Conyza canadensis

Marestail (Conyza canadensis), also known as horseweed or Canada fleabane, is problematic due to vast seed reproduction in two cycles, wind dissemination, lack of seed dormancy, and adaptability to both moist and dry soils.

The small, pea-sized, white, daisy-like flowers of marestail form seedheads similar to tiny dandelion puffballs that produce thousands of wind-dispersed seeds. Marestail can be misidentified as mouse-ear chickweed, shepherd’s purse, or annual fleabane.

Marestail seeds germinate in either the fall or the spring and intermittent germination may occur anytime except under hot summer temperatures. Fall biotypes bolt earlier than the spring-emerging plants that flower beginning in late June and produce mature seeds from August through early October. Spring biotypes begin bolting in midsummer. One mature marestail plant is able to produce up to 200,000 seeds that are wind transported.

Management issues

• Marestail tolerates drought conditions well and continues to grow and produce seed under environmental conditions too stressful for growth of many other plants, preferring undisturbed soil. Conversely, marestail is not very shade tolerant.
• Seeds do not require a period of dormancy to germinate, making it difficult to predict emergence. Mature seed falling to the soil during late summer/fall can sprout as soon as adequate moisture becomes available (usually with the first rains).
• Herbicide effectiveness against horseweed significantly lessens after plants are >6” tall and the stem begins elongating/bolting.

Mechanical control

• Remove marestail seedlings when they are small, before they flower and go to seed. Hoeing, hand weeding, or tilling can be effective, especially in container nursery crops, and is the only option for removing large weeds.
• Remove larger weeds entirely, since marestail can re-sprout from broken stems. Cut the plant root below the soil surface.
• Control weeds in adjacent fields/beds to prevent seed spread.
• Clean equipment before use or between fields with high pressure water or compressed air to prevent spreading cut vegetative pieces/propagules.

Chemical control*

Herbicides should be applied when weeds are small and most vulnerable to control (preferably in the seedling or rosette stage, < 4” tall, and prior to flowering). Since marestail can germinate in the fall and in the spring, you may need to split pre-emergent treatments. Apply fall pre-emergent treatments by early August and do spring pre-emergents by mid-to-late March.

In a container nursery production cycle, herbicide application timing is important: 1) during liner propagation, 2) site preparation before setting containers on ground, 3) at potting, 4) approximately one month after potting.

Residual herbicides can be effective if applied at the seedling/rosette stages. It can provide control through early June. Burndown herbicides can be applied when plants are 4-6 inches tall or before weeds reach the bolting period. Postemergent herbicides should contain three to four sites (mechanisms) of action to manage glyphosate-resistant marestail.

*Disclaimer: Complete reliance on any one herbicide may lead to resistance and population shifts. The listed chemicals exhibit activity against this weed; however, chemicals exhibiting activity will not always provide complete control. Tank-mixing more than one chemical often will improve efficacy, as will delivering sequential applications of a single chemical. Always use pesticides according to directions on the label.

The information given herein is supplied with the understanding that no discrimination is intended and no endorsement by University of Maryland Extension is implied. Read labels carefully before applying any pesticides.

Source: Deborah Smith-Fiola and Stanton Gill, University of Maryland Extension

To read this extension publication in its entirety, including references, visit https://bit.ly/marestail_UMD.