Tag Archives: tree dahlias

Autumn, the first magnolia bloom of late winter, a bridge and the lovely tree dahlias

Autumn down by our stream 

Metasequoia glyptostroboides or the Dawn Redwood in our park

It is indubitably autumn here. The deciduous plants have coloured and are dropping their leaves. The nights are cool enough for us to have entered the time of the year when we light fires in the evening. True, the daytime temperatures are still around 19 celsius and we are enjoying one of our prolonged calm, mild and dry autumns. But autumn it is.

The first blooms open already on Magnolia campbellii in Waitara! On May 15!

This meant I was a little surprised when I ventured out of our home bubble last Friday to see the first blooms opening on Magnolia campbellii down in our local town of Waitara. The tree hasn’t even dropped all its leaves yet but there are several blooms already open. Being right on the coast and surrounded by urban concrete and seal, the temperature is warmer there than in our garden. We won’t see the first blooms on our M. campbellii, which is the same selected clone, until the start of July. Each year I talk about that as the harbinger of spring and the start of a new gardening year for us. I am not sure I can keep saying that having seen it coming in to bloom so early. This is one of the reasons why M. campbellii is not suitable for cold climates. Certainly it will flower later in colder temperatures but it is still so early in the season that it can be taken out by frosts. Waitara is pretty much frost-free.

The wisteria bridge as it was last November when our little dot of a grandson last came to stay

The big project this week, for Lloyd at least, is replacing the decking and railing on the wisteria bridge. That man is worth his weight in gold, I tell you. The wisterias – white Snow Showers on one side and Blue Sapphire on the other – had grown so gnarly and strong that they finally brought the railings down. Now they are both lying on the ground, I can see how big they are and will reduce them by at least fifty percent before we tie them back in, keeping them to a single old trunk and one or two new replacement whips.

Built on an old truck chassis that is outlasting the macrocarpa decking

Dredging the memory banks, we worked out that it is 25 years since the bridge went in. It was constructed by a visiting German engineer who was odd-jobbing around the place. The structural frame is an old truck chassis that was galvanised before it was put in place. That is still in perfectly good condition. It is the timbers that have finally given up the ghost. Initially Lloyd wondered if we could get away with just replacing the uprights and railings that were clearly rotten, but as he deconstructed the bridge, it became clear that all the timbers needed replacing. The original wood used was all untreated macrocarpa (Monterey cypress or Cupressus macrocarpa) so it has done very well to last 25 years.

Progress is being made with new decking and railing supports

Fortunately, ours is a well-stocked establishment with large sheds filled with many useful resources that we may need one day, so we just happened to have a stock of suitable tanalised pine to replace the timbers. Because of my aversion to the appearance of tanalised pine in the garden, it will be stained dark charcoal and I expect it to look very smart. This may even be by the end of the coming week because Lloyd is a project-oriented person. Once he starts something, he likes to keep to the one task in hand until it is completed. This is not a personality trait either Mark or I have and we recognise the advantages of it in other people.

Dahlia imperialis way up in the sky. The white form comes even later in the season.

The tree dahlias are in bloom. Goodness but these are challenging plants to have in the garden. They are magnificent in bloom, that is true. But placing them in the garden is difficult. They are brittle, rampant in growth, frost tender and way too large to stake. Some of ours can tower up to four or even five metres in the sky so they are dependent on surrounding plants to hold them more or less upright. If they fall over, they then smother everything around them and If I go in to try and support that low growth away from surrounding plants, they snap off in my hand. Then when they are dormant, they leave a big gap.

These are certainly not plants for everybody and every garden and there are good reasons why you rarely, if ever, see them for sale.

From a previous season, ‘Chameleon’ I think at the front and ‘Orchid’ (which I hope we haven’t lost but I can’t see any flowers of it yet where I think it should be) at the top. Both Keith Hammett hybrids.

But is there a lovelier autumn sight than their blooms set against a blue sky? We only have half a dozen different ones – the pink and white forms of the species D.imperialis and four from breeder, Keith Hammett. ‘Chameleon’ is a good performing, more compact hybrid of Keith’s that does not shoot for the sky so is more amenable as a garden plant with pure yellow flowers in abundance but it still needs plenty of space.

One autumnal wind will blow the taller ones over but they are a seasonal delight while they last.

A beautiful deep colour on one of Hammett’s hybrids growing through the raspberry coop.

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

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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.
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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.