Issue 7, July 15, 2019

Marestail (Horseweed) in the Landscape

Horseweed or marestail (Conyza canadensis) is a fairly common weed in Illinois for corn and soybean fields, nurseries, and orchards.  It is less commonly thought of as a landscape weed, but it can find its way to ornamental plantings and control can be challenging. Both names are commonly used throughout the state.

This plant is a bit peculiar considering its life cycle.  It can grow as a winter annual or sometimes as a summer annual. Seeds germinate in late summer or early spring.  The seedlings develop a rosette of leaves, which can be easily confused with other rosette forming weeds such as Virginia pepperweed and shepherd’s-purse. From the rosette of leaves, plants will then “bolt” and produce a flowering stem.  Stems can reach 7 feet in height.  The rosette may go entirely unnoticed in a bed due to it’s low stature or the fact that once the stem forms, the basal leaves will deteriorate and disappear altogether with time.  The tall stems make marestail more noticeable.

An easy way to describe a mature marestail or horseweed is that of a single stem with many dark green, whorled leaves.  The stems is hairy with many small, flowering branches toward the top. I suppose it could look like the tail of a horse, given the name.  This weed should not be confused with horsetail (Equisetum arvense), which is an entirely different weed.

To be more accurate in our description, we must consider the leaves further. They are approximately 4 inches long by 1/3 inch wide, hairy, alternate but numerous, and crowded around the stem. Stem leaves are lanceolate to linear, with minimal toothing on the margins.  Flowers are present July to October and consist of dense panicles of many small flower heads with white ray flowers and yellow disk flowers.  The seeds have white slender bristles at the end which aid in wind dispersal.

Marestail or horseweed has a slender taproot and can be removed by hand easily most of the time.  However, be sure to pull this plant at the base or it can break off.  For best results, pull when plants are young and soil is moist.  Removing this plant from the landscape before seed set is important in preventing future large infestations.  One plant can produce around 200,000 seeds.  Herbicides can be used as well, but be aware that resistance to certain herbicides (ALS inhibitors and glyphosate) has been found in this weed.  You may not have a history of using glyphosate to control this plant, but if the seed moved in from an agricultural area that had that history, the plant may not be well controlled with this herbicide.  Preemergent herbicides can be used as well but proper timing can be difficult given the extremely wide  germination period.

Young marestail.

The stems and leaves are hairy.

Mature marestail in flower.

Weeds of the North Central States
Weeds of the Northeast

Michelle Wiesbrook

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