By Kristen Callow - Weed Management Program Lead, Horticulture - OMAFRA

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The unusual weather this spring has and will continue to result in serious weed problems in all cropping systems. Many growers applied pre-emergent residual herbicides earlier than normal due to the warm, dry conditions in March. Subsequently, the weather has turned cooler and, in many places, wetter, which has slowed crop growth. Herbicides applied early have not been very effective, and many weeds have emerged.

It is therefore, advisable to apply a second pre-emergent residual herbicide in orchard systems and if possible tank-mix the product with a contact herbicide, such as Aim, Ignite, Gramoxone or glyphosate (depends on crop and label) for emerged weeds. Hooded sprayers will likely be required, but a tank-mix will minimize the number of passes through the field.

For recently emerged or transplanted vegetable crops (onions, potatoes, carrots, etcetera), growers need to be extremely cautious on their herbicide selections. Pre-emergent residual herbicides are no longer an option at this point, therefore, post-emergent herbicides will have to be used ... but the crop is stressed and will likely be more sensitive to herbicide applications. So be cautious and allow the crop time to recover before applying another post-emergent herbicide.

Complete weed control will be a challenge this year, but it is essential to control weeds throughout the critical weed free period to achieve good yields in all cropping systems. The critical weed free period is defined as the length of time that the crop needs to remain weed free to limit yield reduction from weeds. Weeds emerging after the critical weed free period will not affect yield.

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