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Quackgrass (Agropyron repens) is difficult to control. The long running rhizomes can weigh up to 3-4 tons per acre. The seeds are a serious contaminant in cultivated forage seed. This perennial grass spreads by seed and underground rhizomes. The stems are smooth and reach 1 to 4.5 feet in height. The leaves are flat and slightly hairy on the upper surface. There are clasping auricles at the base of the leaf blade. The spikelets have 3-7 florets, attached broadside to the flowering stem. There are 2 horizontal rows on each spike. The seeds are yellow or white, and can remain in the soil for more than a year before germinating.

However, Quackgrass propagates and spreads mainly by rhizome formation. These rhizomes also secrete a toxic substance that suppresses the growth of surrounding plants and enhances the competitiveness of Quackgrass (known as an allelopathic effect). Quackgrass thrives under cool, moist conditions. The rhizomes often get tangled in harvesting equipment.

When present in dense patches (more than 900 shoots per square yard), Quackgrass can reduce wheat yields by up to 100%. For example, 500 shoots per square yard cause about a 50% loss in yield. As a rule of thumb, for every 100 shoots per square yard, the wheat yield will drop by about 10%. The economic threshold in canola is about 20-25 shoots per square yard.

A multi-year integrated control plan must be used for control of Quackgrass. This could include tillage, patch mowing, in-crop herbicides, and pre-harvest and post-harvest herbicides. Proper crop rotation is essential. As with some of the other weeds, the keys to control are in recognizing the problem and being persistent. Tillage may decrease Quackgrass density or may spread the rhizomes increasing spread of the weed. Quackgrass patches should be tilled into the center of the patch to limit the spread of the rhizomes. In the spring, repeat tillage whenever the top-growth approaches 2 inches. Remove rhizomes from tillage and seeding equipment before leaving Quackgrass-infested fields. Herbicides are available for broadleaf crops for suppressing Quackgrass.