Pigweed at the two-leaf stage of growth which is the optimum stage for some post-emergent herbicides.
Lamb’s quarters at the ideal stage for glyphosate applications (top). But most post-emergent herbicides must be applied to smaller lamb’s quarters (bottom).
Many annual grasses, like green foxtail, are most susceptible to post-emergent herbicides when at the two- to four-leaf stage of growth.
Most herbicides that are effective at controlling Canada fleabane specify that it must be less than 10 cm in height (or diameter) or less than 20 leaves at application. The photo at top depicts what this would look like for a fall-germinating rosette compared to a spring-germinated seedling (bottom).
Table 1: Maximum leaf stage of common weeds in Ontario where they will be sensitive to the herbicide* applied. The herbicide label will provide guidance on optimum application timing. (*Clearly other post-emergent herbicides are also available for use. This represents a snapshot of common products)
A generation of glyphosate use has redefined what we mean by “apply herbicides to weeds that are small and actively growing.” Many interpret “small” to mean “about 15 cm” (six inches). If using “older” herbicides to manage glyphosate resistant weeds and your idea of small is 15 cm, then you may be disappointed with the level of control.
The table included in this gallery outlines the maximum weed stage common post-emergent herbicides. The maximum size of some species may surprise you.
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