Autumn is the best time to wage war on perennial and biennial weeds. Growers will reach for broad-spectrum weed killers like 2,4-D or dicambia (Banvel, Clarity, etc.) and as long as plants have enough leaf tissue left to absorb sufficient herbicide, broad-spectrum is the way to go.
Biennial weeds like burdock, plumeless thistles, and wild carrot are much softer targets in the fall than in spring. Once biennials have weathered a winter and emerge in the spring they are much more difficult to manage.
For perennials, late summer and early fall applications allow herbicides to move freely into the root systems of weeds like horsenettle, smooth groundcherry, and various woody species, granting better control.
William Curran, Professor of Weed Science at Penn State Extension offers the following:
- Make sure that the foliage on the weeds appears relatively healthy and capable of absorbing the herbicide spray. Plants that have been damaged by insect feeding, drought, harvest equipment, frost, or autumn leaf senescence are not good candidates for fall applications. So, if that pokeweed you have been dealing with during season is still standing and the leaves and stems are not too tattered after harvest, then there is still a great chance to control it yet this fall. Make sure to use adequate herbicide rates, high spray volumes, and get good spray coverage over the plant for effective kill.
- Favorable air temperatures should be a consideration immediately before, during, and after application. In general, the warmer the better, with daytime high temperatures in the mid 50s at a minimum. Cold nights and cool, cloudy days will reduce and slow the effectiveness of the applications. The more active the weeds are growing, the better the herbicide performance.
Curran also suggests fall as the best time to deal with sod stands and hayfields. Glyphosate is Curran's herbicide of choice as it is much more effective in fall than spring, but an additional application and tillage may be required once spring rolls around. For grass pastures, check the 2011-12 Penn State Agronomy Guide for specific herbicide performance by weed species information and a current product label for use recommendations and restrictions.
With many farmers harvesting early this year, growers will have extra time to deal with stubborn biennials and perennials this season. The extra time and mild weather this fall will give growers ample opportunity to get a leg up on pesky perennial weeds.
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