Fall weeds, white seed, green seed

October 6, 2010

A great chance for perennial weed control

Perennial weeds such as Canada thistle, dandelion, and quack grass are best controlled in the fall because these plants are moving nutrients down into their roots for winter. This is their most susceptible growth stage. Often by October it’s too cool for effective fall weed control with glyphosate. But with a stretch of warm weather the past week, and in the forecast, the fall weed control window may be widened.

In many areas there has already been a killing frost, which for weeds is -5°C. If weeds are green and the leaf tissue is still relatively pliable, growers may still have an opportunity to control the perennial weeds with glyphosate. To determine whether it is worth spraying after a frost, scout the field and check the amount of damage on the leaves. Control can still be obtained if no more than 40% of the original leaf tissue is damaged. Herbicide should be applied at the higher end of recommended rates in order to get the same amount of herbicide to the root mass, which is the target.

When scouting to determine the value of post harvest weed control:

• Assess if there is a significant population of weeds to warrant a spray and what species are present. Are they annuals, winter annuals or perennials? If the weeds are mainly annuals that have already set seed, it may be better to save the burnoff for next spring when those seeds emerge. Rainfall in August and September will stimulate winter annual emergence, so fall rosettes will likely be present.
• Can those weeds still be controlled? Are they actively growing?
• Is frost damage less than 40% of the leaf tissue?

If these conditions exist then a fall burn off may be of benefit.

When spraying fields in the fall prior to seeding canola, use a herbicide that is registered for use prior to seeding canola.

Annuals: Post harvest spraying also controls late maturing annual weeds such as wild buckwheat and green foxtail. These weeds continue to set seed until there is a killing frost and spraying will help limit weed seed return.

Tillage for weed control. Fall tillage can be effective on small winter annuals but will have little effect on large, established perennials. Consider the erosion potential of the field when determining what amount of fall tillage is appropriate.

Volunteer Canola. Canola seeds left on the  soil surface will deteriorate over the winter. This is an important part of volunteer canola management. Burying canola seeds can induce seed dormancy, resulting in growers having to manage volunteers arising from this year’s harvest losses for longer in rotation.