Quackgrass is found in most parts of Montana and is one of the more difficult weeds to manage in both yards and gardens. It is a coarse-bladed grass that forms clumps of vegetation that displace Kentucky bluegrass and some of the more desirable grasses.
Quackgrass is a perennial, meaning it lives for more than one year. It reproduces both by seed and by underground rootstalks. Stems that produce a slender, wheatlike seedhead are hollow and can grow one to three feet tall. Leaves are narrow, dark green, and rough on the surface, often with a distinctive constriction near the tip of the grass blade. Horizontal rootstalks can grow five feet in length in one season, producing new plants at nodes spaced along the root.
This grassy weed is often confused with crabgrass, which is an annual weed that reproduces each year only by seed, and which is not common in Montana. It is important to determine which weed you have, because what works to manage one, won't work for the other. Crabgrass has wide, coarse blades, but the stems that produce the seedheads are shorter, growing only 6-8 inches tall, and are not hollow. The seedhead on crabgrass is not wheatlike, but rather like 4 strands about 1½ inches long that spread in different directions at the end of the stems. Roots of quackgrass have sharp-tipped points on some of the horizontal rootstalks, and new plants can be seen arising from these rootstalks, some distance from the parent plant. If you are not sure about the identity of the weed, your Extension agent or county weed officer can help.
To control quackgrass, apply spot treatments with a glyphosate herbicide such as Roundup. The glyphosate herbicides are necessary because if applied correctly, carefully following label instructions, they will often travel down into the root system. This is essential in controlling quackgrass to kill the vegetative buds on the rootstalks. Do not apply glyphosate when plants need water because drought-stressed plants slow down all internal activity, thus slowing transport of the glyphosate to the roots. Glyphosate herbicides are relatively safe for both humans and the environment because they are of low toxicity and quickly broken down by soil microorganisms and the ultraviolet rays in sunlight. Remember, however, that glyphosate is non-selective and will kill any plant it touches, including trees. This means it must be applied very carefully and allowed to touch only those plants you wish to kill. Applications can be made using rubber gloves and a sponge, or by dipping a cotton glove worn over the rubber gloves directly into the solution, then touching the plant. Concentrate formulations that you must mix yourself are more effective than ready-to-use formulations. Do not use more than label instructions call for. Reseed or plant the area a week to 14 days after treatment with glyphosate to help prevent reinvasion by weeds.
Another least-toxic herbicide you may wish to try is glufosinate-ammonium (e.g. Finale). Glufosinate-ammonium is a partially systemic herbicide that kills both roots and above-ground portions of weeds. Like glyphosate and herbicidal soap, it will kill both desirable plants (grasses and ornamentals) as well as weeds, so must be allowed to touch only plants you wish to kill. Glufosinate-ammonium is a synthetic version of a naturally-occurring compound produced by soil bacteria. It is of low mammalian toxicity and is rapidly biodegraded.
Avoid tilling quackgrass because even short pieces of the rootstalks that are left in the ground can produce new plants, making the problem even worse. Pulling by hand is usually not effective because root pieces which break off in the ground can produce new plants, possibly more than were originally present on the site.
Fabric mulches, available from yard and garden suppliers, is useful in some settings; quackgrass does not penetrate certain brands of this mulching fabric, but will penetrate others. Ask to be sure. A triple layer of 80-pound grade roofing paper is also useful as a permanent barrier on areas such as driveways or permanent borders. Both types of mulch can be covered with decorative material such as woodchips or gravel. The quackgrass can grow out around the edges however, so either extend the mulch six to eight feet beyond established quackgrass plants, or monitor the edges for new plants, and treat with glyphosate or glufosinate-ammonium any which emerge from under the mulching. Barriers set down into the soil can prevent this, and also keep the gravel or bark chips from migrating into areas where it is not wanted. The barriers must extend deeper in the soil than the quackgrass roots, probably at least one foot deep. Quackgrass is very good at what it does, so persistence is the key to managing it.
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Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997