Experts Advise Early Action to Control Marestail in Soybeans

K-State researcher says warmer spring temperatures mean weeds are also emerging, requiring prompt attention.

Mar. 30, 2026 at 5:14pm

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A highly structured abstract painting in earthy tones, featuring sweeping geometric arcs, concentric circular forms, and precise botanical spirals, conceptually illustrating the complex growth patterns and resilience of the marestail weed.An abstract visual representation of the marestail weed's tenacious life cycle, highlighting the challenges farmers face in controlling this persistent and fast-spreading pest.Manhattan Today

With warmer spring temperatures, weeds like marestail (also known as horseweed) are emerging and posing a threat to soybean crops. K-State weed specialist Sarah Ganske recommends that producers take action now to control marestail before it bolts, as the weed becomes much harder to manage once it grows taller. Ganske suggests using herbicides like 2,4-D or dicamba early in the season, while also noting that a combination of Liberty and Sharpen can be effective on larger marestail plants.

Why it matters

Marestail is a problematic weed that can quickly spread and outcompete soybean crops if not controlled early. Left unchecked, marestail can reduce soybean yields and become increasingly difficult to manage as it develops resistance to common herbicides. Proactive weed control is crucial for maintaining healthy soybean harvests.

The details

Marestail emerges as a rosette in the fall or spring, becoming dormant over winter before bolting and flowering in the warmer months. Once it bolts, marestail becomes much harder to control with most herbicides, including glyphosate. Ganske recommends using 2,4-D or dicamba early in the season, before the marestail has a chance to grow taller than six inches. For larger marestail plants, a combination of the herbicides Liberty and Sharpen can be effective.

  • Marestail typically emerges in the fall or spring as a rosette.
  • The weed becomes dormant over winter and then starts bolting in April and May.
  • Marestail flowers will emerge in July and disperse seeds from August to October.

The players

Sarah Ganske

A weed specialist with K-State Extension who provided expert advice on controlling marestail in soybeans.

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What they’re saying

“Marestail can be a big problem in some situations, as it has fluffy little seeds that can fly around and go long distances – plus it is hard to control once it bolts (grows tall).”

— Sarah Ganske, Weed Specialist, K-State Extension

“Once it bolts, it becomes challenging to control with most herbicides; it also has resistance to glyphosate and other herbicides that we have historically relied on.”

— Sarah Ganske, Weed Specialist, K-State Extension

“Producers who have used Liberty herbicide in combination with Sharpen have said that it is a very effective combination, even on larger marestail plants.”

— Sarah Ganske, Weed Specialist, K-State Extension

What’s next

Producers should scout their soybean fields now to identify any emerging marestail and take prompt action to control the weed using herbicides like 2,4-D, dicamba, Liberty, and Sharpen before it has a chance to bolt and spread.

The takeaway

Early detection and proactive weed control are crucial for managing marestail in soybean crops. By acting quickly with the right herbicide combinations, producers can prevent this problematic weed from gaining a foothold and negatively impacting their soybean yields.