Fall is a good time to spray perennials and winter annual weed species because pre-winter management can help preserve critical soil moisture for the following spring. Controlling winter annuals when they are small also offers better control.
That said, spraying immediately after harvest will not offer the best results.
Perennials that are chopped to stubble height at harvest require four to six weeks to regrow sufficient leaf tissue to absorb the herbicide, and typically require two to three times the glyphosate concentration over pre-harvest rates to get the same concentration into the plant root, according to the Canola Council of Canada.
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