It's Time to Apply Pre-emergent Herbicides

April is an important time to apply pre-emergent herbicides. Weeds are beginning to emerge and most pre-emergent materials should be applied either before weed emergence or when weeds are small. In most southern Ontario locations, mid-April is a good time.

There are quite a few materials to choose from. Choose herbicides based on the age of the plants (young plants are less tolerant than older plants), weeds present and soil type. Some pre-emergent herbicides are stronger on grasses or broadleaf weeds, so use combinations that control a broad range of weeds. Also, consider using herbicides with different modes of action so that there is less chance weeds will develop resistance.

Pre-emergent herbicides are designed to control germinating weed seeds that have not yet emerged from the soil. To get the best results and to avoid wasting time and labour costs down the road, the weeds shouldn't be visible above ground at the time of application.

Pre-emergent herbicides are not designed to control existing weeds or weed seeds. The weed will only be killed when it begins to sprout from the seed and hits the herbicide barrier. It is possible for seeds to remain dormant and not be harmed by the pre-emergent herbicide application. This is why weed control is a constant process. There will always be seeds under the surface and a portion will germinate each season. Annual applications must be made to significantly reduce large infestations.

Pre-emergent herbicides must be watered in. Watering in activates the herbicide, creating a barrier just below the surface. A general rule of thumb is 15-20 mm of rainfall within 7-10 days after application is required to activate pre-emergent treatments. If this has not happened, consider shallow cultivation, rotary hoeing or harrowing. This will control weed escapes and improve herbicide activity in the absence of rainfall. Another option, if available is to irrigate. If there are too many weed escapes it will be time to apply a post-emergent herbicide.

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