Weedy Grasses in Lawns

Control of grass weeds in lawns puzzles many people because products like Weed-B-Gone, or Weed-N-Feed and other products that contain 2,4-D don't work. Those products are designed for broadleafed weeds and won't work for grasses. Control of grass weeds requires different methods than broadleafed weeds. 

Grass weeds that are found in lawns fall into two groups; annuals and perennials. The annual weeds, such as crabgrass, are not that difficult to control if you know what they are and what to do about them. The perennial weeds, such as quackgrass, can be more difficult to control, but they too, can be managed fairly well if you know what they are and if you are persistent and patient. 

The key to managing the weedy grasses is identifying them correctly. For example, quackgrass (a perennial) is often mistaken for crabgrass, which is uncommon in Montana. But what works for crabgrass (e.g. Ortho Crabgrass Killer) won't affect quackgrass. If you are not sure what the identity of a weed is, take a sample to your county Extension agent or your county weed office. These individuals will also tell you if the weed is an annual or a perennial. An annual is a plant that dies at the end of its growing season; new plants are produced each year from seed. Preventing seed production in annual weeds by mowing or other means is often effective, and so are pre-emergence herbicides. A perennial, on the other hand, lives for more than one year and grows back from
the roots each year. Perennials can also be produced from seeds. Deep-rooted perennials, such as Canada thistle or bindweed, are usually not effectively managed by methods that do nothing to control roots. Roots of perennials often have vegetative buds, growing deep in the soil, that can give rise to new, independent plants. Certain herbicides that are "translocated" will move down into the root system. Perennials that have root systems more shallow than healthy grass roots can often be managed by good watering and mowing practices. For more information on using plant competition to manage weeds in lawns, see the fact sheet in this series, "Weeds in Lawns: Healthy Grasses Can Outcompete most Weeds", or watch the video "Proactive Pest Management", available on loan or for purchase through your county Extension agent. 

Some of the more common annual grass weeds in Montana include goosegrass, yellow foxtail, green foxtail, barnyard grass. Crabgrass, is one of the most infamous annual weedy grasses, but it is not found in most parts of Montana. It is similar in appearance to quackgrass, and often mistaken for it. For annual grasses, the key is to keep seeds from sprouting; the seeds of summer annuals germinate in spring and the seeds of winter annuals germinate in fall. In established lawns, the application of a pre-emergence herbicide such as dacthal, applied in either spring (for summer annuals) or fall (for winter annuals), can make management of annual weeds such as crabgrass fairly easy. A pre-emergence herbicide kills germinating seeds of both broadleaf and grass weeds. Do not use pre-emergence herbicides if you plan to seed desirable lawn grasses or ornamentals. Many of the post-emergence herbicides are
also quite effective if the weed identified is on the label and if all instructions are followed carefully. In some cases, annual weeds must be dug out, or simply tolerated. Mowing annual weeds frequently enough to prevent seed formation can reduce infestations substantially over time. Watch for stems that are prostrate and not cut by the mower; they will produce seeds close to the ground, so you might want to dig them up. 

The more common perennial grass weeds in Montana include quackgrass, orchardgrass, creeping bentgrass, and brome. To control these weeds, you will need to apply a non-selective herbicide such as Kleenup or Roundup. These products contain glyphosate, a least-toxic herbicide that is environmentally gentle when used as directed. The term "non-selective" means that it will kill any plant it touches, however, so it must be applied very carefully to avoid damage to the surrounding desirable grass, ornamentals, or trees. Some people wear rubber gloves and apply the product with a sponge. The advantage of glyphosate is that it moves down into the plant and kills the root system, essential in the control of perennials. Digging perennials out by hand is often ineffective because they are usually deep-rooted and if the entire root system is not removed, new plants can grow back, actually worsening the problem. Be sure to read and follow label directions carefully when using any pesticide. 

References: 

Danneberger, T.K. 1993. Turfgrass Ecology and Management. G.I.E. Inc., Cleveland, OH. 201 pp. 

Lajeunesse, S., G. Johnson, and J. Jacobsen. 1997. Proactive Pest Management; Using landscape design and plant vigor to manage weeds, insects, and diseases. Montana State Univ., Extension Service, Video No. 38. Bozeman, Montana. 23 min. 

Shurtleff, M., W. Thomas, and R. Randell. 1987. Controlling Turfgrass Pests. Prentice- Hall, Inc. Englewood Cliffs, NJ 07362. 449 pp. 

Turfgrass Pests. 1989. Cooperative Extension, Publication 4053. Univ. of California, Oakland, CA 94608. 121 pp. 

Watschke, T., P. Dernoeden, and D. Shetlar. 1995. Managing Turfgrass Pests. Lewis Publishers, Boca Raton, FL 33431. 361 pp. 

Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997