Grassy Weeds

Prepared by Bob Polomski, Extension Consumer Horticulturist, and Bert McCarty, Turf Specialist, Clemson University. (New 09/99.)

HGIC 2300

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Grassy weeds are true grasses or monocots. A grass seed germinates and emerges as one single leaf. It develops hollow, rounded stems and nodes (joints) that are closed and hard. The leaf blades alternate on each side of the stem, are much longer than they are wide and have parallel veins.

A weed’s life cycle has great impact on the selection and success of a given control procedure, so it is important to learn the life cycle characteristics of a weed when you first learn its identity.

Annual weeds germinate from seeds, grow, flower, produce seeds and die in 12 months or less. Annual weeds are further categorized by the season in which they germinate and flourish. Winter annuals sprout in the fall, thrive during the winter and die in late spring or early summer. Summer or warm-season grasses such as crabgrass and goosegrass sprout in the spring and thrive in summer and early fall.

Perennial weeds are weeds that live more than two years. They reproduce from vegetative (non-seed) parts such as tubers, bulbs, rhizomes (underground stems) or stolons (above-ground stems), although some also produce seed. Perennial weeds are the most difficult to control because of their great reproductive potential and persistence.

Proper identification of weeds targeted for control is necessary in order to select effective control measures, whether cultural or chemical. Further assistance with weed identification is available from any Clemson Extension office.

Annual Bluegrass

Life Cycle & Description: Annual bluegrass (Poa annua) is a winter annual weed that emerges in early fall, persists through the winter, produces seed in early spring and then dies in late spring or early summer. Annual bluegrass reproduces by seed.

Annual bluegrass prefers shady, moist sites and invades weak, thin lawn areas, especially low spots and flower beds where standing water occurs. It mainly germinates in late summer through early fall when nighttime air temperatures drop to the mid-70s. This usually occurs from September 15 to October 1 in the Coastal Plain and Sandhills areas, and September 1 to 15 in the Piedmont and Mountain areas. Further germination occurs in early winter with warm days and cold nights.

Annual bluegrass produces a white-colored, pyramid-shaped seedhead in the spring. It dies in the summer with the onset of high temperatures and/or dry conditions.

Annual bluegrass has smooth, apple-green leaves with two clear lines, one on each side of the midrib that run down the length of the leaf blade. The edges of the leaf tip curve inward like the front of a boat.

Control: Handpulling is a simple, practical approach for small areas. Improve the health and density of the lawn by fertilizing at the right time and with the correct amount; maintaining an appropriate soil pH; mowing at the recommended height; and watering properly. Apply a 2- to 3-inch layer of mulch to ornamental bed areas to suppress germinating weed seeds. Finally, improve surface drainage. Preemergence herbicides are available depending on the kind of turfgrass and ornamental plants grown. Apply preemergence herbicides to established lawns before the annual bluegrass seeds germinate. Once annual bluegrass emerges, preemergence herbicides are generally ineffective.

Selective postemergence herbicides are available for annual bluegrass control. These are best applied in November or early December when the weed is small, thus most susceptible to control. Apply all chemicals according to directions on the label.

Crabgrass & Goosegrass

Life Cycle & Description: Crabgrasses (Digitaria species) are summer annuals that germinate in the spring at about the time crabapple and forsythia bloom, when the air temperature is warm enough to promote crabgrass seed germination. They produce seed from midsummer to fall and are then killed by the first freeze in autumn. Crabgrass reproduces by seed.

Crabgrass can be identified by its tufted or prostrate growth habit, hairy stems, broad leaves and flower spikes with two to nine finger-like branches. This weed appears in disturbed areas, weak or thin turf areas and in edges of the lawn next to sidewalks and drives.

Goosegrass (Eleusine indica) is a tough, clump-forming summer annual with white to silver coloring near its base. Unlike crabgrass, goosegrass has flat stems and does not root at the lower nodes. It germinates a few weeks after crabgrass in late spring and produces seed from summer to early fall. The flowers and seeds are produced in two rows like a zipper on two to 13 finger-like branches at the top of the stem. Goosegrass is killed at the first freeze, and reproduces entirely from seed.

Control: Handpulling is a simple, practical approach for small areas. Improve the health and density of the lawn by fertilizing at the right time and with the correct amount; reducing soil compaction; maintaining an appropriate soil pH; mowing at the recommended height; and watering properly. Apply a 2- to 3-inch layer of mulch to ornamental bed areas to suppress germinating weed seeds.

Preemergence and postemergence herbicides are available depending on the kind of turfgrass in your lawn. Preemergence herbicides provide about 2 to 2 1/4 months of control. Repeat applications would be required 60 days later for season-long control. Apply preemergence herbicides March 1 from the Coastal Plain to the Sandhills regions, and March 15 to 30 in the Piedmont and Mountain areas. Fall-seeded turfgrasses should not be treated with a preemergence herbicide until the following spring. Apply all chemicals according to directions on the label.

Sandbur 

Life Cycle & Description: Southern sandbur, or sandspur (Cenchrus eschinatus), and field or coast sandspur (Cenchrus incertus) are summer annuals that germinate in the spring, grow during the summer and early fall and die with the first heavy frost. The name "sandspur" describes the sandpapery feel of their leaves and the spurs or burs that are produced from July until the first frost. Both reproduce by seeds. Sandspur tends to be more of a problem on sandy soils from the Coastal Plain westward to the Sandhills.

Control: Handpulling with gloved hands is a simple, practical approach to control sandspur in small areas. Improve the health and density of the lawn by fertilizing at the right time and with the correct amount; maintaining an appropriate soil pH; mowing at the recommended height; and watering properly. Apply a 2- to 3-inch layer of mulch to ornamental bed areas to suppress germinating weed seeds.

This annual weed can be controlled with a preemergence herbicide applied in early spring (March 1 in the Coastal areas to March 15 in Piedmont areas). Repeat in 60 days. Select a herbicide that can be safely used on your lawn. Apply all chemicals according to directions on the label.

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This information is supplied with the understanding that no discrimination is intended and no endorsement of brand names or registered trademarks by the Clemson University Cooperative Extension Service is implied, nor is any discrimination intended by the exclusion of products or manufacturers not named. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.