- Name also: Wormseed Wallflower, Treacle-mustard, Wormseed Mustard
- Family: Mustard Family – Brassicaceae (Cruciferae)
- Growing form: Annual (ssp. cheiranthoides) or biennial (ssp. altum) herb.
- Height: 20–100 cm (8–40 in.). Stem to some extent branching, edged, roughly haired.
- Flower: Corolla regular (actinomorphic), quite pale yellow, approx. 0.5 cm (0.2 in.) wide; petals four, 3–5 mm (0.12–0.2 in.) long. Sepals 4. Stamens 6, of which 2 short and 4 long. Gynoecium fused, a single carpel. Inflorescence a raceme, extending in fruiting stage.
- Leaves: Alternate, stem leaves almost stalkless. Blade narrowly elliptic–lanceolate–lanceolate-ovate, stellate-haired, almost entire margins–shallowly toothed.
- Fruit: Many-seeded, 4-edged, stellate-haired, spreading, 2–3 cm (0.8–1.2 in.) long siliqua. Stalk 10–15 mm (0.4–0.6 in.), spreading.
- Habitat: Cereal and hay fields, potato fields, gardens, fallow land, dry meadows, beside roads and railways, heaps of earth, waste ground.
- Flowering time: June–August.
Treacle mustard is one of the Mustard family’s more memorable weeds and has probably been making a nuisance of itself to European farmers since the very beginning of agriculture. As is typical of weeds, it makes a lot of seeds within its quick life cycle. A short gap between generations and a number of mutations promote its ability to adapt to new conditions.
There are two sub-species of treacle mustard in Finland. Ssp. cheiranthoides is an annual that has a long time since spread across the whole country apart from Lapland, in southern parts of which it is an alien species. Its original homeland, habitat and time of arrival in Finland are all open to debate, but possibly it has adapted to become a weed from an original biennial form. Its typical habitats are cultivated ground, gardens, banks and many kinds of waste ground. It disappears quickly from undisturbed places.
Ssp. altum is a biennial that is on average larger, hairier and flowers less than ssp. cheiranthoides, and the remains of the previous year’s rosette are easy to see at the base of the former. It also has a more northerly habitat and is a newer arrival, growing only casually on waste soil heaps, in dumps and beside railways in the south of the country. It probably arrived in Finland prior to the 17th century from Russia, where it grows native to Siberian river banks. As a biennial it thrives in hay fields, untended fallow fields and beside railway tracks and roads.
Tall wormseed wallflower (E. strictum) is native to Finland and can be told apart from treacle mustard by e.g. its shorter, more erect fruit-stalks and more clearly-toothed margins. It is also common in completely undisturbed places, while treacle mustard is only common in places that have been affected by humans.