Tropaeolum - Species in Cultivation

Species in Cultivation

The most common species in cultivation is a hybrid of T. majus, T. minus and T. peltophorum and is commonly just known as a nasturtium. It is mostly grown from seed as a half-hardy annual and both single and double varieties are available (doubles are mostly sterile and need to be propagated from cuttings). It comes in a range of habits and colours including cream, yellow, orange and red, either solid colour or striped and often with a dark blotch at the base of the petals. It is vigorous and easily grown and prefers a sunny spot. It thrives in poor soil and dry conditions whereas in rich soil it tends to produce much leafy growth and few flowers. Some varieties adopt a bush form while others scramble over and through other plants and are useful for planting in awkward spots or for covering fences and trellises.

The following cultivars have gained the Royal Horticultural Society's Award of Garden Merit:-

• 'Alaska series'
• 'Hermine Grashoff'
• 'Whirlybird series'

Tropaeolum azureum is a tender species from Chile which, unusually for this genus, has flowers in an intense shade of blue.

The Canary creeper (Tropaeolum peregrinum) is a trailing and climbing species with wiry stalks and palmately lobed leaves. The pale yellow, fringed flowers are borne on long stalks. It originated from Peru but may first have been cultivated in the Canary Islands before being introduced into Western Europe.

Tropaeolum polyphyllum is a prostrate plant originating from Argentina and Chile. It has silvery, deeply-lobed leaves and a profusion of small, bright yellow flowers on long trailing stalks. After flowering, the plant dies back. It is a perennial with underground rhizomes which send up new shoots at intervals. In a suitable sunny location with well drained soil, it will survive for several years.

The flame flower (Tropaeolum speciosum) is well adapted to cool, moist climates and does well in Scotland. It sends up shoots which thread their way through hedges and shrubs and which, when they emerge into the light, bear brilliant red flowers among small, five or six-lobed leaves. It needs a cool location for its roots and is difficult to establish, but when it thrives, it is a glorious sight. This plant has gained the Royal Horticultural Society's Award of Garden Merit.

Three-coloured Indian cress (Tropaeolum tricolor) is another tuberous, climbing species grown for its attractive red, purple and yellow tubular flowers. It comes from Chile and Bolivia and is a reliable winter-growing species.

Mashua (Tropaeolum tuberosum) is a perennial climbing plant from the Andes, grown for its tuberous roots. It has been cultivated since ancient times and depictions of it are found at archaeological sites pre-dating the Incas. It has leaves with five to seven lobes and small, red and yellow, long-spurred flowers. The tubers have an unpleasant smell when raw which disappears on cooking. It is frost-hardy and produces crops of 30 tons per hectare at a height of 3,000 metres (9,800 ft) above sea level. The cultivar T. tuberosum lineamaculatum 'Ken Aslet' has gained the Royal Horticultural Society's Award of Garden Merit.

Species that originated from the coastal areas and from the lower foothills make most of their growth in winter whereas the true alpine species are summer growers. Tuberous Tropaeolum species are well known for occasionally remaining dormant for one or more years. The species with underground rhizomes and tubers can be propagated from these while other species are best raised from seed. Fresh seed is favoured by many growers but dried seed is also often successful. Seed from the winter growing species should be sown in the autumn while the summer growing species are best sown in the spring in well-drained compost and covered with 1 to 2 cm (0.4 to 0.8 in) of grit or sand. The containers should be kept at below 4 °C (39 °F) until the seedlings appear in about a month, as too high a temperature inhibits germination.