Cool-season perennials include dandelion, Canada thistle, and quackgrass. Herbicides can be more active in these weeds after a frost, which triggers the plant to increase movement of carbohydrates into the roots or rhizomes. Systemic herbicides move to these same areas with the translocating carbohydrate.

 

The other key to effective control is to make sure the weeds have recovered fully from harvest, late summer mowing or earlier droughty conditions, and are fairly sizable. Canada thistle should be 8-12 in. tall for best results, and dandelion should have a healthy rosette with a number of fully expanded leaves. Delaying herbicide application until late October or November can allow more time for the needed recovery.

 

While winter annuals can be treated into early winter, the colder conditions will reduce herbicide activity in cool-season perennials. Cool-season perennials will generally be most effectively controlled with combinations of glyphosate and either 2,4-D or dicamba, although combinations of 2,4-D with Basis or Canopy are among the most effective treatments on dandelion . Glyphosate is most effective on Canada thistle when applied without other herbicides, and is really more effective than 2,4-D or dicamba on dandelion in the fall. However, combinations of glyphosate and 2,4-D will be more effective for control of the many populations of glyphosate-resistant marestail.