News

post harvest weed control

By Harry Brook

It doesn’t happen every year but this year might be a good year. I’m talking about post-harvest weed control. In general, most annual weeds are best controlled in the spring, before they get too big. Perennial weeds such as Canada thistle, perennial sow thistle and quackgrass are best controlled in the fall. There are a group of weeds that are not necessarily either annual or perennial weeds. They are called winter annuals.

Winter annual weeds can grow in the spring, but they also start growing in the fall. When fall germinated, they merely form a rosette of leaves, overwinter, then quickly mature and go to seed in the early spring. Thusly, they avoid the spring burndown of herbicide. Some of the common winter annual weeds around are scentless chamomile, cleavers, stinkweed, shepherd’s purse, narrow-leaved hawk’s-beard, blue bur, stork’s-bill, chickweed, flixweed, all members of the mustard family and common groundsel, among others.

Not only do winter annuals create problems next spring, they also use nutrients and moisture that could be used for the subsequent spring crop. Often spring herbicide control is too late to effectively control the weed as it has already gone to seed and provided more weed seeds for the seed bank.

Before glyphosate was cheap, these winter annuals were easily controlled with a cultivation prior to freeze up. They can be controlled with herbicide in the fall while they are small. However, not every fall lends itself for post-harvest weed control. Herbicides only work when the plants are actively growing. Even after a frost, you can still get good control but you do need to have some green, actively growing plant material to absorb the chemical and effectively stop the plant.

Temperature is also an issue. If you’ve had a frost, you may have to let the weeds recover a bit before spraying. Temperatures at spraying should be above 12˚C and sunny weather is good, if you can get it. Moisture conditions are currently good. We have almost ideal conditions for winter annual weeds to germinate. Dandelions can also function as winter annuals. You can get seedlings started in the fall, which can quickly establish the following spring and flower early. Once they’ve been through a winter they are much harder to kill.

Herbicides to control these winter annuals are usually fairly economical. Glyphosate can work well as can 2, 4-D, MCPA and dicamba. Be aware that some herbicides available may have residual effects for next year’s crop when used in the fall. As always, make sure you are applying the herbicide to an actively growing weed that has some ability to absorb your chemical. This year, with the early harvest, might be an ideal time to get the jump on some of these weeds. It’s not every year that post-harvest weed control is effective.

Take advantage of the opportunity to get the jump on weeds this fall and have a safe harvest.


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.