As temperatures begin to warm up, I notice many fields covered with purple henbit flowers in Southeast Nebraska. Henbit is one of the most common winter annual weeds in the region, along with weeds like downy brome, field pennycress, marestail, tansy mustard, and prickly lettuce.
Winter annual weeds typically emerge in late summer and fall, survive through the winter, and then grow actively in the spring of the following year. One major impact of winter annual weeds is their role as important alternative hosts for pests. For example, henbit and field pennycress serves as host for soybean cyst nematode. Common chickweed is another example of alternate host, they attract black cutworms to lay their eggs in early spring. Other than serving as an alternate host for pests, winter annual weed may also cause delayed soil warming in spring which results in difficulties during planting and issues with the establishment of summer crops.
Controlling winter annuals is relatively easy in the fall and early spring but becomes harder once they begin flowering, as they are less susceptible to herbicides. Herbicide application should be targeted before stem elongation and flowering. Flowering occurs earlier in weeds like henbit, field pennycress, and downy brome than in marestail, prickly lettuce, and musk thistle. While musk thistle is typically a biennial weed, it can occasionally behave as an annual weed. It is particularly important to control musk thistle because it is a noxious weed in Nebraska, meaning it is unlawful to let it grow on your property without attempting to control it.
Between late March and early April, you have a good opportunity to tackle winter annual weeds. While treating them in the fall generally yields the best results, some may go unnoticed during that time, and even if they do emerge, they often do not survive the winter. Field experiments conducted near Clay Center and Lincoln from 2007-2009 compared corn and soybean yields when winter annual weeds were removed in different months before planting. Failure to control these weeds prior to planting resulted in yield losses exceeding 5% in five out of six years, with losses exceeding 10% in four years. Critical dates for weed control ranged from April 15 to May 5 for corn and March 20 to May 5 for soybeans when planting in mid-May.
Herbicides such as 2,4-D, Acuron, atrazine, Aim, and glyphosate are effective for corn, while 2,4-D, authority first, and glyphosate is suitable for soybeans. When applying these herbicides, it is important to check herbicide labels for planting restrictions. When using 2,4-D at a rate of 16 fl oz/acre in a burndown program, there is a seven-day pre-planting interval for both corn and soybeans. However, if the application rate exceeds 16 fl oz/acre, the planting intervals increase to 14 days for corn and 30 days for soybeans.
For more information on weed control and herbicide efficacy, you can purchase the 2024 weed guide online https://marketplace.unl.edu/extension/ec130.html or get last year’s weed guide free of cost from your local extension office. Copies are available at extension offices in Nemaha, Johnson, Pawnee, and Richardson counties. Feel free to contact me at rlamichhane3@unl.edu or 402-274-4755 if you have any questions.
Ritika Lamichhane
Extension Educator
Nemaha County
April 2024