Identifying and Controlling Winter Annuals

Identifying and Controlling Winter Annuals

May 4, 2007

Two Winter Annuals Join the Ranks

Photo of Purslane speedwell
Figure 1. Purslane speedwell (Veronica peregrina).
Photo of Purslane speedwell
Figure 2. Purslane speedwell.
Photo of field violet
Figure 3: Field violet (Viola arvensis)
Photo of field violet
Figure 4: Field violet (Viola arvensis)

When we scout fields in early spring, it's always interesting to see the diversity in species present. While many winter annual species, such as henbit, field pennycress, shephardspurse and tansy mustard, are fairly common from field to field and year to year, a couple less common species were recently submitted to the UNL Plant Diagnostics Clinic for identification. They are purslane speedwell and field violet. We have observed both of these weed species in numerous fields this spring. A burndown herbicide treatment that includes a soil-applied broadleaf product or glyphosate likely will give good control of both species.

Purslane speedwell (Veronica peregrinaFigures 1 and 2

Identification characteristics include opposite leaves when the plant is in younger vegetative stages and leaves become alternate as the plant reaches reproductive stages. The stem is round and sometimes has small hairs. It has small, white four-petal flowers and a heart-shaped seed pod.

Field violet (Viola arvensisFigures 3 and 4

Field violet starts in a rosette growth habit and produces a more erect stem with distinctive leaves as it reaches reproductive stages. Leaves are opposite on the stem as the plant matures. There are two large, deeply lobed, leaf-like stipules at the base of the primary leaf. The seed capsule will split into three valves as it matures and dries. This weed is reported as a problem in many high value crops in Europe and in small grain and canola fields in Canada.

Factors Affecting Increase in Winter Annuals

Winter annual weeds have become a topic of increased interest and discussion in the world of weeds in recent years. A number of factors over the last 5 to 10 years have contributed to higher winter annual weed populations in row crop production fields.

Such factors include milder winters and warmer early springs that can encourage robust early season growth of winter annuals. No-till adoption also can create a beneficial environment for winter annual weeds. In the past fall tillage and spring tillage to prepare the seedbed often controlled these winter annuals. With this said, it is important to point out that the benefits of no-till, including reduced soil erosion and environmental contamination, far outweigh any winter annual weed control gained from increased tillage.

Another factor contributing to the increase of winter annuals is the rapid adoption of glyphosate-resistant crops and the use of only glyphosate, which means there are fewer residual chemicals for weed control. While these winter annuals probably were not the application target, it is likely that their populations also were reduced. Increased winter annual weed species simply represent a shift in weed species due to changes in management and environmental conditions over the recent past. It is an excellent example of how nature exploits niches in our production systems and why there likely will never be a single foolproof weed management approach.

If you have questions on winter annual species identification and control, contact your nearest extension educator or submit samples to the UNL Plant and Pest Diagnostic Clinic.

Lowell Sandell
Extension Weed Science Educator, Lincoln

 

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A field of corn.