Tropical colour

Thursday, 12 February 2009

Dahlia and Canna - tuberous-rooted perennials from Central and Southern America - give us some of the most brilliantly coloured flowers of all. Although they belong to quite different plant families, they share some common characteristics and like the same growing conditions. They bloom throughout summer and autumn and provide a tropical, exotic touch to our gardens, mingling happily with the flowers of shrubby Salvia, Buddleja and sun-hardy Fuchsia, as well as bold foliage plants such as Melianthus, coleus, Iresine, ornamental grasses, silvery Plectranthus argentatus and Phormium cultivars.

Shorter-growing, single-flowered Dahlia seem to fit best into garden borders rather than the multi-petalled giant ones, and they come in every imaginable hue; some also have stunning dark foliage. Scarlet-flowered Dahlia 'Bishop of Llandaff' was one of the first of these, discovered as a chance seedling in the 1920s; many others have followed. Dahlia 'Mt Noddy' is a particularly beautiful form with rich burgundy flowers amidst its chocolate-coloured foliage. Dahlia 'Moonfire' is a favourite new dark-foliaged cultivar, its single orange blooms flushed with a red at their centres.

Many Canna also have striking leaves: all-purple or striped with yellow (for example 'Striata', syn. 'Pretoria'), or patterned orange, red, green and bronze in the cultivar 'Tropicanna' (syn. 'Phasion'): giving scope for colour echoes with companion flowers to match these hues. The blooms of Canna come in many possible colours and can be gladiolus-flowered (flower spikes arranged close together on the stalk with wide petals) or orchid-flowered (flower spikes arranged loosely with narrow petals). One very beautiful specimen is Canna iridiflora, with elegant, drooping cerise flowers. Dwarf versions, growing to no more than 1m in height, are suitable for more compact gardens; they can also be grown in containers.

Dahlia and Canna share a love of rich, moist soil in full sun. Both benefit from deadheading regularly in the flowering season, and they enjoy regular applications of fertiliser through the warmer months. Mulching is beneficial. Snails and slugs enjoy the fresh shoots of cannas and dahlias in spring, so these need to be watched out for then. If the plants are well fed and watered, there should be few other problems.

The plants of both types can be cut back to the ground in June. They both benefit from being dug up, divided and replanted into replenished soil every three years or so. The best time to do this is around September. When dividing Dahlia tubers, ensure that each portion has a part of last year's stalk attached, or it will not regrow. Canna are less fussy: simply chop up the young, healthy parts of the tuber into pieces with some roots and growing points, and discard the old gnarled sections.