Controlling Summer Annual Grassy Weeds in LawnsBy Donn Meindertsma, Fairfax Master Gardener
As we enter the summer season, you might notice some new outlaws sauntering onto your lawn. Gone are the turf weeds of late winter and early spring, such as hairy bittercress and chickweed. A new crop is in town, the “summer annuals.” What are summer annual turfgrass weeds, and what can you do about them? This Article discusses some of the more common summer annual weeds found in our lawns that have a grass-like appearance, sometimes referred to as “annual grasses” or, as here, “grassy weeds.”
Summer annuals germinate in spring and grow until the first frost, completing their life cycle in one season. The self-seeding grassy weeds discussed here produce copious quantities of seed that, once dispersed, remain dormant in the soil in winter, ready to sprout when warm weather returns. While we might not ever be able to beat these formidable yard nuisances, we can control them.
Types of grassy weedsCrabgrass (Digitaria sanguinalis, D. ischaemum) is readily distinguishable from desirable turfgrasses due to its scraggly growth habit and flat, relatively wide blades. It is a prolific self-seeder. Numerous sources accept the notion that one plant can produce up to 150,000 seeds per year. The seeds germinate, if exposed to light, when spring soil temperatures reach the mid-fifties. When summer temperatures arrive, crabgrass—as a warm season grass—is very competitive against the cool season, less-heat-tolerant turf grasses common in our region.
Crabgrass lookalikes include goosegrass (Eleusine indica). Goosegrass sprouts about a month later than crabgrass because its seeds require somewhat warmer temperatures for germination. If you successfully reined in your crabgrass early in the season, only to see it mysteriously reappear, you might in fact be looking at goosegrass. Goosegrass features a tight center rosette and silvery or whitish flattened sheaths. Viewed from above, it looks like spokes from a central hub.Barnyardgrass (Echinochloa crus-galli) is a tufted, erect grass that germinates around the same time as goosegrass (soil temperatures of 60 to 65 degrees). It is an even more prolific self-seeder than crabgrass; a healthy plant can produce from 750,000 to one million seeds. In a lawn, it will look coarser than the turfgrass, with blades protruding from a central crown. It prefers open, unshaded areas, and is intolerant of dense shade. It occurs in both moist and drier habitats and tolerates a variety of soil types.
Another crabgrass lookalike, the foxtails, including yellow foxtail (Setaria pumila), features long leaf blades and erect, red-tinged stems. It is the last of the grassy weeds discussed here to germinate, requiring soil temperatures of about 65 degrees. Foxtails can grow in moist or dry soils and tolerate a wide range of environmental conditions. Don’t fret if you’re not sure which of these weeds is what because most are controlled in the same way.Another grassy weed is Japanese stiltgrass (Microstegium vimineum), but it is distinguishable from the weeds discussed above. While most grassy weeds thrive in sunny, hot conditions, stiltgrass prefers moist shade. Stiltgrass is an invasive species in Virginia and stands out for its bamboo-like, stalky appearance, although in lawns its seedlings are nondescript and are less easily identified.
Control of grassy weeds
If your weeds are few and far between, pluck them by hand. Summer annual grassy weeds (especially stiltgrass) are among the easiest to pull because they usually lack creeping stems that root. Some might think that a close buzz with a mower (fitted with a catch bag) will nip off the seed heads of a mature plant, but crabgrass can produce seed even in very short turf. Also, mowing low will expose seeds already in the soil to sunlight, encouraging germination.
Chemical controls are also available. Look for products with the active ingredients mesotrione, fenoxaprop, fluazifop, or sethoxydim, which will control most of these weeds. Quinclorac controls crabgrass, but not goosegrass. (Pre-emergent controls are another option, although summer is not the season to apply them. Those controls must be applied before seeds sprout. Crabgrass pre-emergents should be applied in our area in mid-March.) The more mature your grassy weeds, and the denser and more prevalent they are, the more difficult it will be to control them with chemicals. Repeated applications may be required. As with any pesticide, users of herbicides are required by law to follow the product label.
The best control is to maintain healthy turf in the first place. Vigorous turf inhibits seed germination by shading seeds already present, while turf struggling in poor soil or shade gives these weeds an opportunity to germinate — giving space and sunlight settle in. Avoid fertilizing your lawn when weeds are sprouting; you don’t want to feed them. Also, water your lawn, as necessary, with deep, infrequent watering, which aids turfgrasses with deeper roots, and avoid light, infrequent watering that will benefit short-rooted weeds.
• Spring and Summer Lawn Management Considerations for Cool-Season Turfgrasses, Virginia
Cooperative Extension Publication 430-532
• Best Management Practices in Spring Crabgrass Control in Lawns, Virginia Cooperative Extension
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• Top 5 Weeds — Headaches in our Summer Lawns, Sharon V. Smith, Fairfax Gardening
• We Don’t Have Enough Weeds, Barbara Shepard, Fairfax Gardening
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Section 5, Lawn: Weeds and Table 5.7
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Station Cooperative Extension (Rutgers) Fact Sheet FS1237
• Japanese Stiltgrass, United States Department of Agriculture, U.S. Forest Service
• Biology and Management of Crabgrass, University of Massachusetts Extension
• Summer Annual Weeds, Missouri Botanical Garden