Winter annual weeds can emerge in the fall, survive the winter as low rosettes, resume growth in late winter or early spring, and flower and go to seed in late spring or early summer. Several factors can cause an increase in winter annual weeds such as common chickweed, Canada fleabane, wormseed mustard, prickly lettuce and wild carrot:

• No-till tends to promote winter annual populations, since there is no tillage in fall to disrupt their emergence.
• A switch from pre plant/pre emergence herbicide programs to post programs can increase winter annual populations. And if post sprays are delayed to catch later-emerging weeds, winter annuals may have already bolted and be to advanced to achieve good control.
• Warm weather in late fall can result in higher populations, since most winter annuals emerge in the fall.
• Early soybean harvests can allow earlier than typical fall tillage in some fields, which provides a post-tillage window for winter annual emergence.

Winter annuals, spring burndown
Control of winter annuals with a spring burndown is often more variable and more difficult than in fall, due to weed size and weather conditions. Herbicides are more active under the warmer temperatures in late spring, but winter annuals should probably be controlled in early spring to allow maximum time for soil drying and herbicide activity that can be very slow.

Because winter annuals are generally are completing their life cycle in late spring, one management strategy when winter annual populations are low is to till or apply burndown herbicides around the time of planting, and plant the crop regardless of how effective the tillage/herbicides were on the weeds.

This can be an adequate strategy, since the winter annuals are most likely reduced in vigor and are unlikely to interfere with early crop establishment. However, as crop planting is moved earlier and winter annuals increase in density and size, a more aggressive management strategy will be needed and fall or early spring applications should be considered.

Continued problems with winter annuals may warrant reconsideration of the utility of total post emergence programs.

Fall treatments for winter annuals
In general, herbicide treatments or tillage in late fall seem to be much more consistently effective than spring treatments, especially in dense stands of winter annuals. Weeds are smaller and more susceptible to herbicides in the fall, and weather may be more conducive for herbicide activity. Soil conditions may also be better suited to sprayer traffic.

Does fall treatment eliminate the need for a spring burndown treatment?
A late fall herbicide treatment that controls all of the weeds present at that time should often provide near complete winter annual control through spring crop planting. Some emergence of winter annuals can occur in spring, but this will usually be minimal.

However, since a number of annual weeds can emerge in early spring and reach considerable size by planting, a burndown treatment will probably still be needed. The Dow AgroSciences No-Till PowerPlay system provides a powerful, one-shot combination of Broadstrike* Dual' Magnum' and a burndown, for pre-seed burndown and season-long control of annual grasses.


Source:
"Managing winter Annual Weeds: Are Fall Treatments Necessary?" by Mark Loux and Jeff Stachler, Perry County , Ohio, Agriculture & Natural Resources Newsletter, November 2000.


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