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28 winter annual MARESTAIL (Conyza canadensis) PLANT DESCRIPTION Marestail, or horseweed, is a broadleaf summer or winter annual weed that can be seen in late spring to early summer as an upright, 3–5 foot tall, single-stemmed weed with an open, pyramidal flower head. Young plants can also be seen in early spring or in fall as low-growing seedlings or rosettes. Marestail is easiest to identify after growing tall and producing a terminal flower head that resembles a horse's tail. LEAF: Young marestail rosette or seedling leaves are dark green, hairy, and have shallow, rounded leaf lobes. When marestail begins to grow vertically, the leaves become bright green, remain covered in hairs, but usually have no lobes. The leaves are produced directly on the main stem and are densely arranged. STEM: The stem grows vertically during the vegetative stage and is also covered in hairs. As marestail matures and prepares to flower, branches develop at the top of the plant, tapering towards the top. FLOWER: Marestail flowers are inconspicuous and very small. They look like miniature daisies with a round green base, short white petals, and a yellow center. SEED: After pollination, white cottony seed tufts develop in round puffballs similar to dandelion seeds. REPRODUCTION: Seeds are the primary means of reproduction and are dispersed long distances by wind. Mature marestail plants can produce hundreds of thousands of seeds that can remain viable in the soil for two to three years. WHY IT IS NOXIOUS: Marestail is resistant to common herbicides. It is highly competitive at the road edge, can disperse seeds long distances, and is a major threat to agriculture. SEASONAL IDENTIFICATION WHEN TO FIND: The ideal time to find marestail is summer, since it is easily identified by leaves or flower and seed heads. SPRING IDENTIFICATION = EASY TO MODERATE Young maresetail rosette or seedling leaves are dark green, hairy, and have shallow, rounded leaf lobes. SUMMER IDENTIFICATION = EASY Single stem plants lack branches and are covered in hairs. Leaves are slender and hair covered. Leaves attach directly to the stem. In midsummer, a spreading flower head develops at the top of the plant. Flowers are inconspicuous with white petals and yellow centers. Cottony seeds are produced from midsummer to late summer. FALL IDENTIFICATION = EASY Seeds that germinate in midsummer may have a chance to produce flower and seed before frost. Fall horseweed is usually much shorter than those that germinate in spring. WINTER IDENTIFICATION = EASY TO MODERATE Dead plant skeletons remain standing through winter. Rosettes may also be found but can die from extreme cold weather. SIMILAR-LOOKING SPECIES GOLDENROD (Solidago spp.) form and leaves look similar. The flowers are typically yellow and grow from many points along upper branches. Leaf hairs are very short and not as stiff. DWARF MARESTAIL (Conyza ramosissima) flowers and seed heads look similar but are much smaller. It has many branching stems with no central axis. It takes the form of a short bush. OXEYE DAISY (Leucanthemum vulgare) seedling or rosette form can resemble marestail seedling or rosette but it grows as a low mound rather than tall. Oxeye daisy leaves are hairless. LOW HIGH ohio distribution roadside distribution growth seeds ability to dominate difficulty of control WHERE TO FIND: Marestail is common in Ohio at the edge of the road and under rails. Usually found in large groups or colonies, marestail prefers full sun and dry disturbed soil. Marestail is most often found in areas that have been sprayed in the past for bare ground. CONTROL METHODS: MECHANICAL: Mow in the summer only to maintain visibility or to reduce seed production. Hand pull individual plants. Grade road edge to control young plants. CHEMICAL: Make broadcast applications to young plants from late summer to fall with selective herbicide and surfactant. Broadcast pre-emergent herbicide to soils of infested areas in spring before seeds germinate. Marestail has shown an ability to develop resistance to various herbicides. CULTURAL: Install paving when appropriate near roadway. RECOMMENDATION SUMMARY: Marestail is an aggressive, herbicide- resistant, summer or winter annual weed that becomes more difficult to control as it matures. For best control results, make foliar herbicide applications in late spring to early summer. Marestail is more susceptible to chemical control when young and hairs are not as dense. When making foliar applications add surfactant to increase the herbicide's ability to penetrate leaf hairs and reach the leaf surface. Marestail has shown an ability to develop resistance to many commonly used herbicides including ALS-inhibiting herbicides and glyphosate. Always monitor herbicide applications for effectiveness, and if a second application is required for control use alternate herbicides with different modes of action. If a large colony produces seed, prevent future germination by making pre-emergent herbicide applications to soil in spring before seeds germinate. Mow or cut only to maintain visibility or to reduce seed production. Failure to control marestail will allow this aggressive, herbicide-resistant, heavy seed-producing weed to spread along roadways, under guardrails, and other disturbed areas. The wind-blown seeds can easily spread to neighboring private properties and farm fields and remain viable for two to three years. summer annual OR RIVM LEVEL OF CONCERN

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