Weed of the Week: Crabgrass

May 4, 2018

Weed of the Week: Crabgrass

Type: Crabgrass is a common lawn invader that most people are already familiar with. This is a low growing, grassy summer annual weed that's a troublesome pest that lowers turf quality and appearance. Crabgrass is a prolific seed producer; one plant can produce over 150,000 seeds, making it a difficult weed to eliminate once it has started growing in turf.

There are several species of crabgrass that are problematic to lawns. The two closely related species being discussed throughout this article are large or hairy crabgrass and small or smooth crabgrass. These species of crabgrass roots develop at nodes on the prostrate stems.

How to Identify:

Large (Hairy) Crabgrass - A rapid growing, coarse textured weed that appears as yellowish-green grass. It is particularly easy to spot against turf that is fine textured and dark green. A bunch like type grassy weed, large crabgrass features seed head spikelets in 2-9 fingerlike branches along the stalk. The weed's leaves appear to be rolled in the bud while the collar is broad with long hairs. Large crabgrass has tall, membranous ligules outgrowth from the sheath that are jagged at the edges and have no auricles. If left alone, this species of crabgrass can grow as tall as 2 ft. tall! Picture source: NY State IPM Program at Cornell University | Flickr

Smooth (Small) Crabgrass - This low-growing weed appears as a light green grass. It can be distinguished from large crabgrass for its smooth appearance or absence of hairs on the leaves and sheaths. Smooth crabgrass offers a prostrate growth habit as well as seed heads that feature two-six fingerlike spiked branches. Although both large and smooth crabgrass species can grow under close mowing heights, smooth crabgrass can survive as well as set seed at grass heights as low as 0.25 inches!
Picture Source:  https://extension.umass.edu/landscape/weeds/digitaria-ischaemum

NOTE: Crabgrass is often mistaken for goosegrass. Click to learn more about goosegrass to properly identify grassy lawn invaders.

Where it Grows: This weed prefers areas of lawns where turf is weak and experiencing bare spots. Crabgrass favors lawns that have been mowed too short, as well. This weed can be spotted in turf, lawns, ornamental landscapes and vegetable gardens.

Growing Season: Both large and smooth crabgrass species begin to germinate when soil temperatures reach 55°F for at least 3 consecutive days. Growth and germination will continue throughout summer and into fall. After the first heavy frost, crabgrass will die.

How to Manage: Because crabgrass is a prolific seed producer, it's difficult to kill once it takes hold in the soil. The best method for removing crabgrass is to follow these good cultural methods. First, maintain a healthy, dense turf with a strong root system and a good lawn fertility/care routine to prevent bare spots or weak turf that allows for crabgrass to move in. Next, ensure the correct mowing height for your turf species and avoid lawn scalping since this weed can survive in low-mowed turf. Finally, establish proper irrigation, aeration and thatch management practices so crabgrass cannot establish itself into lawns.

For intense crabgrass infestations, use a herbicide that contain season long pre-emergent and post emergent control. Look for products that contain the active ingredient (AI) dithiopyr, a proven herbicide control technology that provides pre- and post-emergent control for crabgrass and other weeds (NOTE: Avoid the use of herbicides in the heat of summer, as they will cause more stress to turf). Apply the herbicide in early spring when soil temperatures reach 55°F. Once applied, DO NOT mow or cut back the crabgrass in any way; allow the herbicide to absorb into the shoot tissue so that it may be transported down to the roots. Because crabgrass is an invasive, tough weed to control throughout the growing season, repeat applications of herbicides may be needed until the infestation is eradicated.

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