Successful control of any weed population requires knowledge of how that particular weed develops and survives. Marestail* is generally regarded as a winter annual weed north of I-70 in Illinois, but can adopt a summer annual growth habit south of I-70. Recall that winter annual weeds usually germinate from August through November while summer annual weeds generally germinate from April through June. Marestail is a small-seeded broadleaf weed that has minimal requirements for either fall or spring germination. Soil temperatures above 50 degrees Fahrenheit, with adequate soil moisture and sunlight are all that is needed for successful germination.
Once marestail germinates, it forms a small rosette and overwinters in this stage. Figure 1 shows a photo of a rosette stage of marestail. Generally, this rosette will not be larger than the size of a silver dollar. It often begins growth under crop residue, making it difficult to find. Upon return of favorable growing conditions in the spring, the rosette will enlarge to about six inches in diameter. Soon after this, the weed will “bolt” or begin rapid stem growth, as displayed in Figure 2. When marestail exceeds six inches in height, it becomes very difficult to control. If the marestail plant happens to be glyphosate resistant or ALS resistant, the only practical methods of control are tillage or fall-applied herbicides.
Figure 3 was taken three weeks after an application of glyphosate tank mixed with an ALS herbicide. The plant recovered following the herbicide treatment, producing multiple branches. This surviving marestail plant has the potential to produce up to 200,000 seeds. Marestail survival following treatment with post-emergence herbicides was common in 2013. Successful marestail control in the future will require development of a “Weed Management System.” This will likely necessitate the use of plant growth regulator herbicides (2,4-D, dicamba, etc.), applied in mid-October or later, followed by application of a spring burn-down herbicide (prior to bolting) that includes a soil-residual herbicide.
The soil residual effects of plant growth regulator herbicides are extremely effective in controlling most winter annual broadleaf weeds when applied in the fall. These herbicides are degraded primarily by soil microbes. Once soil temperatures drop below 50 degrees, microbial activity is inhibited. For this reason, growth regulator herbicides retain considerable viability through winter, until soil temperatures increase in the spring.
Most winter annual weeds, like marestail, henbit, purple deadnettle, butterweed, dandelion and the mustard species are very sensitive to post-emergence applications of growth regulator herbicides. Marestail is also highly susceptible to the residual effects of these herbicides, especially as the weeds break winter dormancy.
If plant growth regulator herbicides were not applied during the fall of 2013, we basically have 3 remaining choices for managing marestail:
1. Some form of thorough tillage
2. An early application of a plant growth regulator herbicide
3. An early application of saflufenacil or paraquat + metribuzin
* Marestail is classified as “Horseweed” by the Weed Science Society of America, and is also known as Canada fleabane throughout much of Canada. [The images, included here, were provided by Dr. Aaron Hager, University of Illinois.]