Going up – with dahlias 5 metres tall!

Tree dahlias
Photo courtesy of Lesley Shaw

Who doesn’t love a dahlia? Richly colourful, and with a seemingly endless array of petal shapes, these late summer favourites graced our colonial ancestors’ perennial borders and are still delighting us today. But one dahlia in particular is currently grabbing our attention like no other. It’s the tree dahlia, which can grow up to up to 5 metres high!

Tender giants

These giants of the dahlia family hail from the warmth of Central America (think Mexico, Columbia and Guatemala), but unlike their bushy cousins, they spend so much of their year gaining height, that they don’t flower until late autumn and into early winter. Although tree dahlias will tolerate a couple of light frosts, they are relatively tender, and a serious chill will cut them back in no time. If you live in mountainous regions, these beauties are not for you, but if your winters are regarded as chilly (rather than severe), it is still worth giving tree dahlias a go, especially as many of us are now experiencing an extension to our autumns and a shortening of our coldest season. Those in warmer climates can grow tree dahlias to their heart’s content, but will still need to provide a sheltered spot well out of reach of strong winds.

Tree dahlias
Photo courtesy of Lesley Shaw

Colour plus

Thanks to the work of well known New Zealand plant breeder Dr Keith Hammett, we have not only the original lilac-flowered tree dahlia to enjoy, but also crosses in a range of colours including candyfloss-pink, orange (a more compact variety) and yellow. There is also Dahlia imperialis Alba, the shaggy white member of the family. Each plant bears multiple heads of flowers, and blooms can be 10-15 cm across.

Where can I get them?

Although we are slowly beginning to see more tree dahlias in gardens around the country, their arrival enmasse has been somewhat held up by the difficulty of raising them in a commercial nursery setting. To begin with, if this dahlia is to be lifted, the best time to do so is in autumn at around the very time the plant is blooming. Then there is the sheer size of the tubers, which are much larger than those of regular dahlias, and which require deep pots to accommodate them. Nevertheless, a quick internet search will elicit a few nurseries which are able to provide plants. Even better, if you happen to know of an acquaintance who has tree dahlias growing in their garden, ask for a stem cutting. Cuttings can be taken from autumn or spring growth, and each section should have two nodes. Lay the cuttings on their sides, and partially bury them in good quality potting mix. Roots should begin developing from the growing points.

Making a home for tree dahlias

Tree dahlias are not fussy plants, except where shelter is concerned (their great height dictates the need for shelter and staking, or they can be grown against the side of a house, outbuilding or tall fence). They do not mind heavy rains provided the soil is free draining, and although you can add fertiliser to their beds if you wish, a modestly rich soil will suffice to keep them happy. Nothing could be simpler – or more rewarding – than growing these colourful talking points!