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Winter Annual Weed Identification

Posted 10/21/2016
Accurately identifying weed species can help determine the right herbicide mix to manage the weed situation.

Winter annual weeds can become a major problem in crop production when not controlled in the fall or early spring, especially in no-till systems. Winter annual weeds typically emerge from late summer through fall, can overwinter and then flower to set seed in the spring/early summer. Seedling winter annual weeds can be difficult to distinguish from other weeds, particularly if they are in the same family (Table 1). Many fallemerging weeds have a rosette growth stage such as dandelion, horseweed, prickly lettuce, Shepherd’s purse, and other members of the Brassicaceae, making identification more difficult.

Accurately identifying weed species can help determine the right herbicide applications needed to manage the weed species in each field. The University of Missouri publishes several good diagnostic resources that include color photos and keys for identification.1,2,3 Consider accessing Early Spring Weeds of No-till Crop Production, NCR 614 Extension Publication at http://extension.missouri.edu/p/NCR614.

The impact of winter annual weeds in cropping systems is sometimes overlooked because these weeds typically complete most of their life cycle prior to or shortly after corn and soybean planting. Dense mats of winter annual weeds may delay soil warming in spring, compete for water and nutrients, and interfere with crop planting. Winter annual weed species are hosts for some pests (Table 2). Henbit and purple deadnettle are hosts for soybean cyst nematode.

Table 1. Winter annual and similar weed species

p = perennial, b = biennial, a = summer annual. Adapted from Early Spring Weeds in No-till Crop Production, University of Missouri.
Family Name Common Names
Asteraceae Marestail, Cornflower, Fleabanes, Prickly lettuce, Dandelionp, Butterweeda
Boraginaceae Corn gromwell
Brassicaceae Bushy wallflower, Field pennycress, Shepherd’s purse, Smallflowered bittercress, Tansy mustard, Virginia pepperweed, Wild mustard, Yellow rocket
Caryophyllaceae Common and Mouseear chickweedp
Geraniaceae Carolina geranium
Lamiaceae Henbit, Purple deadnettle
Ranunculaceae Buttercups, Mousetail
Rubiaceae Catchweed bedstraw
Scrophulariaceae Corn speedwell, Purselane speedwell, Common mulleinb
Violaceae Field pansy
Liliaceae Star-of-Bethlehem, Wild garlicp, Wild onionp
Poaceae Annual bluegrass, Carolina foxtail, Downy brome, Foxtail barleyp

Table 2. Winter annual weed hosts for soybean cyst nematode (SCN) and black cutworm (BCW).

Soybean Cyst Nematode Black Cutworm
Common chickweed Common chickweed
Henbit Henbit
Shephard’s purse Shepherd’s purse
Field Pennycress Peppergrass
Small-flowered bittercress Yellow rocket/Mustards
Purple deadnettle Curly dock

Winter Annual Weed Identification

Although agronomists may overlook the value of accurate identification of weeds in the fall, proper identification can help focus weed management decision-making. Selecting the right herbicides and proper application timing can influence performance.

Common winter annual weeds in the Midwest include: henbit, marestail, shepherd’s purse, field pennycress, prickly lettuce, and purple deadnettle.4

Purple deadnettle and henbit

Purple deadnettle and henbit are often misidentified due to their similar appearance. The best way to identify these plants is by looking at the leaves in the upper portions of the stem; henbit leaves attach directly to the stem while the upper leaves on purple deadnettle have short petioles that attach leaves to the stem (Figure 1).

Figure 1. Purple deadnettle (left) Henbit (right)
Figure 1. Purple deadnettle (left) Henbit (right)
Marestail

Marestail forms a basal rosette after germination and seedlings are covered with coarse hairs (Figure 2). Seedling leaf margins are toothed. Flowers are white to pink with yellow centers. Marestail is more susceptible to herbicide application in the fall compared to early-spring.

Figure 2. Marestail seedling
Figure 2. Marestail seedling
Shepherd’s purse and dandelion

To distinguish between Shepherd’s purse and dandelion, examine the lobes in the rosette leaves, if the leaves come to points towards the center of the rosette, it is a dandelion (Figures 3 & 4). Dandelion leaves or stems will exude a milky sap when broken.

Figure 3. Dandelion rosette
Figure 3. Dandelion rosette
Figure 4. Shepherd's purse seedling
Figure 4. Shepherd’s purse seedling
Field pennycress and prickly lettuce

Field pennycress and prickly lettuce are in different families but can be misidentified in the seedling stage (Figures 5 & 6).

Figure 5. Field pennycress seedling.
Figure 5. Field pennycress seedling.
Figure 6. Prickly lettuce. Steve Dewey, Utah State University, bugwood.org
Figure 6. Prickly lettuce. Steve Dewey, Utah State University, bugwood.org
Common chickweed

Common chickweed is an annual, whereas mouseear chickweed is a perennial with a similar growth habit. Common chickweed leaves are bright light-green, nearly rounded with pointed tips, and hairless (Figure 7). Mouseear chickweed leaves are dark green, elongated, and covered with soft hairs. Common chickweed has a single straight row of hairs along the stem.

Figure 7. Common chickweed. Ohio State Weed Lab Archive, The Ohio State University, bugwood.org
Figure 7. Common chickweed. Ohio State Weed Lab Archive, The Ohio State University, bugwood.org

A fall herbicide application can effectively reduce weed populations from overwintering and help reduce the number of seeds set in the spring. Visit with your Monsanto Crop Protection Representative, local retailer, or visit www.RoundupReadyPLUS.com for solutions and recommendations.

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