- Name also: Narrowleaf Pepperweed, Roadside Pepperweed, Narrow Leaved Peppergrass
- Family: Mustard Family – Brassicaceae (Cruciferae)
- Growing form: Annual herb.
- Height: 10–20 cm (4–8 in.). Stem branching profusely from base, glabrous–short-haired. With strong fragrance.
- Flower: Corolla regular (actinomorphic), very small; petals vestigial–lacking. Sepals narrow, with membranous wings, sparely haired, commonly slightly reddish, 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. Basal leaf and lower stem leaf-blades pinnately lobed, lobes narrow, with toothed margins–lobed. Upper stem leaf-blades narrow, with entire margins or at most with toothed margins, almost glabrous.
- Fruit: 2-seeded, quite elliptic (basal part widest), flat, only shallowly notched, almost wingless, approx. 2.5 mm (0.1 in.) long silicula. Stalk quite erect, short-haired all over, at least as long as silicula.
- Habitat: Yards, gardens, edges of streets, paths, market squares, fields, waste ground, occasionally railways.
- Flowering time: June–August.
Narrow-leaved pepperwort is native to the Mediterranean region or southern Russia. In Finland it accompanies people, but its stands are quite disjointed and rare. Narrow-leaved pepperwort is at its best in places where old culture is still visible: pavements in areas with wooden houses, between cobble stones and around harbours. When narrow-leaved pepperwort has settled in somewhere it hangs on tenaciously although it does not usually spread. Modern constructed environments do not suit it, and it has declined in recent years and disappeared in some places altogether.
This small, fragile annual, which has a less-than-impressive flower, grows on the edges of inhabited areas, and not much attention has been paid to the changes in how abundant it is. Sometimes it cannot even be differentiated from its abundant American relative greenflower pepperweed (L. densiflorum). In most cases, however, there are clear markers and it is easy to tell the species apart. Narrow-leaved pepperwort begins to branch in many directions from the base – eventually it looks like an indefinable mess. Greenflower pepperweed only branches from its upper part and forms an orderly, dense infructescence which is reminiscent of a candelabra. Full-grown greenflower pepperweed’s leaves are clearly toothed while narrow-leaved pepperwort’s lower leaves are lobed, and its upper ones usually have entire margins. Greenflower pepperweed’s siliculae are widely obovate while narrow-leaved pepperwort’s siliculae are widest at the base or in the middle. A good identification marker is narrow-leaved pepperwort’s characteristic powerful fragrance, but this can sometimes be misleading because although greenflower pepperweed has no fragrance of its own, it can smell of oil and smoke as a consequence of growing along railway embankments.