Fall Spraying of Weeds

Most producers are done there in-crop pre-harvest spraying. Since then we have had rain. Rains will be starting some new growth in the fields that needs to be looked at. Winter annuals can be a moisture and fertility robber in the fall and spring. In the spring, these weeds are sometimes difficult to control because of lack of growth or poor growing conditions. Weeds become very difficult to control if they get big in the spring. It may be too early to be putting your sprayer away. Late fall control for perennial weeds like dandelion and Canadian thistle is very effective with proper growing conditions. There has to be moisture to ensure that the weeds are actively growing. There has to be leaves that are green. Frost damage to the leaves of these plants usually starts around -4 degree Celsius. With certain conditions, I have seen these plants withstand up to 8 degrees of frost. There has to be growing conditions that will allow the herbicide to penetrate the plant. If these conditions are present after harvest, it is worth considering a post-harvest application of a herbicide. Winter annual weeds like Shepard’s purse and stinkweed are easy to kill in the spring even if spraying is delayed. Weeds like narrow-leaved hawks beard, stork’s bill and cleavers are not easy to control as they get bigger in the spring. Also, short-lived perennials like cinqfoil, wild strawberry and foxtail barley all seem to be harder to kill if the spraying gets late in the spring. These weeds are all easily controlled with the application of glyphosate, sometimes with something added in, in the fall. For producers that have recently taken out hay fields, looking at the field late in the fall can make things a lot simpler in the spring. If there is volunteer grasses in your field that are seedlings, controlling them in the fall will be a more effective way of dealing with these weeds than trying to kill them in the spring. It is difficult to get warm enough temperatures to get activity on grassy weeds in the spring. If the conditions are right for fall post-harvest spraying, it is time to go out and look at a field. There are different rates and different products that can be added into the glyphosate mix that will help improve control of winter annuals. Talking to your local agronomist will help determine which product will be the best fit in your situation. Timing is everything when it comes to weed control. If there is an opportunity to control weeds in your fields after harvest, it should be done. Late fall spraying makes for clean fields in the spring. Most perennial weeds are more easily controlled in the fall than in the spring. Too wet in the spring can make weed control too late and moisture and nutrients are lost. Having a place to start seeding in the spring that is free from winter annual and perennial weeds makes an easier spring. Look for this opportunity in your fields.


By Dave Cubbon, P Ag