At the end of a long growing season with the harvest in and the fields clear it’s tempting to let your guard down against weeds. You may see a few spots of Canadian thistle and dandelion and feel that with frost coming on soon you can wait until spring to deal with them. Before you get complacent you should know that those “few” weeds are getting ready to become a bumper crop once the ground thaws. Killing them will cost you time, money and aggravation before planting can begin. Wouldn’t it be nice to deal with them before the first frost?
Once you’ve brought the crops in you’ll want to start thinking about a post-harvest herbicide application, a “burndown” to kill the weeds that remain before the first freeze. A well-planned burndown can yield many benefits. Cleaner fields in the spring mean earlier planting with less or no need for pre-planting herbicide applications, saving you money. The reduced mats of dead weeds speed soil warming and dry out as winter recedes, meaning earlier planting and more time for growth. Additionally, fewer weeds mean less shelter for destructive insects, reducing your pesticide costs.
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At the same time, you have to be cautious. Like any other task on your farm, you’ll want to plan your burndown carefully. Here are a few tips.
1. Pick your time
Chris Neeser, a weed specialist with the Alberta Government’s Department of Agriculture, knows all about post-harvest burndowns and pre-emptively treating noxious weeds. “In Western Canada our harvest takes place mostly in September,” he says, “and there’s no point in spraying once you have frost. So we have a fairly short window depending on your harvest, basically late September to early October. For most crops you’re talking two to three weeks.”
Neeser advises against being overly zealous in attacking weeds immediately after harvest. You have to allow time for the weeds to start their late-season growth, both to spot them and to most effectively deal with them. Most herbicides need to be applied to the green growing parts of the plant — if the stems are turning brown there will not be sufficient absorption to kill the plant, and you’ll just be wasting time and money. It’s always a judgment call, but if you’re harvesting in early October it’s probably best to wait till spring. It’ll be more labour intensive and expensive but you can be assured you’ll catch every weed that’s in the soil without wasting herbicide.
2. Know your weeds
It’s important to identify the weeds you’re dealing with before spraying. Common late-summer and autumn weeds in Canada are foxtail barley and dandelions. Each of these has different sensitivities to common commercial herbicides. Each provincial ministry of agriculture’s website provide extensive weed identification guides, and lets you know their sensitivities and resistances to other chemicals.
Glyphosate is a good all-around herbicide for dealing with fall annuals, being low cost, broad spectrum and non-residual. At the same time, glyphosate resistance is becoming a more and more common problem. If you sprayed glyphosate earlier in the year your weeds in the fall are likely resistant to it and a second spraying would be ineffective — consider spraying something your weeds have not been exposed to yet.
3. Beware residuals
While post-harvest herbicide applications can be very effective against weeds, they can also hurt next year’s crop. Herbicides are broken down primarily by two factors: microbial action and photo-decay. With soil temperatures low and less light through the long Canadian winter, the spray you put on the ground in fall could still be there come the spring, inhibiting the growth of many crops.
This is less of a problem with non-residuals like glyphosate, which undergo rapid breakdown upon contact with soil. Others, like 2,4-D, have significant residual effects from one to four weeks or longer, as well as a broad inhibitory effect on many different crops. Again, timing is a very important — you’ll want to allow for a window between your application and the first frost.
For the economic health of your farm you’ll want to have maximum recropping options. This means you’ll want to avoid locking yourself into a set of crops because you used an overly strong or inhibitive herbicide. To avoid re-cropping problems, Neeser has some simple yet sound advice: always read the label, and all the documentation that comes with your herbicide. This will contain extensive information about residual risks, re-cropping restrictions and other important information.
By keeping in mind these factors you’ll be able to plan a successful time and money saving post-harvest burndown without restricting your options for what to plant in the spring.