Horseweed
Also known as
Canadian horseweed, Canada fleabane, Fleabane, Marestail
Keep this plant’s care guide close at hand.
Get the appAbout Horseweed
Erigeron canadensis, commonly called horseweed (syn. Conyza canadensis), is a fast-growing North American annual in the Asteraceae. It often begins as a low basal rosette in cool weather, then bolts into a slender, bristly, upright stem 30–200 cm tall with narrow, alternate, lanceolate leaves that are slightly toothed and aromatic when crushed. From late spring into fall, it produces many branched panicles bearing hundreds of small heads with short, greenish-white rays and yellow discs. Each head yields a tufted achene with a silky pappus, enabling efficient wind dispersal and prolific self-seeding. Exceptionally adaptable, it thrives in poor, compacted, or sandy soils and colonizes disturbed sites, prairies, and open edges. Ecologically, it offers nectar and pollen for small bees and flies and can support beneficial predatory insects; finches may take its seeds. While useful in restoration or wild-style plantings and known in traditional medicine, it readily becomes weedy and includes herbicide-resistant biotypes. In gardens, grow deliberately, contain its spread, and deadhead before seed set.
Taxonomy
Properties & Requirements
How to care for Horseweed
Water
Water to establish, then sparingly. Keep the seedbed evenly moist through germination and the rosette stage, avoiding waterlogging. After roots extend, switch to deep but infrequent irrigation, letting the top 2–3 cm (about 1 inch) of soil dry between waterings. In containers, water when the pot feels light and the top inch is dry, then drain thoroughly. Once established, horseweed tolerates drought and is more harmed by overwatering than by short dry spells.
Light
Thrives in full sun (6–10+ hours daily), producing sturdier stems and better flowering. It tolerates light, open shade in hot climates, but dense shade causes weak, elongated growth and scant bloom. Because seeds need light to germinate, avoid thick mulches during sowing. For container or small-space culture, choose the brightest, most open exposure with free air movement, and avoid deeply shaded north-facing walls or enclosed courtyards.
Nutrients
As a pioneer adapted to lean soils, horseweed typically needs no fertilizer. In the ground, avoid feeding; excess nitrogen encourages lanky, lodging-prone growth and heavier seeding. For containers or nursery culture, blend a small amount of compost into a sharply drained medium and, if desired, apply a balanced, low-nitrogen liquid (e.g., 5-5-5) at half strength once during early vegetative growth. Do not fertilize after buds appear. Overfertilization can increase pest/disease pressure and reduce sturdiness.
Temperature
Horseweed germinates in cool to mild weather and grows best between roughly 10–30°C (50–86°F). Basal rosettes can tolerate light frosts, though bolting stems are more cold-sensitive. It endures summer heat if soils are well-drained and airflow is good. Outdoor culture is strongly preferred; it is not suited to heated indoor conditions. In cold climates, treat as a summer annual and re-sow after the last hard frost; in milder areas, autumn-germinated rosettes may overwinter.
