- Family: Daisy Family – Compositae, subfamily Asteroideae
(formerly Aster Family – Asteraceae)
- Growing form: Annual herb.
- Height: 2–25 cm (0.8–10 in.). Stem branching–branchless, usually densely tomentose, sometimes completely glabrous, lacking basal shoot.
- Flower: Single flower-like approx. 3–4 mm (0.12–0.16 in.) capitula surrounded by involucral bracts. Capitula’s ray-florets lacking; disc florets yellowish white, tubular, small. Stamens 5. Pistil of 2 fused carpels. Involucre slightly spherical, involucral bracts overlapping, lanceolate, tapered, membranous, shiny, mostly dark brown–straw-coloured. Capitula 3–10 in axillary clusters, subtending bracts clearly longer than capitula.
- Leaves: Alternate, short-stalked. Blade narrowly lanceolate–narrowly obovate, sharp-pointed, with entire margins, densely haired, both sides greyish green–light grey.
- Fruit: Elliptic, glabrous, brown, achene less than 1 mm (0.04 in.) long, tip with unbranched hairs.
- Habitat: Shores, puddles, ditches, paths, small roads, yards, arable land, wasteland.
- Flowering time: July–September.
Marsh cudweed often grows as a field weed in quite damp, clay-rich land. It has adapted well to life as a field weed: its seeds are able to ripen just before threshing time and autumn ploughing, and the land makes a good seed-bank until the following summer. During spring ploughing some of the seeds end up buried deep in the earth, but others rise up to take their place. The achenes have flying hairs and are sticky and so move efficiently to new habitats. Unlike members of the Poaceae family marsh cudweed is dicotyledonous, meaning that it has two cotyledons when it begins to grow. This is significant to the farmer because monocotyledonous and dicotyledonous plants are repelled by different chemicals, and marsh cudweed can be prevented from running amok by weedkiller. Other cudweeds that grow in Finland do not form large stands, but in the right conditions marsh cudweed can grow abundantly. The most handsome stands usually grow beside ditches and roads. Marsh cudweed’s abundant seed production increases the amount of hereditary mutations and helps it adapt to changing conditions, which is very important for an annual plant in an unstable environment.
Marsh cudweed in Finland can be divided into two subspecies: ssp. uliginosum has achenes which are glabrous, while on ssp. pilulare they are short-haired. The latter grows mainly in northern Finland. Marsh cudweed’s common northern limit is inside the Arctic Circle, and the northernmost stands are to be found in Enontekiö and Inari Lapland.