Grassy Weeds

Text adapted from Turfgrass Pest Management Manual: A Guide to Major Turfgrass Pests and Turfgrass
North Carolina Cooperative Extension Service

Weedy grasses are classified as summer annuals, winter annuals and perennials. Some of the most common ones are listed below. Characteristics and diagrams of weedy grasses are given in the previous section on "Turfgrass Vegetative Identification." Annuals complete their life cycles in one season by flowering, maturing seed and dying. Their high seed production serves as a ready source for infestation when conditions are favorable. Summer annuals germinate from late March through July, depending on the location. They flower in the summer and either die in the fall or are killed by frost. Winter annuals germinate in the fall and early winter and may continue to grow into early summer in cool seasons. Perennials live more than 2 years and may produce seed each season.

Weed Management Practices

Selection of adapted turfgrass species and cultivars and the use of cultural practices are important in minimizing weedy grass encroachment and competition. Management practices include: 1) mowing at the recommended height for the selected turfgrass and removing clippings when seedheads of grassy weeds are present; 2) applying the proper amount of nitrogen at the correct time according to the turfgrass present; 3) utilizing soil tests to determine needed potassium, phosphorus and lime; 4) avoiding power raking and aerifying after preemergence herbicides have been applied; and 5) applying preemergence herbicides before weeds germinate. Specific comments relating to selected weedy grasses are included in the following paragraphs.

Large Crabgrass [Digitaria sanguinalis (L.) Scop] Smooth Crabgrass [Digitaria ischaemum
(Schreb.) Muhl.]

Crabgrass germinates from March through early May when soil temperatures reach 53 to 58°F at a 4 inch depth. Alternating dry and wet conditions at the soil surface in the spring encourage germination. Crabgrass germinates and grows best when adequate light and moisture are present. Stems have a prostrate growth habit and may root at the lower nodes. It will grow under close mowing conditions. Crabgrass competition is enhanced by thin open turfgrass stands, improper mowing heights for the desired turf, summer fertilization and light, frequent irrigation.

For crabgrass control, practice procedures outlined to maintain a dense, actively growing turf. In areas where there is a crabgrass history, apply a preemergence herbicide in the spring when soil temperatures approach 53°F, which corresponds to when dogwoods bloom in North Carolina. These herbicides may be used effectively on home lawns, athletic fields, golf fairways, and parks, however it is advisable not to apply to golf course greens because of variable management practices and turf tolerance. A second preemergence treatment 8 weeks after the initial application may be necessary to maintain season-long control of crabgrass. Arsonate herbicides (DSMA, MSMA, CMA) may be applied postemergence for control of emerged crabgrass early in the summer in certain turfgrasses. Since turfgrasses vary in tolerance to preemergence and postemergence herbicides, check labels for tolerance information.

Goosegrass [Eleusine indica (L.) Gaertn.]

Goosegrass germinates when soil temperatures reach 60 to 65°F which is usually 2 weeks later than crabgrass. Like crabgrass, it requires moisture and light for germination; however, it is very competitive in compacted soils. Goosegrass has a prostrate growth habit, but unlike crabgrass it does not root at the nodes. It competes very successfully with warm-season and cool-season turfgrasses during summer months and is most competitive in thin, open turfs and turfs subject to intense traffic or use. Close mowing and frequent watering enhance goosegrass competition. It is a very troublesome grassy weed in athletic fields, golf greens, tees and fairways and other turfgrasses that are mowed short. Preemergence herbicides used for crabgrass control are less effective on goosegrass. Goosegrass may also be controlled by postemergence application of an arsonate herbicide in certain turfgrasses.

Annual Bluegrass [Poa arrnua L.]

Annual bluegrass is a light green bunch-type to slightly spreading winter annual weedy grass. Some subspecies exist which are classified as short-lived perennials. These perennial types are particularly evident in bentgrass golf greens. Management practices encouraging the persistence of annual bluegrass are: 1) excessively close mowing; 2) shallow, frequent irrigation; 3) poor soil drainage; 4) improperly timed fertilization; 5) aerifying or dethatching during primary periods of annual bluegrass germination; 6) use of heavy equipment causing compaction, and 7) shade.

Preemergence herbicides applied in the fall before annual bluegrass emerges give effective control in home lawns, athletic fields and golf course fairways. However, preemergence herbicides cannot be used where cool-season grasses are to be seeded in home lawns or over-seeded in golf course greens. Preemergence herbicides do not appear to be effective on the perennial subspecies. However, postemergence chemicals are now available which will selectively control annual bluegrass in golf course greens and permit the overseeding of perennial ryegrass.

Barnyardgrass [Echinochloa crusgalli (L.) Beauv.] Yellow Foxtail [Setaria glauca (L.) Beauv.] Green Foxtail [Setaria viridis (L.) Beauv.]

Other summer annual grasses, including barnyardgrass, yellow foxtail and green foxtail, may be controlled by preemergence herbicides. These grasses occur infrequently in North Carolina.

Annual ryegrass may be controlled by preemergence materials when applied before seed germination in the fall. In dormant bermudagrass it may be controlled also with postemergence applied herbicides.

Dallisgrass [Paspalum dilatatum Poir.] Nutsedge [Cyperus esculentus L.] Sandbur [Cenchrus spp.]

Dallisgrass and nutsedge, which are perennials, and sandbur are controlled with postemergence applications of an arsonate herbicide. However, only certain turfgrasses are tolerant.

Turfgrasses as Perennial Weedy Grasses

Desirable turfgrasses may be classified as perennial weedy grasses. Tall fescue in a bermudagrass lawn would be considered a weed. Most turfgrasses are difficult to control within another turfgrass. Therefore, select clean seed or vegetative sources for establishment, use an adapted turfgrass species and cultivar for your location, and use proper mowing and fertilization techniques to maintain a dense, actively growing desired turf. Digging or removal with hand or mechanical equipment, for example a sod cutter, is one way to control undesired perennial turfgrasses. You may spot treat an infested area with an appropriate non-selective herbicide, realizing it will also kill the desired turfgrass. The arsonate herbicides can be used to control centipedegrass and bahiagrass if they are considered undesirable.