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» Large crabgrass is a summer annual weed that can compete with vegetable crops for nutrients and moisture.
» Dense patches of large crabgrass can develop quickly during periods with warm conditions.
» Cultivating newly emerged seedlings, hoeing larger plants, and applying herbicides can help manage large crabgrass in vegetable production systems.
Large crabgrass (Digitaria sanguinalis) is a sprawling, summer annual grass. It is native to Eurasia but now occurs widely throughout the US. The plants form multiple stems (tillers) that grow along the ground and develop roots at the nodes. Rapidly growing patches of large crabgrass can compete with vegetable crops for moisture and nutrients, potentially resulting in reductions of yield and product quality.1,2,3,4
Figure 1. Large crabgrass plants: (A) a recently emerged seedling, (B) beginning tiller formation.
IDENTIFICATION
Large crabgrass cotyledons are oval to elongate, somewhat rounded at the tips, and grow parallel to the ground (Figure 1A).1,3 Seedlings initially grow upright and the true leaves unroll as they emerge, forming linear blades with tapered tips. Mature leaf blades can grow up to eight inches long and half an inch wide. The leaves have stiff, erect hairs on both upper and lower leaf surfaces.2,3
Mature stems are flattened and often red at the base. Tillers begin to form when plants reach the four to five true-leaf stage, and dozens of tillers can form on each plant (Figure 1B). Large crabgrass plants produce fibrous root systems that can grow as deep as six and a half feet below the soil surface.1,2,3
Large crabgrass inflorescences (seedheads) are made of finger-like spikes that form one-eighth to one-quarter inch apart near the tip of the stem. Two rows of spikelets are alternately arranged along each spike (Figure 2).3,4 Other grass species that can resemble large crabgrass include smooth crabgrass, southern crabgrass, goosegrass, yellow foxtail, and witchgrass. To differentiate, large crabgrass has stiff hairs on both upper and lower leaf surafaces, has broad collars with hairs on the edges, and lacks auricles.2,3
LIFE CYCLE AND CONDITIONS
Large crabgrass is found in temperate and tropical regions of the world between 50°N and 40°S latitudes. It reproduces mostly by seeds, which can survive in the soil for up to three years.1,2,3 Seed germination and emergence usually start when soil temperatures are in the 50 to 58 °F range, usually after the last frost. The optimal average daily temperature range for seed germination is between 68 and 86 °F. In southern California, large crabgrass can begin emerging from mid-January to early April, while emergence in more northern areas may not occur until late April or May. Adequate soil moisture is also needed to stimulate germination.1,2,3,4 Seeds in the upper 0.8 inches of soil are most likely to germinate. However, seeds can germinate from as deep as three inches below the soil surface.
Flowering is triggered by short day lengths, and flower stalks can start to form as soon as two weeks after emergence on plants that emerge later in the season. The flowers are mostly self-pollinated, but some pollination by wind-borne pollen does occur. Flowers will continue to form and produce seeds until the plants are killed by frost.2,3 Individual plants can produce as many as 145,000 seeds. Newly released seeds are dormant, but they can break dormancy over several months, a process that is enhanced by cold temperatures. Seeds are dispersed by the movement of soil and flowing water. Large crabgrass can also reproduce vegetatively. Roots can form at the nodes of horizontal tillers, and these can develop into new plants if separated from the parent plant. In areas where frost does not occur, large crabgrass plants can overwinter and start to grow again the following year.1,2,4
Large crabgrass grows best in undisturbed, bare ground áreas with full sun and warm temperatures. The plants can grow under a wide range of conditions, but growth is enhanced on well-drained soils with good fertility.1,2,3 Large crabgrass does not grow well under dense shade, during periods of cool temperatures, or in regularly cultivated areas (Figure 3). It can tolerate up to 63% shade with little effect, but higher shade levels usually result in reductions in growth and seed production. during periods of high solar radiation, can also be used to reduce the population of large crabgrass seeds in the soil seed bank.2,4
Figure 2. The inflorescence (seedhead) of a large crabgrass plant.
