Garden Variety: Now’s the time to control crabgrass

photo by: Jennifer Smith/Contributed Photo

Crabgrass is a nuisance in the garden, but it can be controlled by hand-weeding, mowing or careful use of herbicides.

Early August is when crabgrass becomes most noticeable in the Lawrence area, and it is especially prolific this year. The weedy grass grows in spreading clumps and favors curb lines, bare spots in the lawn, landscape beds, gardens and sidewalk cracks. Now is the time to control it or make plans to prevent the plant from regrowing in the same places next year.

Crabgrass is a nuisance in the lawn because its seed heads pop up quickly after mowing, making the lawn look unkempt within a few days. It also competes with the desirable turf grass for water and nutrients. That competition is the main concern in landscape beds and vegetable gardens as well, where it easily takes over areas of bare soil in a few weeks’ time.

In sidewalk cracks, concern about crabgrass may be more personal, but a brick sidewalk is easily covered with unmanaged crabgrass. Plants quickly grow back if a weedeater is used on them, and they are especially difficult to pull in these settings.

A single crabgrass plant can produce about 150,000 seeds in a season, according to weed science experts, so keeping it from producing seeds is most important. Leaving it unmanaged means many more plants next year.

Physical removal of crabgrass is the most effective. Pull it by hand or use a hoe or other weeding tool to remove it. Each runner that has rooted into the ground may have to be pulled separately. Physical removal is usually easiest within a few days of a rain while soil is moist.

Although hand-weeding is generally reserved for landscape beds and vegetable gardens, crabgrass can be weeded or hoed out of the lawn as well. The bare areas that remain can be covered with mulch or seeded in a few weeks with desirable turf grass or ground cover.

For larger patches of crabgrass, you can use a mower or weedeater every few days to prevent seed production or treat smaller spots with a selective herbicide. If mowing, avoid the urge to mow at a lower height — this will actually make the problem worse. A short mowing height stresses the desirable grass, making it less likely to outcompete the crabgrass. The short mowing height also allows more sunlight to reach the soil surface, and the sunlight makes more crabgrass seeds germinate.

Use herbicides as a last resort. In a landscape bed or vegetable garden, any herbicide that kills grassy plants will work. If using a broad-spectrum product, be especially careful to avoid drift or contact with nearby desirable plants. For a lawn, look for products that contain the active ingredient quinclorac or fenoxaprop-p-ethyl and have a label that say they can be used on crabgrass. Other products on the market for crabgrass in lawns are only effective before the seed has germinated (pre-emergent) or when crabgrass plants are young and tender.

To minimize regrowth of crabgrass next year:

• Consider renovating lawns in September to minimize bare space where crabgrass could grow. (Look for more information on lawn renovation in next week’s column.)

• At a minimum, plant grass seed in bare areas of lawn in September.

• Mow lawns at 3 to 4 inches to ensure healthy, desirable turf grass that outcompetes crabgrass.

• Mulch over bare areas in landscape beds or vegetable gardens, or plant a ground cover. Use woodchips, compost, pine needles, etc. for landscape beds. Prairie hay and straw are more desirable in vegetable gardens.

• Avoid excess irrigation.

• If using a pre-emergent product, ensure proper timing of the application in the spring.

Crabgrass is identified by its growth habit, leaves (blades), and seed heads. It grows in short clumps, producing runners that root into the ground and make additional clumps a few inches from the main plant. Leaves or blades are much wider than those of desirable turfgrasses and are lighter green than fescue or bluegrass (the most common lawn grasses in the area). Leaves also have silvery hairs that make them feel soft and fuzzy.

Seeds are borne on erect stems that extend several inches above the grassy clump. At the top of each erect stem are five shorter stems lined with seeds. The shorter stems with the seeds have a feathery appearance. They wave easily in the wind.

There are two species of crabgrass that are common in northeast Kansas that look very similar. Smooth crabgrass is the most common. Large crabgrass has larger leaves, and its stems have a larger diameter.

— Jennifer Smith works in regulatory horticulture and has worked as a horticulturist for various government entities. She has experience in landscape design and maintenance and as an educator.


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