For problem number one is that these are frost tender plants which is not surprising when you consider they originate from Central American areas like Mexico, Columbia and Guatemala. We are not actually frost free in Tikorangi. We have areas of the garden that are so protected now by overhead cover that we can grow the most tender material, but out in the open we still get sufficient frost to require placing tender plants carefully. We may only get three visible frosts each winter, but the air chill on a calm night can get low enough to wreak havoc. And because these tree dahlias don’t start flowering until May and continue into June, they can get hit late in their season.A hint to the second problem lies in the name – the ‘tree’ part. These are not trees. They have nothing to do with trees. They are a fully deciduous herbaceous perennial but their rapid growth in summer and autumn sees them take on tree-like proportions. It is nothing for them to be 3 metres high, sometimes 4 or even 5 metres. Being dahlias, they are plants for sunny, open positions but they also benefit from some support and shelter from wind which can knock their brittle stems over. They have the hollow stems that are typical of dahlias. Some we grow against sheds or to the side of frames already in place for runner beans and frost protection frames for the bananas and sugar cane. Some we fence in with heavy duty bamboo cross bars – hitching rails, Mark calls them.
Below ground, they have big, chunky tubers which mean that they are difficult to grow amongst other plants and they take up quite a bit of space for their six weeks of glory.
Not many gardens have both the space and the conditions that suit such particular requirements, along with a tolerance for their scruffy off-times. But if you have and can, they are as easy to grow as your more modest dahlia but with more spectacular results.
New Zealand plant breeder, Keith Hammett, has done a lot of work with dahlias, including tree dahlias. The orange starburst variety which he named ‘Orchid’, with its twisted petals is more compact than any of the others we grow. It only reaches about 2 metres maximum though that is 2 metres high and 2 metres wide. We have it by a big mandarin tree whose fruit are ripening as the dahlia blooms. It is a lovely combination.My favourite is the simple Dahlia imperialis species and it is the most commonly available plant. When it first comes out, it looks like a clematis from a distance. Yes the blooms are a little floppy and the petals are larger and soft, so easily damaged, but I like the somewhat pendulous form and I think the lilac pink colouring is pretty.
Being a species, there are a fair number of different selections of D. imperialis. Our late season double white is Dahlia imperialis alba plena. ‘Alba’ of course means white and ‘plena’ means full and is applied to fully double flower forms. This one towers above a shed and puts on a wonderful display with its shaggy blooms but usually gets cut back by the cold when still in bloom in early June.
While tree dahlias can be grown from tubers in the same way as their smaller dahlia cousins, they are also commonly propagated from cuttings which are easier to handle than their oversized tubers. I admit I have yet to try it – there is a limit to how many tree dahlias we can place here – but the advice is to cut the stems that flowered in autumn, making sure that you have at least two nodes per cutting. Lay it flat because the new roots form from the nodes and cover to a depth of about 10cm. Or you can take spring cuttings from fresh growth. It does not appear to be difficult. I may report back on this because we are taking cuttings this year. We have a newly available position where a large tree fell, opening up what looks to be an ideal space for a tree dahlia or maybe two.
First published in the May issue of NZ Gardener and reprinted here with their permission.