MANAGEMENT
Combining of early detection with the use of an integrated strategy is usually the most effective way to manage large crabgrass. Knowing the field history of crabgrass infestation and scouting when conditions favor emergence can help with management efforts. Avoid introducing large crabgrass into a field by cleaning possibly contaminated soil from tools and equipment before bringing them into another field.1,2,3
Control is usually most effective before large crabgrass plants have set seed, and it is usually easiest to eliminate plants in the seedling stage when they are two to three inches tall and before tillers have started to form.2 Plants are harder to eliminate once they produce roots at the base of the shoot.3 Multiple cultivations of fallow fields during the first four to five weeks of warm (>50 °F) weather can help eliminate emerged seedlings. The use of flaming or steam-based weeding can also be used, and these methods are most effective on young seedlings.2 Since large crabgrass seedlings are likely to emerge after crop plant establishment, selecting crops that can tolerate repeated cultivation close to production rows can help facilitate better crabgrass control.2 Selecting crops and planting/harvest times that allow for cultivation during the optimal large crabgrass emergence period can also help provide better crabgrass control.2
The application of organic mulches or the use of cover crop biomass to shade soils and keep them cool can help suppress crabgrass seedling emergence and development, but these methods are not as effective if applied after soils warm up.3,4
If larger crabgrass plants need to be managed, flat, shallow cuts that leave the entire plant and root system on the soil surface, where it will dry out, are most effective. Partially buried plants can re-root and survive.2,3 If established large crabgrass plants cannot be effectively eliminated, mowing to remove flower stalks before they set seed can help reduce the number of new seeds deposited into the soil. Since seeds can only survive in the soil for a few years, keeping new seeds from entering the soil seed bank will reduce the seed population over time.1,2 Soil solarization, applying clear plastic mulch during periods of high solar radiation, can also be used to reduce the population of large crabgrass seeds in the soil seed bank.2,4
Figure 3. Large crabgrass seedlings can be shaded out by dense crop canopies.
Herbicides can also be used to help manage large crabgrass in vegetable production systems. Depending on the crop species, several pre-emergent herbicides are available to help inhibit the germination and emergence of large crabgrass. Because large crabgrass seedlings emerge during the warmer parts of the season, herbicide application and crop planting/ transplanting dates need to be considered to help maximize the effectiveness of the treatment.4,5,6 Post-emergent products, including grass selective herbicides, are also available to help manage emerged crabgrass seedlings. Post-emergent applications are most effective on small seedlings. Large crabgrass plants become more difficult to manage with herbicide applications as they mature. Check regional vegetable production and pest management guides for lists of herbicides currently registered to control large crabgrass on the specific crops being grown. Always follow all product label instructions and restrictions.4,5,6
Selecting appropriate crop rotations can help in the long-term management of large crabgrass. Rapid rotations of short-season crops that allow for disruptions of crabgrass seedlings by multiple cultivations are helpful. Cultivation of fields after early harvested crops can also be used to help prevent crabgrass seed production. Planting cover crops between cash crops can also shade the soil and help suppress crabgrass emergence and growth. Additionally, vegetable crops that develop dense canopies early in the season can help suppress the development of large crabgrass infestations.1,3
SOURCES
1 Melendez, M. and Besacon, T. 2023. Large crabgrass management strategies for no/low spray specialty crop farms. Rutgers New Jersey Agricultural Experiment Station.
https://youtu.be/eSUZJ4MO4y4?si=aBCxBhQpALC1JYav.
2 Melendez, M. and Besancon, T. 2022. Large crabgrass life cycle disruptions for effective control in specialty crops. Rutgers, New Jersey Agricultural Experiment Station. Cooperative Extension Fact Sheet FS1347.
3 Cornell Weed Science. Large crabgrass. Weed Profiles. https://cals.cornell.edu/weed-science/weed-profiles/large-crabgrass#:~:text=a%20hairy%20ligule.-,Management,they%20 are%20harder%20to%20kill.
4 Molinar, R.H. and Elmore, C.L. 2019. Crabgrass. UC IPM, How to Manage Pests. UC ANR Publication 7456.
5 Phillips, B., Nair, A., Egel, D., Cloyd, R., and Meyers, S. (Eds.) 2024. Midwest vegetable production guide for commercial growers 2024. https://mwveguide.org/.
6 Wyenandt, C., man Vuuren, M., Owens, D., Johnson, G., Hamilton, G., VanGessel. 2022/2023 Mid-Atlantic commercial vegetable production recommendations.
Websites verified 2/1/2024
ADDITIONAL INFORMATION
For additional agronomic information, please contact your local seed representative.
Performance may vary, from location to location and from year to year, as local growing, soil and environmental conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on their growing environment. The recommendations in this article are based upon information obtained from the cited sources and should be used as a quick reference for information about vegetable production. The content of this article should not be substituted for the professional opinion of a producer, grower, agronomist, pathologist and similar professional dealing with vegetable crops.
BAYER GROUP DOES NOT WARRANT THE ACCURACY OF ANY INFORMATION OR TECHNICAL ADVICE PROVIDED HEREIN AND DISCLAIMS ALL LIABILITY FOR ANY CLAIM INVOLVING SUCH INFORMATION OR ADVICE.
5023_358906 Published 02/26/2024