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stinkweed

By Harry Brook

It doesn’t happen often but fall control of winter annual weeds and problem perennial weeds can be an effective and cost-effective way to clean up fields prior to next spring.

What are winter annual weeds? These are weeds that germinate and begin to grow in the fall, forming a rosette. They survive the winter in this form and restart growth early, as soon as the snow melts. Often, they go to seed before you can get out there and do a pre-seed burndown. There is a lot of winter annual weeds out there. They include stinkweed, shepherd’s purse, narrow-leaved hawk’s-beard, flixweed, and all weeds from the mustard family.

This doesn’t mean you can’t spray them in the spring, but it does make it easier for good weed control, especially when pulses are going to be seeded the next year. As most herbicides used with pulses, in-crop herbicides are mostly group 2 herbicides. Fall application gives you a different opportunity to hit them before they become a problem in the peas or lentils. If you have issues with group 2 resistant weeds, it might be a very useful, alternative, control measure.

An essential condition for fall control to work is that the plant must have green leaf material and be actively growing. If the frost has killed off the leaf material or it is too cold, then fall spraying is a waste of time and resources. One advantage of fall spraying of these weeds is that they are relatively easy to control with a low dose of a phenoxy herbicide, like 2,4-D. At low doses it encourages the plants to keep growing, thus depleting the energy reserves in the plants and causing winter kill. You still need to be aware of potential residue effects on next year’s crop.  Under the right conditions, this can be very cost effective and an efficient way of controlling some prolific weed problems.

When it comes to perennial weeds, fall time is also an ideal time to get the jump on these weeds. The thistles respond well to being sprayed in late August to September, especially if there has been no or little frost. As with winter annuals, the key ingredient to successful control is having an actively growing, green plant material being sprayed when temperatures and conditions ensure some growth. The susceptible perennial weeds start to send nutrients down into the roots as daylight starts to decline and fall approaches. When sprayed at this time, you get much better translocation of herbicide into the roots and that leads to better control for next year.

Dandelions are noted for their ability to withstand herbicides in the spring. However, if you spray them late in the fall, in October, you get much better control. That is because dandelions recharge their root energy reserves late in the fall, usually after a light frost or two.  An herbicide application in October can be very effective at controlling dandelion. Also, dandelions have a flush of new plants germinating in the fall. They are easy to kill then. If they survive the winter, they are much more difficult to control.

If the stars align correctly, and good weather conditions prevail, consider a little timely herbicide application in the fall. Get the jump on those problem weeds and give yourself a break next spring.


Harry Brook is Flagstaff County's Agricultural Fieldman. He can be reached via email at: This email address is being protected from spambots. You need JavaScript enabled to view it. or by phone at: 780-384-4138.