Tag Archives: tree dahlias

Late Bloomers – the tree dahlias in autumn

Tree dahlia 'Orchid', bred by Keith Hammett

Tree dahlia ‘Orchid’, bred by Keith Hammett

The last clarion call of the autumn flowers here are the tree dahlias, wildly impractical plants to grow but I absolutely love them. There is nothing like their over the top blooms soaring skywards in late autumn.  At least we are lucky in this country that we get clear blue skies with strong light all year round. Otherwise they might be soaring up to the gloom of lower light levels of other climates.

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.

Left to right: 'Chameleon" , 'Orchid' (both Hammett varieties), D. imperialis and an unnamed Hammett variety.

Left to right: ‘Chameleon” , ‘Orchid’ (both Hammett varieties), D. imperialis and an unnamed Hammett variety.

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.

Dahlia imperialis, my personal favourite

Dahlia imperialis, my personal favourite

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.

Dahlia imperialis Alba - soaring skywards as winter descends upon us

Dahlia imperialis Alba – soaring skywards as winter descends upon us

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.

022 - CopyFirst published in the May issue of NZ Gardener and reprinted here with their permission. 

Plant Collector: tree dahlias

The only resemblance these dahlias have to trees lies is in the astonishing size they can reach in the space of the summer growing season. Some can get up above 3 metres high. They are just dahlias, growing from the usual tuberous roots and because they are deciduous, they die back completely every year. The best known tree dahlia is the species D.imperialis which is also the largest variety and hails from Central America. Prominent NZ plant breeder, Dr Keith Hammett, has been working with dahlias over time and using other so-called tree dahlia species in ever more complex crosses to bring different colours, flower form and sometimes more compact growth into the range.
Tree dahlias are not the easiest plants to place in the garden. They need full sun and protection from wind because their growth is somewhat brittle. They also need some sort of support to hold them upright. They flower now, in mid to late autumn, much later than the summer dahlias and this makes them vulnerable to early frosts. If you have the right conditions, plenty of space and are willing to build some sort of structure to support them (we are fans of thick bamboo corrals), they can be a welcome addition to the autumn garden but they are never going to be tidy, well behaved plants.

Reasonably sure this is the one named Timothy Hammett

Reasonably sure this is the one named Timothy Hammett

First published in the Waikato Times and reprinted here with their permission.

Tried and True: Tree Dahlias

•Flower from mid to late autumn when few other perennials flower.
• Fill a large space in the garden.
• Many of the best new varieties available here have been bred in New Zealand by our own expert, Dr Keith Hammett.
•Deciduous, so the foliage dies away completely over winter and returns afresh.
• Easy to grow in good conditions which don’t get too dry over summer.

You do need space for these late autumn beauties and they will be badly affected by heavy frosts.

But if you have a suitable position, they are an easy-care delight. These two varieties are both from the breeder, Keith Hammett and alas we no longer have the names. The big, floppy pastel lilac is so pretty against our shed and I thought at first I was looking at a clematis from afar (it is about 2 metres tall). The golden orange sunburst bloom (love the slightly twisted petals) is a little more compact (a little shy of two metres) and has delighted us for a number of years, planted as it is by the mandarin tree whose fruit are colouring to match.

They are not called tree dahlias because they grow up like a tree but rather because they grow much larger than the usual type of perennial dahlias. In windy conditions they need a bit of support – some of ours we fence in with heavy duty bamboo cross bars. Otherwise, they are like any other dahlia with typical hollow stems and dahlia leaves, growing in a large clump from tubers below ground. Most tree dahlias come from D. imperialis which is native to Columbia and Guatemala which explains why they are not keen on cold and frosty conditions.