Marestail Weed Identification: How and When to Scout

The season is winding down, temperatures are cooling and everything green is wilting. Everything except for winter annual weeds like marestail (also known as horseweed). During the last warm, high-humidity days of fall, weeds like marestail emerge, growing slowly and storing energy to survive the winter. Uncontrolled corn weeds can cause up to 50% potential yield loss and uncontrolled weeds in soybeans can cause up to 52% potential yield loss.¹

Identification is the first step in controlling weeds and setting yourself up for a better chance of returning to a clean field ready for planting next spring.

Marestail weed identification and life cycle

Marestail can emerge essentially any time conditions aren’t too cold. Typically considered a winter weed, marestail that emerges in the fall overwinters until late April and grows rapidly in the right conditions, making it difficult to control with herbicides. It can also germinate in the spring as a summer annual.

Marestail tends to grow in two main phases:

• The first phase occurs in the fall when marestail seeds germinate and new weeds emerge.
• The second phase occurs in the spring and early summer when warmer weather returns. Once environmental conditions are met, marestail shoots upward and produces seed heads, quickly compromising your seedbed potentially for years to come.

Marestail weed identification

There are two distinct visual profiles of marestail. The first version is a young marestail weed rosette, a circular fan of club-shaped leaves covered in small white hairs that sits close to the soil line. Emergence is most likely at soil depths of less than ½”, but it can emerge from depths of up to 1”.² Rosettes can be present almost any time of year.

The second version appears once the plant begins to bolt as an elongated stem of densely grouped leaves that eventually produce conical seed heads. One plant can produce an average of 200,000 seeds that disperse on the wind, similar to dandelion seed. Mature plants can reach 6’ high and form dense groupings, crowding out all other vegetation. Stalks produce flowers in July or August, and up to 86% of seeds germinate immediately after falling from the seed head. The rest lie dormant in coarse, well-drained and fertile loam soils. Rosettes typically emerge in October and survive the winter. From mid-March to early April, the rosette starts to elongate into an upright growth. Seed longevity has not been established. However, one report found viable seeds in the seedbank of a 20-year-old abandoned pasture.³

Identifying features compared to weeds similar to marestail

To distinguish marestail from other similar weeds, ask yourself these questions:

1. Are the leaves covered in fine white hairs? This may be marestail.
2. Are the leaves short or elongated? Elongated leaves could be field pennycress, but you may need to do more research.
3. Are the leaves club-shaped or heart-shaped? Heart-shaped leaves likely belong to a plant in the mint family, like henbit.

Where and when to scouting for marestail

Marestail is prevalent in no-till fields, cultivated areas and pastures, and along roadsides.Fall scouting provides the best opportunity to get ahead of marestail management. Locating young marestail weeds in the fall allows you to catch them before seed production. Marestail has seeds that disperse in the wind, so talk to your neighbors about what they’re seeing in their fields; the problem next door may soon become your problem.

Experts are available to help you identify and manage marestail. Reach out to your seed retailer, a nearby extension office agent or a seed company professional like your regional BASF representative.

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Endnotes

1. Singh, M., and AJ Jhala. “Perspectives on the economics of herbicide usage in corn and soybean production in the United States.” Weed Technology. Cambridge, 2025, www.cambridge.org/core/services/aop-cambridge-core/content/view/84C81E804E9D0B4C7070DF1E38389D41/S0890037X25000193a.pdf/perspectives_on_the_economics_of_herbicide_usage_in_corn_and_soybean_production_in_the_united_states.pdf. Accessed 5 Mar. 2026.
2. “Marestail (Horseweed).” Department of Plant, Soil and Microbial Sciences, Michigan State University, 2023, www.canr.msu.edu/weeds/extension/marestail-horseweed. Accessed 5 Mar. 2026.
3. Zimmer, Marcelo, and Bill Johnson. “Fall Applied Herbicides and Winter Weed Control.” Pest&Crop Newsletter, Purdue, 15 Sept. extension.entm.purdue.edu/newsletters/pestandcrop/article/fall-applied-herbicides-and-winter-weed-control-2/. Accessed 5 Mar. 2026.