop combinations could be substituted here (e.g., spinach for lettuce and Brussels sprouts for tomatoes).
- Sweet corn, although it is tall, lets a lot of light reach the ground and this allows weed growth. A strategy for improving late season weed control is to allow winter squash or pumpkins to run in under the sweet corn to compete with the weeds. Squash or pumpkin yield will be greatly reduced (e.g., by 1/2 to 3/4) under the corn so the total planting should be increased accordingly.
- Finally, where skips occur in a row due to poor emergence or early death of the crop, some other crop should be planted to fill in the gap. If you do not have a crop that fits appropriately into the space, sow a rapidly growing cover crop like oats or buckwheat in the gap (Summer cover crops).
When you harvest a crop early in the season (for example, radish, peas), you have an opportunity to follow it with a warm season cover crop. Also, many experienced organic growers keep a supply of cover crop seed on hand to provide a quick cover when they experience the inevitable occasional crop failure. Species commonly used for summer cover in the Northeast include oat, buckwheat and sudex (sorghum-sudangrass hybrid. These are all relatively inexpensive, and often can be obtained locally. Cowpeas (black eyed peas) can be planted to provide nitrogen for a succeeding crop. All of these species have relatively large seeds that should be worked into the soil with a rake or hoe to insure successful establishment. If the weather is hot or dry, irrigation will speed establishment and improve density.
Working the seeds into the ground will simultaneously remove any small weeds that have established in the previous crop. Large weeds should be pulled or dug out prior to sowing the cover crop. A dense sowing of a warm season cover crop will smother most species of annual weeds, and also add organic matter to the soil. A summer cover crop should be cut or incorporated before it goes to seed to keep it from sprouting up and competing with crops next year. Also, be wary of small weeds that may persist in a suppressed state while the cover crop is in place and then set seed after it is cut. Often the best strategy is to shallowly incorporate the summer cover crop in late summer and then plant a winter cover crop.
If a crop will not be planted until mid summer and no winter cover crop was planted the previous year, you can use a spring sown cover crop to protect the soil, suppress weeds and add organic matter to the soil. Oat, grain rye and field pea are all used for this purpose in the Northeast.
For more information on cover crops, see USDA-SARE's Managing Cover Crops Profitably.
Sowing a cover crop into or after the final vegetable crop of the year has several advantages. Benefits for the soil include prevention of erosion, reduced leaching of nutrients, and improvement of soil structure by the dense root systems of the cover crop and the green foliage that will be incorporated into the soil in the spring. As explained in Tilth & weeding, maintaining good soil properties by using cover crops makes weeding easier. Cover crops also have a direct competitive effect on weeds, particularly species like common chickweed and shepherd's-purse that thrive in cool weather.
Various crop species in a garden are harvested on different dates, and this will determine to a large extent the type of cover crop that you will want to use. In New York and New England only two commonly used cover crop species survive the winter well. These are grain rye and hairy vetch. Annual rye grass (a finer-leaved and smaller seeded grass than grain rye) can also over winter, but the seed is so commonly contaminated with perennial rye grass that the species cannot be recommended. For information on cover crops suitable for other parts of the U.S.A., see Resources.
Hairy vetch is a legume that can convert nitrogen gas in the air into fertilizer nitrogen (nitrogen fixation). It grows in a lush, tangled mat that by May may be three to four feet thick. It must be sowed by the last week of Aug. to have a good chance of surviving the winter, but sowings in early Sep. may be successful if the autumn is long and mild. If it is sowed earlier than late July, the plants may flower and then die during the winter. Birds and mice eat the seeds, and the germinating seeds are attacked by many species including slugs and earthworms. Consequently, hairy vetch seed should be incorporated into the soil by light hoeing. It is a good cover crop to follow early crops of potatoes, sweet corn or cole crops, and can be undersown into any crop that is not casting dense shade. Hairy vetch seed is relatively expensive ($1.50 to several dollars per pound) and usually has to be ordered from a seed company. The seed is cheaper if purchased in bulk, and if kept cool and dry it will stay viable for at least a decade. Large scale farmers typically sow hairy vetch at 0.05 to 0.2 lb/100 ft2, but in home gardens, rates of 0.2 to 0.5 lb/100 ft2 are financially practical, and these higher rates quickly smother out most annual weeds.
