- Name also: Common Pepperweed, Prairie Peppergrass, Prairie Pepperweed, Prairie Pepperwort, Dense-Flower Pepper-Grass, Miner’s Pepperwort, Miners Pepperweed
- Family: Mustard Family – Brassicaceae (Cruciferae)
- Growing form: Annual or biennial herb.
- Height: 20–30 cm (8–12 in.). Stem usually branching from stem like a candelabra, with dense, spreading hairs, greyish green. With almost no fragrance.
- Flower: Corolla regular (actinomorphic), very small; petals vestigial–lacking. Sepals 4, narrow, with membranous wings, glabrous–hairy, approx. 1 mm (0.04 in.) long. Stamens 2(–4). Gynoecium fused, a single carpel. Inflorescence a raceme, extending in fruiting stage.
- Leaves: Alternate, lowest soon falling, central and upper leaves stalkless. Lowest leaf-blades lobed, Upper lanceolate–linear, with narrow base, lobeless, at least tip with toothed margins, 3-veined.
- Fruit: 2-seeded, roundly obovate (widest at tip), flat, with shallowly notched tips, tip narrowly winged, approx. 2.5 mm (0.1 in.) long silicula. Stalk quite erect, upper surface short-haired, at least as long as silicula.
- Habitat: Railway embankments and yards, harbours, occasionally yards, streets and roadsides.
- Flowering time: (June–)July–August.
Greenflower pepperweed is originally native to North America, and it was found in Finland for the first time in a rubbish dump in Turku at the beginning of the 20th century. Greenflower pepperweed’s infructescence is relatively large compared to other parts of the plant, and it spreads efficiently via its massive seed production. Its kingdom is synonymous with railways, and in Finland its habitat is like a loose network whose threads follow the railtracks. It is found mainly in southern Finland, however, while in northern parts of the country which have a rail network it has always been rare and is now becoming rarer. Stands outwith the rail network are usually short-lived.
Although greenflower pepperweed spread to Finland late it is more common in many places than its relative narrow-leaved pepperwort (L. ruderale). Both plants are quite small and fragile with modest flowers, but they can usually be easily told apart: narrow-leaved pepperwort starts to branch at its base and the branches grow in all directions giving it a rather messy appearance. Greenflower pepperweed only branches from its upper part and the stems eventually take on the regular appearance of a candelabra. It also has almost no fragrance while by comparison narrow-leaved pepperwort has a clearly repulsive smell.
Greenflower pepperweed can be confused, especially when the plants are young, with other close relatives of both European and American origin. The matter has yet to be fully resolved: less pepperwort (L. neglectum) is sometimes classed as a subspecies, and sometimes they are regarded as separate species. Less pepperwort’s leaves have entire margins and in its quite lax infructescence the siliculae are rounder and slightly larger than greenflower pepperweed’s. In Finland there is not usually much need to ponder this issue: less pepperwort spread profusely to Finland in the 1920s mixed in with imported rye, but like many other alien species it was unable to establish a foothold apart from on the Åland Islands, where it appears to have settled in.