The potential for soybean losses due to uncontrolled weeds in the U.S. and Canada adds up to $17.2 billion annually. Getting a jump on controlling winter annual weeds is especially important to minimize losses. To top it off, weed control is doubly important for fields at risk for soybean cyst nematode (SCN), as many common winter annual weeds host SCN.
Learn how to get ahead of winter annual weeds (and help control soybean cyst nematode in the meantime).
How soybean cyst nematode and winter annual weeds are connected
Six winter annual weeds have been identified as overwinter hosts for soybean cyst nematode, allowing SCN to overwinter and reproduce in the field on alternative hosts:
- Purple deadnettle
- Henbit
- Field pennycress
- Shepherd’s-purse
- Small-flowered bittercress
- Common chickweed
Of these six, purple deadnettle and henbit have proven to be the strongest SCN hosts. In fact, in greenhouse trials, both these weeds hosted SCN so efficiently that their SCN populations equaled or exceeded SCN populations on SCN-susceptible soybeans.¹
Control strategies for winter annual weeds in soybeans
Many weeds look similar from afar, but their control methods vary. Learning how to properly identify these SCN hosts is critical.
Here are the characteristics to look for when scouting these SCN hosts:
Purple deadnettle
One of the strongest hosts for soybean cyst nematode, purple deadnettle has square stems that are sparsely hairy, with leaves that are egg-shaped and slightly pointed. This weed has long petioles (the stalk that attaches the leaf to the stem) on lower leaves and, in contrast to henbit, short petioles on upper leaves.
The top leaves of purple deadnettle are reddish-purple and smaller than its lower leaves, which are greener. Leaves hang down and overlap other leaves, resembling evergreen boughs. Small purple flower clusters can be found at the top of the stem in spring.
Henbit
Like purple deadnettle, henbit is a strong host for SCN. The similarities don’t stop there. Purple deadnettle and henbit are often confused: Both have square stems, slightly hairy egg-shaped leaves, and small purple flowers nestled in the upper leaves. However, unlike purple deadnettle, henbit has petioles on lower leaves but not on upper leaves. Henbit leaves are also more heart-shaped than egg-shaped.
Field pennycress
Field pennycress is considered a moderately strong host for soybean cyst nematode. When scouting for field pennycress early in the season, look for a rosette (similar to marestail) with egg shaped, light green, hairless leaves with wavy margins. After plants bolt, leaves on the hairless stems have no petioles, but are oblong with toothed edges and emit a strong odor when disturbed. Flowers are white and clustered with each flower featuring four petals. Seed pods are round and notched.
Shepherd’s purse
Shepherd’s purse is a relatively weak host for soybean cyst nematode. This weed can easily be mistaken for field pennycress; however, shepherd’s purse’s oblong leaves have deeply lobed margins instead of wavy margins.
Although both weeds produce small, white flowers that grow as rounded clusters at the end of their stems, seed pods differ. Field pennycress seed pods are round and notched, while shepherd’s purse seed pods are heart shaped.
Small-flowered bittercress
Similar in appearance to field pennycress and shepherd’s purse, small-flowered bittercress is a weak yet feasible host for soybean cyst nematode. Another winter annual weed that starts as a rosette, small-flowered bittercress leaves are typically hairless but can be slightly hairy. Stems are branched with two to six pairs of oval leaflets, and flowers are white with four petals and have a long and narrow seed pod.
Common chickweed
Even though it is a relatively weak host for SCN, common chickweed cannot be overlooked as a winter annual that can harbor SCN. When scouting your soybean field, look for plants that grow low to the ground with branching stems. Stems have rows of hair (but unlike mouse-ear chickweed, they are not densely covered with hair at all growth stages). Both stems and leaves are light green.
Winter weed control options
Understanding how to scout for and properly identify winter annual weeds is critical. As already noted, though many winter annual weeds may share similar physical features, methods for controlling them often differ.
Apply fall burndown herbicide for winter weeds
The ideal time to apply fall burndown herbicides is after harvest but before heavy frosts. The window for optimal fall weed control is when winter annuals have started to emerge, but good weather still remains, enabling optimum weed control via burndown herbicides.
Sharpen™ herbicide offers good burndown of shepherd’s purse, henbit, chickweed, and field pennycress. Tank mixing Sharpen with 2,4-D and or glyphosate will widen the weed spectrum and result in good control of most winter annual weeds.
Examine tillage practices
Tillage has a large impact on weed pressure, and winter annual weeds are no exception. Unplowed fields in no-till and reduced-till operations will likely have increased winter annual weed populations and often require more herbicide treatment than would be necessary in conventional tillage.
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Endnotes
- Mock, Valerie A., et al. “Winter Annual Weeds and Soybean Cyst Nematode Management.” Purdue Extension, www.extension.purdue.edu/extmedia/ws/ws-36.pdf. Accessed 6 Jan. 2025.
