Weed ecology

Seed weight indicates much about the biology of an annual weed species and hence its response to a wide range of management practices. Seed weight is important because it indicates the resources available to the seedling during establishment. Consequently, seed weight correlates with the depth from which the seedling can emerge from the soil, its ability to grow up through organic mulch, and its ability to overtop young crop plants. Since seed weight governs the species' ability to establish under adverse conditions, it also indicates how likely the species is to respond to environmental cues that indicate that the seed is near the surface of the soil and that the soil is free from dead organic matter and competing crops. Germination of small seeded weeds is commonly controlled by multiple environmental signals associated with near surface conditions, recent soil disturbance and bare soil (Tillage & germination). The small seeded species balance these cues physiologically to determine whether or not to germinate. Large seeded weed species, on the other hand, are usually insensitive or only weakly sensitive to many of these cues.

Seed weight is also correlated with the species' rate of growth. The growth of small seeded weeds (weight gain/day) as seedlings is usually slow relative to crops and large seeded weeds simply because the larger seedlings have more photosynthetic area per plant. The tiny plants that emerge from little seeds, however, tend to have more photosynthetic area relative to the weight of non-photosynthetic tissues like roots and the inside of stems. Consequently, their relative growth rate (weight gain/unit weight/day) as seedlings is high. In fact, this factor combined with others (like long thin roots that rapidly exploit a large volume of soil) give agricultural weeds some of the highest relative growth rates recorded. Consequently, small seeded weeds start out life weak and non-competitive, but if given a chance, they rapidly outgrow the crops.

Implications of seeds size for management are many. Weeds are always easier to control by hoeing when they are young, but the smaller the seed of a species, the easier they are to kill, and the shallower the hoeing has to be to give good control. Moreover, just because a weed is tiny does not mean that it can be safely ignored. The smaller the weed, the more rapidly its size multiplies each day.

Seed weight indicates the species' ability to grow up into and through the canopy of an established crop. Species that grow from tiny seeds tend to stagnate in the shade under a crop. Those from larger seeds can rely on seed reserves, however, to get above the lower leaves of the crop into partial sunlight and continue growth. The ultimate extension of this pattern is to perennial species that rely not on seed reserves but on substantial tubers, rhizomes or storage roots for their early growth. Enhancing crop competitiveness helps control all sorts of weeds, but whereas a highly competitive crop may slow new shoot growth of a perennial weed and check the growth of a seedling from a large seed, it can devastate the seedling of a small seeded species.

Finally, smaller seeded weeds are more easily controlled by organic mulch than large seeded species. First, fewer seeds of the small seeded species will receive the cues necessary for germination if covered with organic mulch (see Tillage & germination). Second, those that do emerge from the soil will be more likely to starve or become so weak that they succumb to disease before growing through the mulch and into the light.

All of the patterns discussed above are descriptions of general trends across many species. Exceptions can be found to any of these generalities that depend on the unique growth form or physiology of individual weed species. Nevertheless, seed size tells much about the biology of a weed and how to manage it.

This indicates the nature of the photosynthetic mechanism of the species. In general, C3 plants thrive under cool moist conditions, though they may tolerate warmer, drier conditions as well. Their rate of photosynthesis reaches a maximum at some fraction of full daylight. In contrast, C4 plants reach peak performance at high temperatures, are often drought tolerant, and their photosynthetic rate does not reach a peak even at full sunlight intensity. Thus, photosynthetic pathway is one indicator of the season and habitat favored by a species. Most broadleaf weeds and cool season grass weeds like quackgrass have the C3 pathway. Most warm season grasses like barnyardgrass and yellow foxtail have the C4 pathway. In addition, a few broadleaf weeds, including redroot pigweed and purslane have the C4 pathway.

Annual weeds differ in their approach to reproduction. Some species flower near the end of their lifespan so that death and reproduction occur more or less concurrently. Redroot pigweed and lambsquarters are examples of species that conserve resources for seed production until late in their lifespan. This type of species is usually capable of growing to enormous size, and if not controlled, most of these can reach 6 to 8 ft tall in favorable conditions. Most gardeners weed them out long before they get so large, but occasionally an individual is missed around the edge somewhere, perhaps in a neglected corner where an early harvested crop was grown or in the middle of a patch of pumpkins where it is essentially inaccessible without trampling the crop. The massive seed production from even a 3 or 4 ft plant will ensure plenty of weeding for years to come.

Other species, like hairy galinsoga, purslane and common chickweed, begin to flower and set seed while still small plants. Under favorable conditions these species will begin releasing seeds when just a few weeks old. They do not make many seeds at a time, but they continue to grow, flower and shed seeds throughout most of their lifespan. These little plants may hide among larger crop plants making seeds for weeks until they are eventually discovered and rooted out.