Rye does not "fix" nitrogen, but it is an effective scavenger of any nutrients that were not used by the crop and produces huge amounts of green matter. By late May it will be five to seven feet tall on fertile ground, so cutting it back in late April or early May is wise. If you do not keep it cut back, the long, coarse flowering stems will be hard to incorporate into the soil. Rye can be planted from late Aug. to mid Sep. in New York State. Consequently, it can be planted after frost killed crops like squash, tomatoes and peppers. Although rye will sometimes establish if sown onto the surface of loose soil, it establishes best if it is hoed lightly into the ground. This also has the salutary effect of removing weeds after the crop has been harvested, a time when most gardeners ignore that part of the garden. Rye can also be interseeded into fall cole crops and late varieties of sweet corn at the final hoeing. The seed is usually available locally at farm and garden stores. Large scale farmers sow rye at 0.25 to 0.40 lb/100 ft2, but since the seed is usually cheap ($0.10 to $1.00/lb), using 1-2 lb/100 ft2 is often financially feasible and is more effective for smothering weeds. Always check cover crop seed for contamination with weed seeds before you sow it, but be especially cautious of inexpensive, locally produced rye seed.
Winter wheat can be used as a substitute for grain rye. It is less competitive than rye but it produces less growth in the spring, and the flowering stalks form later in the season, so it is easier to incorporate. Untreated seeds are often difficult to obtain locally and expensive to order from seed companies. Planting and management practices generally follow those for rye.
Both rye and hairy vetch should be cut before incorporating the plants in the spring. This can be done with a string trimmer, a grass whip or a scyth. If the cover crop has grown tall, cut the top first and then cut closer to the ground. Rye stems longer than 12 to 18 inches will make rototilling the ground completely impractical! Hairy vetch is much easier to incorporate. Most experienced growers incorporate their cover crops about two weeks before they plan to plant. This gives the material time to rot and release nutrients so that the young crop plants are not starved by competition with decomposers. Hairy vetch, however, rots much more quickly than rye. A lag between incorporation and seedbed preparation also gives many weed seeds an opportunity to germinate and be killed before the crop is planted (Tillage & germination).
Red, white, alsike and sweet clovers will overwinter in New York and New England. Crimson clover will overwinter in Maryland and Delaware. Clovers usually must be established before about July 1 to be successful. Consequently, they are usually established by interseeding into a summer crop. They do well in sweet corn, but tomatoes and squash are often too competitive to allow the clover to survive. Since the seeds are small, they should be sown onto the surface of loose soil after the crop is well established and competitive. Summer sowings of clover should be watered to ensure establishment. Since the young plants are small and slow growing, they are ineffective for smothering out weeds until well into the fall. Since hoeing kills the young clover, hand pulling of weeds will probably be unavoidable unless your garden is exceptionally weed free.
Oats usually winter kill in New York and New England, but if planted by early Sep. and sown thickly they will provide substantial soil cover before death. An advantage of oats as a cover crop is that they are already partly decayed by early spring and so make a good cover to precede early-planted crops. Sowing rates for home gardens range from 0.1 to 1.0 lb/100 ft2.
For more information on cover crops, see USDA-SARE's Managing Cover Crops Profitably.
Most weed species have very small seeds. Hence the newly emerged seedlings are tiny and incapable of competing with established vegetation. Consequently, these species have been naturally selected to respond to environmental cues that indicate the absence of competing vegetation. In natural situations, vegetation is usually only absent if the soil has been recently disturbed. Thus, weeds often respond to cues associated with soil disturbance (e.g., tillage). These cues include light, and especially red light (in contrast to the green light that passes through a plant canopy), fluctuation in temperature between day and night, high soil temperatures, the absence of volatile substances released by anaerobic metabolism, and the presence of nitrate in soil water. High light levels at the soil surface occur when competing vegetation is absent. Similarly, high soil temperatures and large day to night fluctuations in soil temperature occur when the soil is exposed to direct sunlight by day and radiative cooling to the sky at night. Turning the soil vents ethanol, aldehydes and other substances that accumulate within soil particles due to insufficient oxygen for complete digestion of carbohydrates by seeds, roots and microbes. Finally, the increase in warmth and oxygen associated with tillage stimulates microbes to consume organic matter and thereby release nitrogen containing compounds that other microbes turn into free nitrate ions.
Many species respond to several of the germination cues discussed above. Thus, for example, a few lambsquarters seeds will germinate in the dark at constant temperature and no nitrate. More will germinate if any one of these three cues are present, and most will germinate if all three cues are present.
As a result of the changes in soil properties following tillage, many weed seeds are able to detect (i) that they are near the soil surface, and (ii) that competing vegetation and dead organic materials have been removed. Consequently, a flush of germination usually follows tillage provided the soil is moist enough for seed germination.
