- Written also: Hairy Rock-cress
- Family: Mustard Family – Brassicaceae (Cruciferae)
- Growing form: Usually short-lived perennial herb.
- Height: 20–60 cm (8–25 in.). Stem unbranched, usually stiff-haired.
- Flower: Corolla regular (actinomorphic), white, under 0.5 cm (0.2 in.) across; petals 4, narrow, approx. 4 mm (0.16 in.) long. Sepals 4. Stamens 6, of which 4 long and 2 short. Gynoecium fused, a single carpel. Inflorescence an elongating raceme in fruiting stage.
- Leaves: In a basal rosette and alternate on stem, stalkless, stem leaves virtually amplexicaul. Basal leaves’ blade narrowly obovate, blunt-toothed, usually hairy, stem leaves long, cordate based, blunt-tipped, serrated.
- Fruit: Many-seeded, terete, 2.5–3.5 cm (1.0–1.4 in.) long siliqua, parallel to stem. Stalk approx. 0.5 cm (0.2 in.) long, erect.
- Habitat: Meadows, meadow slopes, juniper hills, pastures, precipices, rocky outcrops, roadsides, sometimes harbours.
- Flowering time: June.
A group of short-lived, white-flowered Mustard family plants, many of which are easily confused with each other, grow on dry meadows and rocky outcrops. Of these, whitlow grass, hairy bittercress, hoary whitlow grass, shepherd’s purse, Alpine pennycress, thale cress, Swedish cress and yellow-flowered tower mustard are presented elsewhere in this site. Hairy rockcress can also be added to this list – what a diverse species this is! Hairy rockcress only grows in south-westernmost parts of Finland. It is quite common only on the Åland Isles and southwestern archipelago, where it grows on rocky outcrops, but it grows casually here and there on mainland Finland, too. Hairy rockcress demands a lime-rich soil, so most of the meadows, rocks and juniper ridges in Finland are unsuitable.
It is easy to recognise the Mustard family, but the borders between species are often unclear and their differentiating characteristics are difficult to observe. It is no wonder that botanists’ opinions of the family have varied. Nowadays Arabis, Cardaminopsis and Arabidopsis and a few other Mustard family genera have sometimes been combined into a large, loosely defined genus Cress; occasionally they are divided into their own genera, which are smaller than they are at present. The definition of hairy rockcress compared to its close relatives is unclear and information on mutations within the species is incomplete. Hairy rockcress seems to be sensitive to its habitat in such a way that it will look different if it grows e.g. in the shade or in the sunlight. It is probably worth mentioning that a completely hairless variety (var. glaberrima) also exists.
Of the species that grow in Finland, hairy rockcress perhaps bears the greatest resemblance to hoary whitlow grass (Draba incana ; also known as twisted whitlow grass), although their fruits are completely different. Apart from the colour and size of the siliquae, the species can also be differentiated by their leaves: hairy rockcress’s rosette leaves are larger, broader and blunter, and the stem leaves are quite blunt-tipped, while with whitlow grasses they are sharp.