Large crabgrass

Images above: Upper left: Large crabgrass seedling (Scott Morris, Cornell University). Upper right: Large crabgrass young plant (Randall Prostak, University of Massachusetts). Bottom left: Large crabgrass rooting at stem nodes (Randall Prostak, University of Massachusetts). Bottom right: Large crabgrass panicle (Scott Morris, Cornell University).

Identification

Other common names:  purple crab-grass, finger-grass, Polish millet, crowfoot-grass, pigeon-grass, hairy crabgrass, hairy finger grass, northern crabgrass

Family:  grass family, Poaceae

Habit:  Sprawling summer annual grass with stems rooting at the nodes.

Description:  Seedlings have an upright habit and begin tillering when they have only 4-5 true leaves.  Seed leaves are parallel to the ground, oval to elongated in shape (only 3-4 times longer than wide), and rounded at the tip.  True leaves are rolled in the bud and uncurl as they emerge.  Ligules are membranous, translucent, and jagged.  Auricles are absent.  Collars are broad with hairy edges.  Blades are linear, 2-4.75” (5.1-12.1 cm) long by 0.1-0.2” (0.25-0.51 cm) wide, and tapered to a point.  Stiff, erect hairs are present on both blade surfaces and on the sheath.  Mature plants have dozens of tillers; stems are 12-48” (30.5-122 cm) with erect central stems and sprawling outer stems with rooting at the nodes.  Mature stems are flattened and red at the base.  Sheaths are compressed, hairy, and closed.  Leaves are similar to young seedling leaves, but blades are up to 8” (20.3 cm) long by 0.6” (1.52 cm) wide.  Fibrous roots extend up to 6.5 ft (198 cm) deep.  Inflorescences are terminal, composed of up to 13 (usually 3 to 5) flat, fingerlike spikes arranged in a spiral near the stem tip.  Two rows of 0.1” (0.25 cm) spikelets are arranged alternately along each spike.  Seeds are glossy, yellow-brown, elliptic or lance shaped.

Similar species:  Smooth crabgrass [Digitaria ischaemum (Schreb.) Schreb. ex Muhl.] is closely related and has a non-jagged, translucent ligule and few hairs on leaf blades or sheaths.  Goosegrass [Eleusine indica (L.) Gaertn.] is also mat-forming, but goosegrass leaves are folded in the bud.  Southern crabgrass [Digitaria ciliaris (Retz.) Koeler] has hairs on the sheaths, but none on the blades.  Yellow foxtail [Setaria pumila (Poir.) Roem. & Schult.] and witchgrass (Panicum capillare L.) seedlings are also rolled in the bud and absent of auricles, but they have a hairy ligule.

Management

The tiny, fragile seedlings of large crabgrass are easily killed by tine weeding or rotary hoeing just before or shortly after they emerge.  Once the seedlings have developed roots at the base of the shoot in addition to the primary root from the seed, they are harder to kill.  Early planted corn may grow too large for in-row weed management when most crabgrass emerge.  In crops that tolerate hilling-up, try to do so before the first true leaf elongates since an incompletely buried plant can recover.  When cultivating to kill larger plants, set up cultivators to cut flat and shallow so that the shoots quickly dry on the soil surface.  Burying the plants is less effective because complete burial of larger plants is nearly impossible, and large crabgrass re-roots exceptionally well.

Since most large crabgrass seeds do not persist long in the soil, rotation into a sod crop for a few years greatly decreases the seed bank, provided seed production is prevented during establishment of the sod.  Cutting sod as high as possible will facilitate competitive suppression of crabgrass by these crops, including turfgrass.  The prostrate growth habit and tolerance of hot, dry summer conditions make this species well-suited to gaps in regularly mowed hay crops, so good hay stands are essential during the hay phase of rotations.  Vegetable rotations that include early, short-season crops tend to reduce the population by allowing many seedlings to emerge shortly before harvest.   Subsequent tillage for the next crop or cover crop then destroys the young crabgrass before they can set seeds.  A cultivated fallow period during the first 4-6 weeks of warm weather can effectively decrease severe infestations since most seeds will germinate if provided with warm, moist conditions.  Avoid growing a short season soybean cultivar that drops leaves too early in late summer since this creates a window for late season seed production (Oreja et al. 2021).