A clean fallow is a period during which no crop is planted and the soil is regularly hoed or cultivated to eliminate weeds. The bare soil and regular stirring of the soil provide cues that prompt seed germination (Tillage & germination) and the young weeds are then destroyed. Similarly, the rest period between hoeings gives perennials time to move carbohydrates from storage roots or rhizomes into shoots which are subsequently removed, thereby depleting the storage organs (Exhaust perennial roots).
A period of clean fallow prior to planting is sometimes referred to as a 'stale seedbed' though properly, this term only applies if the weeds are killed without disturbing of the soil surface (e.g., with a propane flame). Other opportunities for clean fallows exist between harvest of early crops (e.g., of spinach or lettuce) and fall crops (e.g., of broccoli), and after the harvest of early crops of potatoes or sweet corn. The optimal time to use a clean fallow for weed management depends on the seasonality of the weeds that are giving you the most severe problems.
Clean fallow tends to destroy soil structure. Usually, only a shallow cultivation is necessary to kill the weeds, and this reduces the problem. Nevertheless, soil-building practices discussed in Tilth and weeding should be used to counteract the damaging effects of clean fallow periods.
Rotation between spring, summer and fall planted crops tends to reduce overall weed problems by interfering with the life-cycles of species that have preferred seasons of germination (Timing of germination). For example, spring germinating weeds will be destroyed during seedbed preparation for summer planted crops, and few spring germinating weeds will replace them because the season is not favorable for their germination. During the summer, fall and winter, some of the remaining spring germinating weed seeds will be eliminated by accidental germination deep in the soil and by consumption by earthworms, carabid ground beetles and other soil fauna (Seed longevity). Hence a summer planted crop decreases future pressure from spring germinating weeds. Similar processes act when rotation occurs between spring and fall planted crops and summer and fall planted crops.
Mulches of organic materials are highly effective for suppressing small seeded (i.e., <2 mg) annual weeds. Since most garden weeds are annuals and most annual weeds in the Northeast have small seeds, the use of mulches is broadly effective against many species. Mulches are nearly useless for controlling perennial weeds because these have sufficient energy stores in the roots or rhizomes to push shoots up through even very thick layers of mulch. Large seeded weeds (e.g., > 5 mg) may also emerge through substantial layers of mulch.
Weed seeds in mulch
The most commonly used mulch materials are straw, hay, compost, leaves, grass clippings and bark chips. Regardless of the material used, it should be thoroughly checked for weed seeds. Hay, and particularly, late cuttings of grass hay, often contains mature weeds and perennial grasses that you do not want in your garden. Straw is generally free of weed seeds but may have thistle seed heads and grain seeds. Grass clippings often contain dandelion flowers that subsequently turn to seeds, and may also contain crabgrass and annual bluegrass seeds. Tree leaves are generally free from weed seeds but may contain acorns and other tree seeds that subsequently germinate and compete with crops.
When to mulch
Although large seeded and transplanted crops can be mulched almost immediately after planting, it is helpful to delay mulching until the soil is fully warmed and the first flush of weeds has been removed by shallow weeding. Although a thick mulch eliminates most light at the soil surface, even a homogenous appearing mulch layer has partial "windows" through which some light penetrates. If the density of weeds emerging under the mulch is high, more weeds will be positioned to exploit such windows and emerge through the mulch.
How much mulch
The amount of mulch needed depends on the dominant weeds in the garden and the type of mulch. Larger seeded weeds require more mulch than smaller seeded weeds. Hay and straw should be fluffed up rather than applied as slabs from a bale. Although the slabs are very effective at blocking weed growth, they quickly rot and provide a prime seedbed for any crop seed in the mulch and for windblown weeds like dandelion. A 3 to 5 inch layer of straw, hay, or leaves (which subsequently settles to about 2-3 inches) is generally effective against most of the small seeded annual weeds in the Northeast. Because grass clippings and compost are denser, 2-3 inches is generally effective.
Notes on particular mulches
Some mulches pose special problems and advantages. Bark and wood chips cannot be recommended for use in vegetable gardens (except on permanent paths) because their high carbon to nitrogen ratio encourages decomposer microbes to take nitrogen from the soil, thereby starving the crops. Similarly, straw has a high C:N ratio and can temporarily immobilize nitrogen. Whereas the nitrogen taken up by bark chips may be stored for years, the nitrogen taken up by straw is largely released again to the crop later in the season. For heavy feeding crops, hoeing in a nitrogen source just before laying the straw mulch can avoid N deficiency in the crop.
Unlike hay and straw, tree leaves do not tangle into a mat and thus sometimes blow to other parts of the garden where they may smother small crops. Applying the leaves after the crop is up helps hold them in place.
