The fall harvest season offers an opportunity to make note of problem weeds, and fields where weed control did not meet expectations.  The next step is to make plans to use this fall as an opportunity to control some problem weeds.  Fall is a good time to use herbicides to control problematic perennial, biennial and winter annual weeds because those plants are moving carbohydrates down into their root systems as part of the overwintering process and so herbicides are also translocated to the root system.  This week Mark Loux, OSU Extension weed specialist offers the following recommendations for some fall weed control targets:

Marestail in wheat: The plants of concern are the seedlings that emerge in late summer into fall, which can overwinter. A few options to consider follow. This is not an all-inclusive list of herbicide options, but some that make the most sense to us.  Following any of these will take care of most winter annual weeds as well.  It’s possible that some of the newer broadleaf products for wheat also have a fit, although none have residual activity.

• Tillage. Does not guarantee the complete absence of marestail but usually takes care of the problem for the season. Tillage should thoroughly and uniformly mix the upper few inches of soil to uproot existing plants and bury any new seed. Scout in spring to make sure control is adequate.
• Preemergence burndown + residual. The combination of glyphosate + Sharpen + MSO will control existing marestail and also provide residual control into fall. We suggest Sharpen rates of 1.5 to 2 oz/A. Spray volume of 15 to 20 gpa is required.
• Late fall POST. We have generally applied these in early November, and wheat should have 1 to 2 leaves depending upon the product. Options include Huskie, and combinations of dicamba (4 oz) with tribenuron (Express) or similar product. Do not apply products or mixes containing 2,4-D POST to wheat in fall.
• Spring POST. In our research, spring herbicide plus the competition from an adequate wheat stand has been effective, even though 2,4-D can be weak on overwintered marestail plants. Options include Huskie, 2,4-D, 2,4-D + dicamba, or combinations of 2,4-D with an ALS-inhibiting products, such as thifensulfuron/tribenuron (Harmony Xtra etc). The rate of dicamba that can be used in spring is too low to control marestail on its own. Most marestail populations are ALS-resistant, so in the ALS mixtures indicated above, the partner herbicide is carrying the load for marestail control.

Poison Hemlock: Fall is also a good time to work on poison hemlock infestations. Hemlock is a biennial (2-year life cycle). The large plants that become evident in spring were actually present in a low-growing form the previous fall, when they are in their first year of growth. Control of this weed is often ignored until late spring when it is large and fairly difficult to control, but it is much more easily controlled in late fall. In areas, such as fence lines, ditch banks, etc where poison hemlock is known to occur annually, consider a late fall application of 2,4-D + dicamba, glyphosate + 2,4-D, etc.