Believe it or not, weeds are the number one pests of our field crops. Based on our recent pre-harvest weed survey of soybeans, and questions received throughout the year, I firmly believe farmers need to pay attention to all management techniques to combat these persistent “plants out of place.” Fall treatment is a tool to consider.

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 according to Ohio State University Extension Weed Specialist Dr. Mark Loux. 

•Weeds that must be treated before frost; pertains to all warm-season perennials, including johnsongrass, pokeweed, milkweeds and hemp dogbane, and horsenettle. The first frost shuts these weeds down, if they have not already matured and senesced, greatly reducing herbicide effectiveness.

•Weeds that can be treated after frost, and in some cases even after a hard freeze; include winter annuals, biennials, and cool-season perennials. 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 remain sensitive to herbicides even after cold weather in December, based on OSU research. Herbicide activity in these weeds slows down in cold weather, but the effective treatments still eventually kill emerged weeds.

Treatment too early in fall can miss the plants that are still emerging; and 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 (see also C.O.R.N. Newsletter http://corn.osu.edu for previous articles).

Biennials, such as poison hemlock (frequently seen along open ditches and woods boundaries) and wild carrot (fence row favorites), are most effectively controlled in the fall at the end of their first year of growth, when they exist as a low-growing rosette.
Those weeds are dormant under very cold conditions, so a mid-October to mid-November application window should be utilized. 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 to 12 inches tall for best results, and dandelion should have a healthy rosette with a number of fully expanded leaves. 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.

Finishing harvest? Great! Now attack those weeds.

The views and opinions expressed in this column are those of the author and not necessarily those of Farm World. Readers with questions or comments for Roger Bender may write to him in care of this publication.