Rye straw commonly releases allelopathic compounds that are toxic to other plants. Since small seeded weeds are more susceptible than large seeded or transplanted crops, this can be advantageous. However, in some circumstances, the toxins may also slow crop growth (e.g., sweet corn during dry years).
Many synthetic mulch materials are marketed for use in gardens, including plastic films of various colors, spun materials that are permeable to water, and plain and oiled paper. In addition, some gardeners use newspapers, cardboard, old carpeting and other materials as weed barriers. All of these can be effective, but they all pose certain problems as well.
Weeds along edges
All synthetic mulches must be anchored along all edges to prevent the material from blowing in the wind. Usually this is accomplished by piling soil along the edge of the mulch material. This soil is, of course, above the mulch and so tends to become weed infested. Since these weeds cannot be hoed without damaging or uncovering the mulch, much hand weeding may be required to prevent seed production by annuals and vegetative propagation of perennials. Weeds also tend to grow thickly in the holes made for the crop, again requiring hand weeding.
Irrigation & mulch
Some, synthetic mulches like plastic film, cardboard and oiled paper are impermeable to water. Usually they are most effective if a drip irrigation system can be placed under the mulch.
Most synthetic mulches pose significant end-of-seasop disposal problems. The labor and cost of disposing of large amounts of dirty and possibly wet mulch material at the end of the growing season should be considered when contemplating the use of these materials. Newspaper, brown kraft paper and kraft paper treated with vegetable oil are all biodegradable, but unless gathered, chopped up and composted, they may leave the garden unsightly through the winter.
Spun ground covers
Spun cloth ground covers similar to floating row covers but colored brown or black to block light from weeds are reasonably effective for preventing the growth of annual weeds. Many perennials, however, can penetrate these materials. Pulling these weeds pulls on the cloth, and that may disturb crops planted in holes in the material. Moreover, great masses of quackgrass and other perennials may cling to these ground covers when they are collected, thereby greatly increasing the expense of disposal. If perennial grasses are present, plan on several large garbage cans of debris per roll of ground cover.
Old carpeting is very effective at blocking the growth of even vigorous perennials for several years, but it cannot be recommended for use in vegetable gardens due to the toxic compounds (e.g., formaldehyde) that are released as the carpeting weathers. Depending on one's outlook, this may seem less of a problem in beds of ornamentals. After a few years however, weeds will establish in dust and decayed organic debris that collects on the surface of the carpet. These plants will grow through the decaying fabric and effectively anchor to the soil what has then become a large piece of ugly debris.
Although plastic mulches are effective for producing early crops (e.g., of tomatoes), in general, synthetic mulch materials tend to be more trouble than they are worth for weed control. The one exception is that paper mulches laid under straw, compost or other organic materials can increase the effectiveness of the natural mulch, and may decay sufficiently by the end of the season to avoid disposal problems.
Weeding is much easier in a healthy, well-structured soil. If the soil is loose and porous, hoeing is much less laborious, weed roots easily shake free from the soil resulting in faster death, and more of the root system of perennials will pull up along with the top when hand weeding. Three elements are key to obtaining good soils structure.
Stay off the soil
There are several ways to avoid walking on soil that will be growing crops. One is to create permanent beds with paths between the beds. The paths can be covered with bark mulch or sown with grass and frequently mowed to prevent weed growth. Regularly edging the walkways to prevent encroachment of sod into the beds may be necessary. Raised beds have advantages (early warming in the spring and less stooping) but the sides of the beds may be difficult to keep weeded. Another approach is to till the entire area and lay old (unpainted) boards on the ground to make paths that support foot traffic. The ease of weeding soil that has not been trampled has to be experienced to be appreciated.
Add organic matter
When compost, cover crops and mulch materials like straw and leaves from the previous year are incorporated into the soil, the decomposing organic matter and the beneficial fungi that grow on it bind soil particles into small, stable crumbs. The spaces between crumbs allow easy penetration of water into the soil and rapid root growth for the crops. The decomposing organic matter provides a source of nutrition for crop growth. Cover crops are particularly valuable because the fibrous root types (e.g. grasses) help aggregate the soil into crumbs and tap-rooted types like sweet clover penetrate and loosen the subsoil.
Keep soil covered
Keeping the soil continuously covered with a crop, cover crop or mulch prevents rain drops from breaking up soil crumbs. It also prevents the soil from baking hard in the sun. Moreover, organic materials from cover crops and mulches provide food for earthworms. These create soil pores with their burrowing and cement soil particles into crumbs with their slime.