Straw mulch applied in the spring keeps the soil too cool for germination and is highly effective.  The same mulch material applied after the soil warms will be relatively ineffective because large crabgrass seedlings can worm through up to 3” (7.5 cm) of straw.  Accordingly, cover crop residue suppressed large crabgrass early in the season, but not for the full season (Pittman et al. 2020).  However, mulch materials that impose a suitable physical barrier of 2” (5 cm) or greater in depth can reduce large crabgrass by over 90% (Saha et al. 2020).  Green manure cover crops in the mustard family incorporated in the spring provided 48-79% control of large crabgrass at four weeks after transplanting bell peppers in South Carolina, but no control was detected later in the season (Norsworthy et al. 2007). 

Ecology

Origin and distribution:  Large crabgrass probably originated in Asia or Africa and was introduced into North America during European settlement.  It occurs in both temperate and tropical climates and is currently distributed from 50° N latitude to 40° S latitude around the world (Holm et al. 1977). 

Seed weight:  0.51 mg (Gaba et al. 2019), 0.52 mg, including chaff (Mohler unpublished), 0.46-0.59 mg (Turner 2012).

Dormancy and germination:  Seeds are dormant when shed from the parent plant (Burnside et al. 1981, Masin et al. 2006), but gradually lose dormancy over a period of several months (Halvorson and Guertin 2003, Oreja et al. 2020, Turner 2012).  Cold treatment accelerates the loss of dormancy as does puncturing or damaging the seed coat (Halvorson and Guertin 2003).  Daily alternating temperatures between 68 and 86 °F (20 and 30 °C) are required to maximize germination (Halvorson and Guertin 2003, Hsu et al. 1985).  Light is not essential, but it can enhance germination (Masin et al. 2006).  Nitrate modestly increases laboratory germination and field emergence of fresh (dormant) seeds, but otherwise has no effect on germination or emergence of after-ripened seeds (Turner and Van Acker 2014).  Seeds undergo a periodic annual cycle in soil whereby dormancy is low in spring following cold after-ripening and secondary dormancy is induced in late summer when temperatures exceed 81-86 °F (27-30 °C) (Masin et al. 2006).

Seed longevity:  No seeds survived longer than three years when buried at 1.7” (4.3 cm) under turfgrass, but approximately 55% survived for one year (Masin et al. 2006).  The annual mortality rate of large crabgrass seeds buried in an agricultural field was computed to be 54% (Egley and Chandler 1978).  More than 95% of seeds remaining on the soil surface for 9 months in a no-tillage system germinated or died (Oreja et al. 2020).

Season of emergence:  In Canada, large crabgrass emerged primarily in June and July (Turner 2012).  In Ohio and the mid-Atlantic area, most seedlings emerged in May and June, with some emergence continuing through the growing season (Cardina et al. 2011, Myers et al. 2004).  In warm climates such as Florida, soil tillage in February to April led to maximum establishment (Busing and Clebsch 1983).  In all cases, these emergence periods generally follow the last frost for the respective area.  Emergence occurs when rainfall exceeds 0.2” (4 mm) of rain (Gallart et al. 2010) and when soil temperature exceeds 50-58 °F (10-14 °C) (Halvorson and Guertin 2003, King and Oliver 1994, Steinmaus et al. 2000).

Emergence depth:  Large crabgrass emerges best within the top 0.8” (2 cm) with some emergence from as deep as 3” (8 cm) (Benvenuti et al. 2001, Peters and Dunn 1971).

Photosynthetic pathway:  C4

Sensitivity to frost:  Large crabgrass is frost sensitive (Halverson and Guertin 2003, Turner 2012).

Drought tolerance:  Large crabgrass grows best at high temperatures and thrives in conditions that cause heat or moisture stress in many other plant species (Holm et al.  1977).  It develops a disproportionally larger root system very quickly, enhancing its ability to compete with neighboring species (Halvorson and Guertin 2003).  The roots penetrate to 6 ft (2 m) (Holm et al.  1977), which allows the plants to tap residual soil moisture that is unavailable to many crops.

