The optimum timing of fall herbicide treatments can vary based on life cycle, and we can roughly lump the various life cycles into one of two categories:
1) Must be treated before frost, which pertains to all warm-season perennials, including johnsongrass, pokeweed, milkweeds and hemp dogbane and horsenettle. The first frost will cause these weeds to shut down, if they have not already matured and senesced. Herbicides are no longer effective after this occurs.
2) Can be treated after frost, and in some cases even after a hard freeze. Winter annuals, biennials and cool-season perennials fit into this category, and they are often most effectively controlled when herbicides are applied between mid-October and mid-November.
Winter annuals, including chickweed, purple deadnettle, mustards and cressleaf groundsel among others, emerge in late summer into fall. They survive frost and are still sensitive to herbicides even after cold weather in December, based on our research. Herbicide activity in these weeds slows down in cold weather, but the effective treatments still eventually kill emerged weeds.
It’s not necessary to wait until frost to apply herbicides, except that: 1) treatment too early in fall can miss the plants that are still emerging; and 2) for treatments that include herbicides with residual activity (metribuzin, simazine, Canopy, etc.), the soil temperatures in early fall are still warm enough for herbicide degradation to occur. This reduces the amount of herbicide present in spring, potentially allowing weeds to emerge earlier in spring than intended. It’s not necessary to use glyphosate for control of winter annuals, unless winter annual grasses are present. They can be controlled with combinations of 2,4-D and either glyphosate, metribuzin, Canopy, Basis or simazine.
Biennials, such as poison hemlock and wild carrot, are most effectively controlled in the fall at the end of their first year of growth, when they exist as a low-growing rosette. We do not have experience trying to control these with herbicides in winter under very cold conditions, but they are dormant then and should probably be treated in the mid-October to mid-November application window. Fall treatments for biennial weeds will generally be most effective when they include glyphosate and 2,4-D.
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.