Perennial Wall Rocket
- Name also: Perennial Wall-rocket, Wild Rocket, Sand Rocket, White rocket, Lincoln Weed
- Family: Mustard Family – Brassicaceae (Cruciferae)
- Growing form: Perennial herb.
- Height: 30–60 cm (12–25 in.), stem branching, almost glabrous, basal part woody.
- Flower: Corolla regular (actinomorphic), sulphur yellow, 1.5–3 cm (0.6–1.2 in.) wide; petals 4, sepals 4. Stamens 6, of which 4 long and 2 short. Gynoeciem fused, a single carpel. Inflorescence an elongating raceme during fruiting stage. Flower-stalks approx. 2 cm (0.8 in.). Flowers fragrant.
- Leaves: Alternate on stem. Pinnately lobed, lobes narrowly linear, terminal lobe longer than others, margin glossy–gently curved. Fragrant when crushed.
- Fruit: Many-seeded, 3–5 cm (1.2–2 in.) long siliqua, with clear central vein and base with short, seedless part. Seeds in 2 rows. Spreading stalk 10–35 mm (0.4–1.4 in.). Siliquae almost erect.
- Habitat: Ballast soil deposits, sea shores, mills, wasteland, rubbish tips, rarely inland.
- Flowering time: June–September.
Perennial wall rocket’s genus’s habitat stretches from the Mediterranean countries to central Europe, Crimea and south-east Russia and India. In the wild in Finland they have spread with people as established aliens. In its homeland in the western Mediterranean countries perennial wall rocket has got used to dry, sandy soil that contains a lot of minerals, so it is able to grow without any problems along the Mediteranean coast. A more significant reason for its coastal habitat, however, is historical: the species’s seeds arrived in Finland in the sand that sailing boats used for ballast. When the age of sailing boats came to an end the stands had to survive and make it on their own.
Perennial wall rocket’s scientific species name tenuifolia describes its leaves well; tenuis means narrow and folium means leaves. Apart from perennial wall rocket, Finland is also home to its close relative wall rocket (D. muralis, also called stinkweed and annual wall-rocket). Wall rocket is an annual and depends completely on its seed production to survive, but in practice all the observations that were made in the 20th century show that the plant is able to ripen its seeds during the short Finnish summer. Long-term stands have survived in e.g. Reposaari in Pori and Lappohja in Hankoniemi. New observations of the species have been made in recent years, but these could be alien species, and wall rocket failed to gain a foothold anywhere even in the age of sailing boats. A close examination of old loading areas for the species can reveal nice surprises, however. Wall rocket’s leaves grow as a basal rosette rather than along the stem, as is the case with perennial wall rocket. Both species can be differentiated from genus Brassica plants by their fruits: their seeds are in two rows, as referred to by their scientific name diplus, means double and taxis, means line. Compared to genus Sinapis plants there are more differences: the siliquae halves have a single vein and its beak is quite short.