Mycorrrhiza:  Large crabgrass is mycorrhizal (Harley and Harley 1987).

Response to fertility:  Large crabgrass tolerates infertile soils but it is highly responsive to fertility, both when growing alone and in competition with a crop.  It is a strong concentrator of most major nutrients, particularly K which may reach > 6% of foliage dry weight (Peters and Dunn 1971).  Nevertheless, strong competition from corn prevented a response to nutrient rate in one experiment (Peters and Barrett 1976).  It tolerates low pH soil, with maximum emergence and growth at pH 4.8 to 5.8 (Buchanan et al. 1975, Pierce et al. 1999).  Magnesium carbonate is more effective than calcium carbonate at reducing emergence and growth of large crabgrass when raising soil pH to neutral levels (Pierce et al. 1999).

Soil physical requirements:  Large crabgrass tolerates a wide range of soil conditions.

Response to shade:  Large crabgrass is partially shade tolerant.  In field conditions it has been found to tolerate up to 63% shade without reduction in productivity or seed set (Peters and Dunn 1971).  Greater degrees of shade, however, greatly reduce growth.  A 95% reduction in light penetration for approximately 2 months by a soybean canopy reduced large crabgrass biomass and seed production by an average of 97% and 94%, respectively. 

Sensitivity to disturbance:  Seedlings are tiny and easily damaged.  Completely uprooting larger plants is difficult due to the extensive, fibrous root system.  Medium to large plants re-root readily in moist soil.  Like all grasses, the growing points are near the soil surface, so simply slicing off the plant at ground level will result in vigorous re-sprouting.    Covering the lower portion of stems with soil during cultivation or hoeing promotes rooting of the stems at the nodes.

Time from emergence to reproduction:  Large crabgrass generally flowers from July to September (Peters and Dunn 1971).  Plants that emerged in May first flowered in July about 10 weeks after emergence (Doll 2002, Peters and Dunn 1971), whereas plants that emerged in June flowered only 2 weeks later (Peters and Dunn 1971).  The species flowers in response to short day length (Halverson and Guertin 2003, Peters and Dunn 1971).  Once a plant begins to flower, it will continue to flower and set seed until frost (Holm et al. 1977).

Pollination:  Large crabgrass is primarily self-pollinated (Volenberg and Stoltenberg 2002), but sometimes cross pollinates by wind (Turner 2012).

Reproduction:  Reproduction is principally from seeds, but large crabgrass is one of the few annual weeds that also can reproduce vegetatively.  The outer stems of the plant lie on the ground and tend to form roots where the nodes (especially the first node) come into contact with the soil.  Consequently, single plants up to 10 ft (3 m) in diameter have been observed (Holm et al. 1977).  Well-spaced plants emerging in May in Connecticut produced an average of 145,000 seeds per plant, although production at high plant densities range from 100 to 6,800 seeds per plant (Gallart et al. 2010).  At the time of soybean harvest in Virginia, 60% of large crabgrass seeds were shattered (Schwartz-Lazaro et al. 2021).

Dispersal:  Large crabgrass seeds have no apparent adaptations for dispersal. However, because they often reach high densities in soil, they are easily spread from one site to another in soil clinging to tires and machinery.  It also disperses in irrigation water (Kelly and Bruns 1975).  The species is common on waste areas and in lawns and frequently has to travel only short distances to invade a field. 

Common natural enemies:  Larvae of the flea beetle, Chaetocnema denticulate, bore into the top of the seed head while it is in the boot stage and feed on the stems and immature seeds.  In Virginia, 26% of seed heads were infested, and the insect completed three generations per year.  Various species of Oscinella fruit fly larvae mine the stems (Beisler et al. 1977).  Loose smut (Ustilago syntherismae) can significantly reduce the proportion of reproductive plants (Gallart et al. 2010).  Drechslera gigantea leaf spot can cause significant injury and has been explored as a bioherbicide (Shabana et al. 2010).

Palatability:  The seeds are edible and can be ground for flour or cooked as porridge.  The species is excellent cattle fodder (Bosworth et al. 1980) and the species is sometimes harvested for hay (Holm et al. 1977).

References:

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