Species specifics: Marestail

Fast facts

Commonly referred to as horseweed
Marestail is a native annual weed found throughout North America that has developed some herbicide-resistant biotypes in parts of the United States
A prolific weed that can produce over 100,000 seeds per plant
Seeds have a fluffy pappus that allows easy travel by wind, often many miles from the parent plant

What to look for

Marestail (Conyza canadensis), often referred to as horseweed, is a tall growing opportunistic annual forb that emerges as a small rosette in fall or early spring. Other common names of this weed include coltstail, fleabane and butterweed.

Ranging from one to six feet tall, marestail has a single stem with many alternating lace shaped leaves, almost willowlike, up to the top where numerous tiny greenish white flowers bud out in summer. The stem has short bristly white hairs giving it its common name of marestail or horseweed. An average sized single plant can produce more than 100,000 parasol seeds that can spread a great distance by wind and can remain viable for many years.

It’s most common for marestail to first emerge in the fall – a majority of which often survive the winter and come out of dormancy around the same time that new rosettes begin to appear in early spring. Marestail will bolt in April or May, begin to flower in June, seed from August to October and then die off. The opportunistic weed favors dry, disturbed or bare land, however, it’s highly adaptable and will thrive in waterlogged soils as well. Pastures that have been overgrazed or impacted by drought tend to be most susceptible to marestail infestations.

While marestail doesn’t generally cause a toxicity issue for cattle if consumed, it does contain toxins that help inhibit germination and growth of other plant species that can cause skin and mucosal irritation in horses.

Where it is found

Found throughout North America, marestail is a native species that is common in all parts of the contiguous United States, especially in agriculture heavy regions such as the Midwest and Great Plains where winds and disturbed soils are abundant. In many states including Ohio and Oregon, the species is considered a noxious weed.

How to treat it

Not only is this opportunistic weed becoming a larger issue across the country, but it has also developed some herbicide-resistant biotypes. Because of marestail’s aggressive and hardy nature once established, treatments tend to be most effective before the plant bolts in early spring.

Apply 16 to 20 fluid ounces of DuraCor^® herbicide per acre or 24 fluid ounces of NovaGraz™ herbicide plus 1% MSO v/v per acre. Alternatively, apply 20 to 32 fluid ounces of GrazonPD3^TM herbicide per acre.

For season-long control, apply 1.5 to 2.1 pints of GrazonNext® HL herbicide per acre.

GrazonPD3™ is a Restricted Use Pesticide. Under normal field conditions DuraCor® is non-volatile. DuraCor has no grazing or haying restrictions for any class of livestock, including lactating dairy cows, horses (including lactating mares) and meat animals prior to slaughter. Label precautions apply to forage treated with DuraCor to manure and urine from animals that have consumed treated forage. DuraCor is not registered for sale or use in all states. When using NovaGraz™ herbicide, white clover and annual lespedeza exhibit some initial injury (such as lodging and loss of vigor) but recover. NovaGraz™ is not registered for sale or use in all states. GrazonPD3 is not registered for sale or use in all states. GrazonPD3 is not for sale, distribution or use in Nassau and Suffolk Counties in New York State. GrazonNext® HL has no grazing or haying restrictions for any class of livestock, including lactating dairy cows, horses (including lactating mares) and meat animals prior to slaughter. Label precautions apply to forage treated with GrazonNext HL and to manure and urine from animals that have consumed treated forage. Consult the label for full details. GrazonNext HL is not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. GrazonNext HL is not for sale, distribution, or use in New York State and San Luis Valley of Colorado. Always read and follow label directions.