How to Get Rid of the Toughest of Weeds
Weeds have developed traits that make then top-notch survivors, allowing them to colonize and choke out weaker competitors. Learn how to beat the worst weeds.
How to Control Marestail
One of the most imposing weeds that can colonize a landscape bed or meadow is marestail, an annual weed that gets its name from the cluster of wiry flower stems atop a tall, central stalk.
Early observers of it decided that the plant reminded them of a horse’s tail.
When marestail seeds find a hospitable patch, they can send up hundreds of these stalky, hulky plants to form a four- to six-foot-tall mini-forest capable of choking out everything else.
Marestail is a rampant seeder, too. Single plants are capable of producing 200,000 or more seeds that can blow more than a quarter of a mile to create ever-expanding colonies.
No wonder, then, that this North American-native weed has spread throughout the U.S., Mexico, and most of Latin America as well as into southern Canada. It’s even made its way throughout most of Europe and beyond.
Colony of tall marestail plants in early summer. iNaturalist
Getting to know marestail
Marestail (Conyza canadensis) is also called horseweed for its horse-tail looks. It’s also occasionally called Canadian fleabane for its resemblance to the native wildflowers known as common and daisy fleabanes (Erigeron species).
Mature marestail plants are fairly easy to recognize because their hefty height makes them hard to ignore.
Other key identifiers are:
- Narrow, lance-shaped, willow-like leaves.
- Those stalk-topping skinny flower stems that produce pea-sized white, daisy flowers.
- Ultimately, the dandelion-like puffy seedheads that are behind the plant’s ability to seed around so well.
Young marestail plants are much easier to overlook. The seeds germinate into short, hairy, almost fuzzy-looking little rosettes that have two-inch, oval leaves – not very similar-looking to the mature version of the weed.
Besides being such a prolific seeder, marestail has an unusually long germination period. Plants can pop up in fall to overwinter and then flower the following summer or in spring to early summer to flower late summer into fall.
That makes marestail one of the few weeds considered to be both a summer and a winter annual. The plant is able to do that because its seeds don’t need cold or winter dormancy to allow sprouting. Seeds can sprout almost as soon as they drop, so long as daytime temperatures are in its ideal 68-to-86-degree germination range. Seeds also germinate best in the top quarter-inch of the soil.
Depending on when a marestail plant germinates, it can send up those substantial flower stalks any time from April into early summer. Most plants flower in mid-summer and disperse seeds from the puffy tips in August through October.
Marestail thrives best in open, sunny, and well drained sites, such as sunny flower beds, vegetable gardens, meadows, and farm fields. It’s not a fan of shady, wet areas, although you may see a few young marestails give it a shot in those conditions.
Young rosettes of marestail quickly grow tall in spring and early summer, with narrow lance-like leaves. iNaturalist
Solving a marestail problem
To keep a lid on marestail, the first step is getting to know the look of those little rosettes before the flower stalks emerge. That’s because this weed is so much easier to control in its infancy than when it’s grown into a hulking, six-foot green monster.
Early to mid-spring is the time to be on the lookout for marestail “babies.” You’ll often see colonies of young ones since this plant’s seeds are so plentiful.
Both fall-sprouted and new spring-sprouted marestails are easy to pull or hoe, although the fall ones might have a bit better-developed root system. Be sure to dig all plant parts since marestail is capable of regrowing from roots and stem pieces left behind.
Young marestail plants also can be killed with spot-sprays of many liquid herbicides. However, mature plants have developed some resistance to herbicides, including the widely used glyphosate.
Check product labels to be sure marestail/horseweed is listed under “weeds controlled,” and be careful not to allow spray to drift onto desirable plants. Follow all other label instructions as well.
Marestail flowers and seedheads. iNaturalist
Preventing future trouble
As with all rampantly seeding weeds, the most important job is to kill or remove marestail plants before they’ve had a chance to flower and set new seed. Even a few marestails that go to seed can lead to hundreds of thousands of new weeds.
A second line of defense is covering bare ground either with mulch or with groundcover or other dense plantings of your choice. Without exposed soil, marestail seeds have no space to germinate and no light to aid germination.
A third defense is applying granular weed-preventers such as Preen Extended Control Weed Preventer.
Preen Extended Control is labeled for the control of marestail in landscape beds, where it can be applied after planting annual flowers or over top of some 600 species of existing landscape plants. The label lists plant-by-plant specifics. For best results, lightly water after scattering the granules.
Preen Extended Control stays active for up to six months. Since marestail has such a long germination window (fall or spring), the best prevention results come when Preen Extended Control is applied twice a year – once in March and once in early August.
Preen Mulch with Extended Control Weed Preventer is a two-action bagged product that combines wood mulch with a weed preventer.
