Perennials and winter annuals always have a head start on the crop, which is why a pre-seed burnoff is such a benefit for managing these weeds. If no pre-seed burnoff, in-crop control has to be early to prevent these weeds from taking up valuable moisture and nutrients and crowding out canola seedlings that are trying to get established.
Read more about the economics of early weed control.
Flixweed, shown above, presents another possible risk – as a clubroot host. This adds to the incentive to spray it early. Clubroot spores become active with a soil temperature around 15°C and temperatures of 20-25°C are optimal for infection. While soil temperatures may not be at that level yet, if this flixweed is going strong at that time, it will have a lot of root area — which will be optimal for early infestation and possibly large gall formation. Within three weeks, galls can be mature enough to produce spores. So waiting that long to control the weed could allow it to contribute to the clubroot spore-bank. Waiting that long will make it more and more difficult to control the weed in-crop.