Late fall still offers chance to work on marestail

Soybean Pest Beat: Agronomists say it’s hard to overestimate the value of fall herbicide applications.
Tom J Bechman | Oct 11, 2017
How long can we go before it’s time to shut off the sprayer and quit applying fall herbicides? I have a decent crop of winter annual weeds, but I also have marestail up, and that’s what I am trying to get.
The Indiana certified crop advisers answering this question include: Betsy Bower, agronomist for Ceres Solutions Cooperative, Terre Haute; Jamie Bultemeier, agronomist and corporate sales manager for A&L Great Lakes Labs, Fort Wayne; and Steve Gauck, sales agronomist with Beck’s, Greensburg.
Bower: One can apply herbicides fairly late into the fall. We just need good growing conditions to properly translocate the herbicide to the roots. Weeds to target include Canada thistle, dandelion and the annual marestail. Even with new dicamba-tolerant soybeans, getting a jump on marestail control in the fall will allow us to better manage the weed and take selection pressure off spring and early summer applications.
We can spray until the ground freezes, but still use the early spring spraying recommendations of spraying when we have actively growing weeds. Look for at least three days of 50 to 55 degree F high days with 45 degree nights, followed by another 24 to 48 hours of similar conditions. We have more time on our side for the weed control product to work in the fall following application before next spring.
Bultemeier: Fall herbicide programs focused on residual herbicide control are best made after the soil temperature has fallen below 50 degrees F but before the soil freezes. Cooler temperatures lengthen the window of activity for the herbicide. When looking to control weeds up and growing now, they need to be actively growing. Cooler fall temperatures trigger plants to increase downward movement of carbohydrates for storage in the roots. It’s an ideal time to promote root translocation of herbicides.
Gauck: In most years, we target fall herbicide applications through mid-November. You want weeds still actively growing. Fall is a great time to target marestail, dandelions and Canada thistle. Look at labels and recommended adjuvants. Some herbicides will have different recommendations for fall.
Bultemeier: A killing frost can disrupt plant growth and translocation. If a frost has occurred, scout weeds 48 to 72 hours after the frost to verify that plants weren’t killed. It’s also best to give plants a minimum of 48 to 72 hours before another hard frost is expected to allow for herbicide translocation within the plant. Marestail is very hardy and can survive a light frost.   
Gauck: Before making a fall herbicide application, be sure to know what cash crop you’ll be planting in the spring. This is extremely important if you’re using a fall residual program. If you use a residual product, spray as late into the fall as weather allows. This will control weeds that are up, but give better residual control into spring. With weed pressures ever-increasing, don’t count on a residual herbicide applied in the fall keeping fields clean until summer. Be sure to apply a residual herbicide in the spring.