New gardens often have terrible weed problems. If the garden is to replace a well weeded lawn, scalp off the sod, compost it, and return it to the garden later. Alternatively, Lee Reich (Weedless Gardening) recommends closely mowing the grass, covering it with newspapers and a few inches of compost. This smothers the grass while allowing crops to be grown in the compost.
When the garden is being established on weedy ground, it is best to avoid planting vegetables the first year. Instead, till, plant a cover crop, till in the cover crop before weeds get large or go to seed, and repeat this at 4-6 week intervals throughout the summer. This will deplete the weed seedbank and exhaust the storage organs of perennial weeds, while simultaneously building up soil organic matter and soil tilth. Buckwheat, sorghum/sudan grass hybrid, or a mixture of oats and field peas are all fast growing, competitive summer cover crops. At the end of Aug. plant oats on parts of the garden that will be planted early the next year. These will compete with weeds in the fall but frost kill, leaving the garden ready for early planting. On areas that will be planted to vegetables around or after the last frost in the spring, plant hairy vetch in Aug or Rye in Sep. (Winter cover crops).
Often vegetable gardens need to be fenced to keep out rabbits, groundhogs, deer and dogs. Keeping areas next to fences from becoming weedy is important because (i) annual weeds may shed seeds from there into the garden, (ii) some perennial weeds along fences can spread several feet into the garden by underground roots and rhizomes (for example, quackgrass, hedge bindweed, and Canada thistle), and (iii) tall vegetation along fences may block light from neighboring crop plants. Fences form ideal support for hedge bindweed and other vining weeds.
Various control measures are possible. Brick or concrete pavers, or a heavy layer of bark mulch can be used to reduce weeds by forming a path along the fence. Hoeing is often ineffective because the fence protects the weeds. Hand pulling is effective but laborious. Repeated use of a string trimmer is the most usual method for controlling weeds along garden fences. Flame Weeding is effective if done frequently enough to keep the weeds small and provided no straw or bark mulch is adjacent to the fence to catch fire.
Many materials used for making compost are contaminated with weed seeds. Late cut hay will certainly contain weed seeds. Straw can be examined for fruiting stalks of weeds. All manure other than poultry manure should be considered contaminated unless you have tested it. Horse manure and manure from other animals that have access to weedy pastures or pastures along roadsides are most likely to be contaminated with weed seeds.
In general, it is easier to use weed free materials to make compost than it is to try to kill weed seeds during the composting process. The problem with adding weedy compost to your garden is rarely that you will be immediately overwhelmed with weeds: usually the weed seed density of garden soil is higher than that of manure or poorly made compost. Rather, the problem is that you may introduce some new pernicious weed species that will cause management problems for years to come.
You can test manure for weed seeds by mixing several quarts of manure taken from various parts of the pile with potting mix in a 1:1 ratio and spreading it in flats. Keep the flats warm during the day and cool but not cold at night. For example, run the test inside in the winter, outside in the summer and in a cold frame during the spring or fall. Water the flats regularly, and observe any weed seedlings that emerge over the following two to three weeks. This test will usually show if weed seeds are present, but it may not accurately predict their density since some seeds may be dormant.
To kill weed seeds during the composting process, the pile should reach 140? F for at least two weeks. Some of the more resistant species may not be killed even by this treatment. For a small compost pile achieving a high sustained temperature may require insulating the pile (e.g., with loose, straw over plastic). If the weather is warm and sunny, the heat generated by biological activity can be supplemented with solar energy by covering the pile with clear plastic. Since the outside of the pile is unlikely to attain the required temperature for a sustained period in any case, thoroughly mixing the pile several times will probably be necessary. For a very small pile (e.g., < 1 cubic yard) attaining a high enough temperature for a long enough time to kill most weed seeds may be impossible.
A small flock of chickens is an excellent adjunct to a vegetable garden. Composted manure from the hen house provides a regular source of nutrients for the garden while the chickens happily eat weeds and culled produce that could pose future weed and disease problems if placed in the compost pile. Chickens relish dandelion, quackgrass and most other weeds, but will reject some members of the mint and parsley family.
Chickens can also be allowed to run in the garden for a few weeks in the spring before planting or in the fall after most harvest is complete. They will pick out weed seeds, clean up perennial weeds and also eat slugs and insect pests. A low, temporary fence is usually sufficient to keep them away from late season, over-wintered or early planted crops. Chickens should not be left in the garden for long periods, however, because their constant scratching will ruin soil structure. Their potential for damaging the soil is particularly great if the soil is wet.