Perennial weeds, such as Canada thistle, field bindweed, or quackgrass, are most effectively controlled with herbicides when they are fall applied. Applications should be made to field bindweed in the late summer or early fall. Other weeds, such as quackgrass, are more effectively controlled with later applications of herbicides. As plants harden off for winter, they move more of their stored nutrients into underground roots, rhizomes, or tubers to assure regeneration the following year. Many herbicides applied during or just before the weeds harden off, will move with these nutrients and kill underground buds, preventing re-establishment the following year.
Tillage is also effective in controlling most perennial weeds.
Deep tillage, such as moldboard or chisel plowing, can bury perennial weeds deeply enough in the soil to prevent re-establishment. However, shallow tillage, such as disking or cultivating, may not bury perennial weeds deeply enough and can serve to spread or intensify infestations. A healthy crop, forage, turf, or other desired plants established in the fall will provide strong competition against perennial weeds trying to re-establish following tillage. Systemic herbicides and tillage used in combination can prove very effective in controlling perennial weeds. Be sure to allow time between herbicide application and tillage to ensure that the chemical has time to move throughout the plant.