Trachelium caeruleum Black Knight

The Trachelium caeruleum 'Black Knight' deserves to be more widely used for its beautiful dark colour of vegetation and its very long scented flowering in large violet umbels. A perennial in Mediterranean regions, but moderately hardy, this relative of bellflowers is capable of flowering in the year of sowing. This allows it to be used as an annual in less favorable climates. From early summer until the first frosts, this plant produces superb inflorescences that are constantly visited by butterflies and pollinating insects. Planted between the stones of a rockery or a wall, or scattered here and there in flower beds, it forms clouds of unusual colour. It can also be grown in pots and its cut flowers are prized in bouquets. Once well established in the ground, the Trachelium caeruleum can withstand dry summers.

The Trachelium caeruleum 'Black Knight' belongs to the family of campanulaceae. It is a horticultural selection derived from the blue Trachelium, from which it differs by its darker stems, foliage, and flowers. The original species, with blue flowers, is native to southern Europe and North Africa. In Europe it grows on rocky limestone soils, cliffs, and between the stones of old slightly damp walls. Generally grown as an annual, its stump can withstand -10/-12 °C (14/10.4°F) in dry soil. The foliage may partially persist in mild winters. Otherwise, the stems die and re-form in the following spring. It is particularly susceptible to cold and wet weather.

The Trachelium caeruleum forms a clump of angular stems, abundantly leafy, which branch out and lignify at the base over time. The whole measures about 50 cm (20in) in height and 40 cm (16in) in width, depending on growing conditions. The leaves are lanceolate in shape, with serrated margins. In the 'Black Knight' cultivar, the stems are almost black-purple, the leaves are veined with violet on a shiny dark green background, and their undersides are also tinged with violet. Flowering begins in May-June in the south, and in June-July further north. If the soil remains slightly moist and spent flowers are regularly removed, it continuously blooms until October. In our very dry Mediterranean regions in summer, flowering dwindles in July-August and resumes with the return of rain. The corymb-shaped inflorescences, measuring 10 to 15 cm (4 to 6in) wide, are composed of numerous tiny tubular flowers with 5 spreading lobes. The flowers emit a sweet vanilla fragrance and are nectariferous. After pollination by insects, small pear-shaped fruits containing very fine seeds are formed, which spontaneously reseed in dry stone walls or rocky soils. Plants grown from seeds may not be identical to the parent plant.

The Trachelium caeruleum 'Black Knight' will be wonderful in perennial beds, rockeries, on or in walls, in a wild garden, or in pots. This extraordinary plant softens heavy flowering, and is a perfect complement to medium-sized grasses and asters. For example, it can be associated with Alstroemerias, Chinese peonies, and old roses, both in the garden and in a vase. In a Mediterranean garden without watering, let it self-seed wherever it pleases, it will bloom on the stones even in the shade, alongside the ivy-leaved toadflax (Cymbalaria muralis) or the wall rue Asplenium ceterach. Try to grow some of these plants in large pots that you store in a greenhouse or orangery, they will grow sheltered and produce a splendid effect!

Bouquet tip: flowers should be picked when they are 75% open, the stems trimmed, and quickly placed in water. They will last for 10 to 14 days.