The 'big bang' annuals tend to predominate in full season crops that become difficult to weed after a certain point. They are often the worst weeds in fields of corn, spring planted grains and soybeans. The 'dribbling' annuals, in contrast, are better adapted to the frequent weeding that usually occurs in gardens. They produce seeds before they are noticed, and often their seed dormancy (Timing of germination) is such that a new generation can immediately sprout to replace the plants that have just been weeded out. Weeds are disturbance adapted species and these are the most disturbance adapted of the weeds.

Because different types of weeds are adapted to different types of disturbance regimens, gardens frequently undergo a succession of different weed communities. Often perennials and 'big bang' annuals predominate in new gardens as holdovers from previous uses of the land. The frequent hoeing and pulling by a persistent gardener may eventually bring these species under control, though they may also persist indefinitely if the weeding is less rigorous. Meanwhile, the 'dribbling' annuals slowly increase due to their ability to rapidly produce seeds during the inevitable brief periods of neglect. Some people say that the 'dribblers' increase because they no longer face competition from the larger species, but more likely the shift is just a natural but slow response to the change from a pre-garden to a garden type of disturbance regimen.

Stating seed longevity is inherently problematic. Seed longevity is usually reported in literature sources as the percentage of seeds alive after some given number of years or just as the existence of viable seeds after some number of years. Mostly we have echoed this in the entries in the database. However, such numbers do not reflect the nature of seed mortality in the soil. Unlike healthy humans that mostly die of old age, seeds in the soil tend to die at a constant rate. One way to understand that is this: if weed seeds had life insurance policies, their payments would be the same regardless of the age at which they bought the policy. Consequently, seed mortality is best expressed as either the percentage of seeds that die in a year, or the half-life of the seed population. Where data are available to express mortality in this way, percentage loss per year is given. Seed survival is highly dependent on weather (particularly for recently shed seeds), the presence or absence of seed predators that like the particular species, and soil and management conditions, so all types of data should be used only as a general indicator of the species persistence in the soil.

Weed seeds die primarily in one of three ways: (i) they begin to germinate in conditions that do not allow establishment, (ii) they are eaten by seed predators, or (iii) they die from physiological breakdown. The relative importance of the three mechanisms is generally believed to be in the order listed, though most of the species that have been studied systematically have been grasses with relatively large seeds.

Although most weed species possess mechanisms for determining appropriate seasons and conditions for germination (Timing of germination, Tillage & germination), these mechanisms do not work perfectly. Consequently, many seeds germinate too deeply in the soil for successful emergence, they begin to germinate and then dry out and die, or they begin to germinate and are then attacked by soil organisms.

In general, seed predators are most effective against seeds that are on or near the soil surface. Consequently, if weeds have gone to seed in the garden, fall tillage will tend to protect the weed seeds. Ground foraging birds and mice tend to be the major predators on seeds greater than 2 to 5 mg, whereas major predators on smaller seeds include ground beetles of the carabid family. Earthworms consume grass seeds and actually digest a substantial proportion of them; their action on broadleaf weeds has not been studied.

Although seeds are inanimate creatures, they are alive and metabolically active. In the soil, most weed species persist in a moist condition and are capable of metabolic repairs. Eventually, however, damage to membranes, genetic mistakes, toxins and other metabolic problems cause loss of vigor and eventual death. This happens most quickly at warm temperatures and when the seeds are nearly but not completely dry. Since both of these conditions are most likely to occur near the soil surface, seeds near the surface that remain dormant are in a risky position.

Most weed species have a particular season of the year in which they emerge most abundantly. The seeds of many species of weeds are dormant when shed from the parent plant. These must go through some period of aging (after ripening), a period of cold, or some other process before they are capable of germinating. Dormancy in weeds is generally either (i) a physiological process in which biochemical changes occur within the seed that ready it for germination, or (ii) due to a hard seed coat that prevents water from entering the seed. Generally, weeds with large seeds like hedge bindweed and velvetleaf rely on hard seed coats to maintain dormancy whereas small seeded weeds like lambsquarters and common ragweed rely on physiological mechanisms, but many exceptions to this rule occur.

Spring germinating weeds usually either require a cold treatment to release them from dormancy (e.g. common ragweed), or an aging process (e.g. the foxtails). If they do not germinate due to dry soil or other factors, eventually the warm temperatures of summer may restore them to a dormant state. They then have to pass through another winter before they can germinate. Other species may be held in a dormant state by high temperatures but become capable of germinating again in the fall when temperatures are cooler (e.g., spring and winter annuals like shepherd's-purse). Species that germinate only in the fall often require a period of hot temperatures to release them from dormancy and then germinate when temperatures again cool. (e.g., purple deadnettle), but few such species are common garden weeds. Summer germinating species like common purslane, have high temperature thresholds for germination and often will germinate directly after falling from the parent plant if the soil is warm enough.

Species with hard seed coats may germinate sporadically throughout the growing season as the seed coat softens in various individuals. The softening occurs due to physical, chemical and biological activity and may involve the opening of special pores on the seed (e.g., velvetleaf). Since winter provides a long period in which these processes can act on the seed, often species with hard seed coat dormancy show a peak in germination in the spring.