Post-Harvest Weed Control

Post-harvest weed control is a great opportunity to clean up fields and prepare for next year’s crop. Fall is especially suited for effective control of perennials such as dandelion and Canada thistle. As temperatures start to drop, perennial weeds start moving nutrients into their root systems for winter storage. This also facilitates the movement of crop protection products into the roots, enhancing their activity and providing a better kill. Control of winter annuals is also more effective in the fall, when these weeds are small seedlings. There is also an opportunity to use different herbicide modes of action in the fall and to tank-mix glyphosate for more complete weed control and resistance management.

Dandelions are best targeted when they are less than 6 inches wide and are in the pre-bloom to full flower stage. Canada thistle should have 3-4 new green leaves. At least 60% of tissue should be green. Spray on sunny days when temperatures are above 8°C and remain there for 2-4 hours after application. Allow 5 days before tillage in ideal conditions or 10 days in cool and cloudy conditions.

If a light frost occurs i.e. 0°C to -3°C, you can spray later in the day or the next day if temperatures climb to a minimum of 8°C for at least 2-4 hours after application. Heavier frost (<-5°C) can cause more severe damage, including perennials. Wait 1-2 days to assess injury to weeds. Continue to spray if majority of weeds are 60% green, actively growing, and daytime temperatures are forecast to reach at least 8°C for 2 or more hours after application.

Consult with your Shur-Gro agronomist for the best post-harvest treatments for your weed spectrum and crop rotation.