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.