Growing form
Annual or biennial herb, occasionally perennial.
Height
15–30 cm (0.8–1.2 in.). Commonly many-branched, stem branching, lower part hairy, commonly reddish.
Flower
Corolla regular (actinomorphic), white–reddish (sometimes later turning purple), approx. 1 cm (0.4 in.) wide; petals four, 5–8 mm (0.2–0.32 in.) long. Sepals 4, with oval base. Stamens 6, of which 2 short and 4 long. Gynoecium fused, a single carpel. Inflorescence a raceme, extending in fruiting stage.
Leaves
In basal rosette and alternate on stem. Rosette leaf-blades pinnately lobed, hairy, underside commonly reddish, terminal lobe large, angular–roundish. Stem leaf-blades narrowly elliptic, with almost entire–large-toothed margins.
Fruit
Many-seeded, slightly curved, flat, 2–4 cm (0.8–1.6 in.) long, spreading siliqua. Stalk 1/4–1/5 length of siliqua, receptacle clearly thickening at tip.
Habitat
Railway embankments and railway yards, highway embankments, harbours, yards, sand fields, gravel pits, waste ground, fallow land, sometimes gravelly river banks.
Flowering time
May–July.

Tall rockcress is native to dry hillside meadows and banks in central Europe. It was known to be growing in Finland already more than 100 years ago, but it only properly established itself after the Second World War. Several dozen central European species spread to northern Finland with the German army, but most quickly vanished. Tall rockcress was able to establish several bridgeheads during the war around loading and storage areas for provisions and garrison areas. During the following 50 years it began to spread to the sides of highways, railway embankments and gravel pits in the north of the province of Oulu and southern parts of Lapland. The population that has spread to Finland would appear to be from the mountains, where the climate is similar to northern Finland. Tall rockcress has begun to spread along railway tracks to the south and it can be found regularly in railway yards in the more densely populated south of the country, although it has yet to establish itself.

Tall rockcress overwinters with the help of its leaf rosette and is therefore sometimes ready to bloom as early as May. Asphalted railway embankments and yards do not suit many Finnish plants, so tall rockcress can grow with almost no competition. In northern Finland the species is nowadays so common and abundant that nobody with an interest in plants can miss it. It is difficult to believe that it is native to somewhere else, and in fact it has grown in Finland since the end of the Ice Age somewhere along the sandy river banks of Salpausselkä. It cross-bred with thale cress (Arabidopsis thaliana), the chromosomes in the resulting plant doubled, and when it was unable to cross-breed back with the original stand it developed into Swedish cress (A. suecica). The species differ from each other with respect to e.g. tall rockcress’s less purplish hue and its deeply-lobed rosette leaves, the way that it is strongly haired, and its large terminal lobe. Additionally, the base of Swedish cress’s petals and sepals are yellowish: tall rockcress has pure white petals and green sepals.

Arabidopsis arenosa

Taxonomía

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