Crabgrass

Don’t let crabgrass make you crabby!

As lawns and gardens in West Virginia get stressed by the relentless heat of late summer, crabgrasses begin to invade those areas and bloom rapidly. Two species of crabgrasses are prevalent in the Mountain State – smooth crabgrass (Digitaria ischaemum) and large crabgrass (Digitaria sanguinalis). Both species have naturalized themselves in North America and are considered to be weeds in crop and non-crop areas. They are both annual warm season grasses, completing their life cycles during the summer months. In the southern states, crabgrasses are considered to be desirable forages, therefore active management in pastures and hayfields is not necessary here.

Lawns in West Virginia are usually composed of cool season grasses, such as perennial ryegrass, Kentucky bluegrass and tall fescue. These turfgrass species tend to become semi-dormant during the hot summer months making lawns prone to attack by crabgrasses. Bare spots left by crabgrasses as they complete their life-cycles make lawns conducive to other weeds, such as hairy bittercress (Cardamine hirsuta), henbit (Lamium amplexicuale) and purple deadnettle (Lamium purpureum), which are winter annuals that typically germinate in late fall and grow actively during early spring of the following year.

Crabgrass Identification

Both smooth crabgrass and large crabgrass are characterized by a tiny, membrane-like appendage (ligule) at the base of the leaf on the upper surface. Large crabgrass has fine hairs on stem as well as leaves, whereas smooth crabgrass is devoid of hairs. Smooth crabgrass begins to germinate in early spring as the soil begins to warm up, whereas large crabgrass germinates later.

It is important to identify the predominant crabgrass in the lawn to execute the management methods, such as application of pre-emergent herbicide, in a timely fashion. However, both species of crabgrasses can occur side by side in a lawn. As a summer annual, crabgrasses reproduce solely from seeds that are deposited in the soil during later summer months. Smooth crabgrass seeds can remain dormant in the soil for about three years.

Controls for Crabgrass

Similar to other weeds, non-chemical approaches to control crabgrasses are mostly preventative and depend on maintaining a good stand of turf to keep the soil covered by providing optimal conditions to nourish the preferred turfgrasses. Those include maintaining a pH around 6.5 and adequate soil organic matter content for proper establishment, choosing an appropriate turf species, mowing at the appropriate height and frequency, fertilizing the lawn as needed at the proper times of the year, providing adequate drainage and irrigation, and occasional aeration.

Chemical controls comprise of applying a selective pre-emergent herbicide before crabgrass germinates or applying a selective post-emergent herbicide during its active growth phase. A combination of the two can also be applied in late-May while smooth crabgrass is actively growing. If applying a pre-emergent herbicide alone, it is best if applied in early to mid-April. A good phenological indicator is to apply the herbicide while the forsythias begin to bloom. Pre-emergent herbicides consist of pendimethalin (Pendulum), benefin (Balan), oryzalin (Surflan), oxadiazon (Ronstar) or dithiopyr (Dimension), while post-emergent herbicides consist of fenoxaprop (Acclaim) and quinclorac (Drive).

When applied at adequate quantities prior to crabgrass germination, natural products based on corn-gluten can suppress seed germination for two to three weeks in addition to providing nitrogen to boost the turf.

Reseed bare spots with a suitable turfgrass species after disturbing or lightly tilling those areas to facilitate contact with the soil to ensure good germination. Apply a mulch after reseeding, followed by light irrigation for a few weeks to allow swift establishment.

Author:  Rakesh Chandran, WVU Extension Weed Science Specialist 
Last Reviewed: August 2019 

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