For a spring planting mechanical control ideally begins in the late summer or fall season. The soil can be tilled in dry or hot weather, which should dry out many of the rhizomes and reduce the weed load. If time permits, the process may be repeated later that fall. In the spring, after the plant has grown to 2 inches and if dry soil conditions exist, you can repeat the tillage. Yes, tillage will cut the rhizomes and create many daughter plants, but in the long run repeated tillage should bring many of the rhizomes to the surface where they will dry or can be easily removed. Repeated tillage may also exhaust food reserves in the disturbed rhizomes. Chemical control can be used in conjunction with this method. Careful, controlled burning of the quackgrass stand may also be used with other mechanical measures, but it might be best to check with your local fire department first..
A thick, organic, weed free mulch will help to discourage the quackgrass sprouts. It may even help to put down a layer of cardboard first, before adding mulch. Rye straw often contains quackgrass seed, so it should not be used as a mulch. Control will be ongoing and prompt removal of any quackgrass sprouts will save a lot of effort in the long run. Seed germination can occur at any time of the year.
Biological control involves the use of a cover crop to outcompete and smother the weed. The University of Minnesota recommendssmothering quackgrass with winter rye and crown vetch followed by buckwheat. This can clear an area of quackgrass and will add organic material to the soil. Again, this is a long-term process and should be done before the perennial bed is planted. I have also heard that ducks can be used to weed out quackgrass and other weeds.
Chemical control can be difficult. When using any chemical control, IT IS VERY IMPORTANT to read the label thoroughly and to follow directions exactly. Wear gloves, long sleeves, and long pants. Glyphosate, which is a broad-spectrum herbicide that will kill most green plants, should be carefully applied to growing quackgrass, and may be used in conjunction with fall tillage the season before the perennial bed establishment. Apply on a dry, windless day to avoid herbicide drift that might injure your good plants, and when no rain is predicted for 48 hours. If using in a densely planted bed, use a small paintbrush to apply to the weed or shield the perennial plants with a can or other barrier to prevent accidently killing the good guys. Glyphosate affects only actively growing tissue. Since about 95% of the quackgrass rhizome lateral buds are dormant at any one time, even in actively growing plants, repeat application should be done about 7 days after the first treatment to completely destroy the weed. You can try nitrogen fertilization to encourage quackgrass growth and the breaking of lateral bud dormancy. The glyphosate application will then move to the actively growing root buds and may destroy the plant.
Good luck with your control efforts.