Drought can hide perennial weeds and reduce herbicide performance, while wet years can trigger new flushes. Knowing when to scout, spray and contain patches can keep a small problem from becoming a costly one.
Moisture changes which weeds emerge, how well herbicides work and which survivors shape the next problem. For perennial weeds such as Canada thistle, a quiet year above ground may conceal a well-established threat below it.
It’s a scenario Thierry Fonville, a research scientist at Farming Smarter near Lethbridge, knows all too well.
A thin, tired-looking patch of Canada thistle can be easy to discount in a drought year. The shoots are short, the crop has other problems, and the patch may not appear large enough to justify a separate pass.
Then the rain returns.
New shoots fill the gaps, the patch pushes beyond last year’s boundary and a weed that seemed insignificant becomes difficult and expensive to contain. It can look as though the thistle appeared overnight. In reality, much of the machinery was already in place.
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