- Synonym: Turritis glabra
- Name also: Tower Rockcress (USA)
- Family: Mustard Family – Brassicaceae (Cruciferae)
- Growing form: Biennial herb.
- Height: 50–120 cm (20–48 in.). Stem usually unbranched, lower part hairy, upper part glabrous.
- Flower: Corolla regular (actinomorphic), yellowish white, approx. 0.5 cm (0.2 in.) wide; petals 4, narrow, 4–6 mm (0.16–0.24 in.) long. Sepals 4. Stamens 6, of which 4 long and 2 short. Gynoecium fused, a single carpel. Inflorescence a raceme, extending in fruiting stage.
- Leaves: In basal rosette and alternate on stem, stalkless, stem leaves amplexicaul, rosette leaves soon withering. Rosette leaf-blades with winding margins–shallowly lobed, hairy, stem leaves’ with sagittate base, with tapering tip, entire margins, quite erect, glabrous, bluish green (may turn purplish after cool weather).
- Fruit: Many-seeded, 4-edged, 4–7 cm (1.6–2.8 in.) long, parallel to stem siliqua, tipped with approx. 1 mm long bristle. Stalk approx. 1 cm (0.4 in.) long, erect.
- Habitat: Rocky outcrops, gravels, stony slopes, meadows, waste ground, roadsides.
- Flowering time: June–July.
Tower mustard’s unbranched and rigidly erect stem – which is emphasised by leaves that become increasingly small towards the top and siliquae that are held in an erect raceme – is straight and quite evenly thick. The plant’s scientific name glabra means ’glabrous’, but this is not an entirely apt description: straight hairs stick out from the bottom part of the stem and the basal leaves are quite densely stellate-haired, although it is true that the most visible part of the plant – its erect stem with its leaves and siliquae – is indeed glabrous. This lack of hair emphasizes the stem’s bluish green colour, which is a product of the thin wax covering that protects it. The prevention of excessive evaporation is necessary in the dry, sun-baked, thin-soiled places that tower mustard prefers. There is no room for its seed shoots among thick vegetation but in open, dry areas it is able to reproduce. During its first growing season it only grows a modest leaf rosette and stores its nutrient reserves in its strong taproot to use when it blooms the following summer.
Tower mustard is probably native to rocky and hilly banks in Finland, even if it rarely grows there. Most of its habitats have been created by people and it also grows in maintained sloping meadows, stony hillocks and dry banks. It can also sometimes be found in shady places at the edge of forests and broad-leaved forests, but in that case it grows poorly. Tower mustard is quite common as far north as Vaasa, and it also grows in a few places further north.
Tower mustard can usually be easily differentiated from its relatives and other plants due to its singular appearance. It can sometimes resemble its close relative hairy rockcress (Arabis hirsuta), but they can be told apart by the latter’s hairiness and serrated leaves.