Rock Whitlow Grass
- Name also: Norwegian Whitlow Grass, Norwegian Draba, Rock Whitlow-grass
- Family: Mustard Family – Brassicaceae (Cruciferae)
- Growing form: Perennial herb. Tufted.
- Height: 7–20 cm (2.8–8 in.). Stem 1–3-leaved; straight-, forked- and stellate-haired.
- Flower: Corolla regular (actinomorphic), white, approx. 0.5–1 cm (0.2–0.4 in.) across; petals 4, entire, 3–4 mm (0.12–0.16 in.) long. Sepals 4. Stamens 6, of which 4 long and 2 short. Gynoecium fused, a single carpel. Inflorescence a raceme.
- Leaves: Dense in a basal rosette and alternate on stem, virtually stalkless. Rosette leaves lanceolate, quite tapered, 1–3-toothed, with ciliate margins, straight-, forked- and stellate-haired.
- Fruit: Many-seeded, elliptic–lanceolate, quite flat, quite narrow, hairy (rarely glabrous), slightly alternate, 6–8 mm (0.24–0.32 in.) long silicula. Stalk quite erect.
- Habitat: Rock faces in the forest zone, canyons, ravines, river bank rocks. Fell tundra gravels, crevices, moors.
- Flowering time: June–July.
Classifying whitlow grass is regarded as difficult and this is certainly true, especially in the case of fell species. Rock whitlow grass grows in the lower fell tundra in northern Finland and in forested canyons in crevices, rock shelves, fissures and gravels. The bedrock of Finland’s fells is mainly barren, so rock whitlow grass, which favours a fertile, lime-rich growing medium is rare in Finland. The species’ habitats are mainly found around the large Enontekiö Fell, canyons in Kevo, near Inari in Lapland, and gorges in north-eastern Lapland, as well as in Korouoma and Posio. Its southernmost stand is on Ounastunturi Fell.
Most Finnish whitlow grasses grow in Lapland. Rock whitlow grass is most easily confused with smooth draba (D. daurica) and gray-leaved whitlow grass (D. cinerea). All three are relatively large species which have, in addition to a basal rosette, quite large, white flowers and long infructescence with many siliculae. Rock whitlow grass’s stem and toothed leaves have normal straight hairs, branched forked hairs and short stellate hairs. The silicula itself is sparsely haired and rarely completely glabrous. Smooth draba has a stem that is almost solely stellate-haired, and siliculae that are larger than rock whitlow grass’s. These are usually glabrous. Gray-leaved whitlow grass grows in north-eastern Lapland and is densely stellate-haired, including the siliculae.
As a member of a group of whitlow grasses that are difficult to recognize, alpine whitlow grass (D. alpina) is easy to differentiate at least when it is in flower, thanks to its reasonably large bright yellow flowers. It also has all the three hair types that are typical of the whitlow grasses. At the fruit stage it can be easily confused with rock whitlow grass, but they can be told apart by alpine whitlow grass’s leafless hairy stem and completely glabrous silicula. At the fruit stage alpine whitlow grass also bears a great resemblance to Lapland whitlow grass (D. lactea), although its stem is glabrous. White-flowered whitlow grasses, which all have densely-leaved basal rosettes, are the most difficult of all to classify and they often demand close inspection of their hairs. Snow whitlow grass (D. nivalis), which is another member of this group, has a stem that is densely stellate-haired from its rosette to its sepals, and thus appears quite grey. It is completely devoid of unbranched straight hairs. Both milky and snow whitlow grasses are rare, and in Finland they only grow in the fell tundra around Enontekiö.