| | Harvest is underway and many producers are noticing higher weed populations in the fields due to late weed germination. Spraying weeds this year did not give the usual good results due to timing problems. Weeds under stress, both drought and frost stress, do not absorb herbicide well and therefore are not well controlled by the herbicide.
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“This spring we had cold, dry conditions with repeated, late season frosts that stalled both the crops and weeds,” says Harry Brook, crop specialist with Alberta Agriculture and Rural Development. “Thin crops and late rains stimulated weed growth after the time for spraying was over. The thin crops did not provide the normal competition and allowed weeds to flourish. All this adds up to higher than usual weed populations this fall.”
What producers need to focus on is what they can do now to mitigate weed problems next year. Fall can be a good time for controlling winter annual weeds and some perennial weeds. Winter annuals germinate in the fall, over-winter as small rosettes then begin growing again early the next spring. Because they grow early in the spring, they are often too large to be killed off with a pre-seed burn-off. They use valuable spring moisture and nutrients, stealing from the spring seeded crop.
Some of these commonly-found winter annuals include: narrow-leaved hawk’s beard, stinkweed, shepherd’s purse, scentless chamomile, chickweed, bluebur, stork's bill, ball mustard, common peppergrass, knawel, night flowering catchfly and common groundsel.
“If you can catch these winter annuals in the fall, they are relatively easy to control with an application of glyphosate or a phenoxy herbicide,” says Brook. “Controlling them is all a matter of timing. The weeds must be actively growing at the time of spraying. If a frost has occurred, you can still get good control, but the plants have to have some living green material and relatively good growing conditions to absorb the chemical. You might want to wait for a couple of days after the frost to allow the plants to recover and begin growing normally again.”
Some perennial weeds are also easier to control in the fall. Canada thistle is often best controlled with a pre-harvest application of glyphosate. Usually, however, after harvest the thistle doesn’t have sufficient time to regrow and have enough leaf material to absorb the herbicide and be killed. Dandelions are best controlled with a late fall weed spray of tribenuron methyl or even glyphosate.
“With all of these weeds, it is crucial to have good growing conditions for the herbicide to work properly,” says Brook. “Some of the essential elements to getting good control include having enough living leaf material to absorb herbicide, warm temperatures and adequate moisture. With dandelions, a light frost stimulates the plant to store more energy in their roots. Spraying a systemic herbicide at that time will translocate more of the herbicide into the roots and lead to greater weed control.
“Regardless of the problem weeds you have, you need to pick the right time to apply the right product to get the best control results. Know your enemy and pick the time and place to get the biggest bang for your herbicide buck. For some weeds, spring spraying is only the second best time to get rid of weeds.”
Contact:
Harry Brook
310-FARM (3276) |
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