The giant wild tree dahlias of Mexico – and how to grow them – Telegraph.co.uk

the-giant-wild-tree-dahlias-of-mexico-–-and-how-to-grow-them-–-telegraphco.uk

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Plantsman Nick Macer of Pan Global Plants is describing to me the circumstances in which he first encountered the wild giant Dahlia tamaulipana. A plant-hunting excursion back in 2005 had taken him to the far north-east of Mexico (“a stunning spot in the most fantastic country”), where the Sierra Madre Oriental range, passing through the state of Tamaulipas, faces the Gulf Coast and boasts a vast diversity of flora.
“To get further up into the forested slopes we had to ford a river six or seven times, following a dirt road up the valley,” he recalls. Steep, slippery tracks then led beneath the leaves of Monterrey and loquat oaks (Quercus polymorpha and Q. rysophylla), before, high at the top, Macer spotted the lush foliage and violet petals of an unfamiliar dahlia. “I took photographs and a pinch of seed,” he says.
Germination back in the UK was a protracted process and getting the plant formally described even slower. D. tamaulipana was registered as a new species only as recently as 2018.
Now a centrepiece within the Garden Museum’s plant collector’s courtyard – brilliantly conceived by landscape designer Dan Pearson – Macer’s dahlia brings the “new” to a garden that celebrates botanical expeditions both historic and contemporary.
It has, in fact, been a source of intrigue ever since the museum and gardens reopened in 2017, its dark, glossy leaves filling out into a slowly swelling mound in the company of slender scheffleras and a glaucous-silver honey bush (Melianthus major).
By late summer, the dahlia has muscled up and outwards to a staggering diameter – at its most recent peak I raised a tape measure 3.5m to the sky; higher than the scheffleras, even higher than our giant hybrid cannas.
Pearson, like Macer, admires D. tamaulipana for its foliage first and foremost. Between seasonal bursts of flower, the courtyard is a masterpiece of integrated yet contrasting leaf forms, from the variegated chevrons of velvety Persicaria virginiana var. filiformis to the fishbone markings of thornless bramble, Rubus lineatus.
Into this tapestry the dahlia presses its own dynamic foliage and, being a late starter, as all dahlias are, its delayed flourish caps off the year’s steady crescendo.

Larger species dahlias are the original, unadulterated forebears of the innumerable colourful, often gaudy, sometimes delicate, dahlias grown today. Upland and mountainous Mexico is their home and, as is commonly known, their edible and medicinal qualities first brought them into cultivation.
With tubers much like a potato in size and texture, they were eaten by the Toltec and Aztec communities long before European hybridisation.
By the early 19th century, single-flowering species like the crimson D. coccinea and deeply lobed D. pinnata had made their way from Mexico City to the botanic gardens in Madrid, and were subsequently circulated among botanical institutions throughout the continent.
The great ornamental potential for colour, poise and form was quickly recognised and, centuries on, demand has never ceased – new dahlia cultivars are added annually to a list well over 50,000 strong.
But somewhere between the multicoloured, pompon, cactus, water lily and even the elegant ‘Bishop’ series dahlias, the original Mexican giants became sidelined; thankfully a renewed thirst for interesting, architectural foliage in gardens is bringing them back.
In Britain’s temperate and frost-prone climate, species tree dahlias like the revered D. imperialis are predominately grown to add structure to a herbaceous border or a more “exotic” contemporary scheme.
Towards the back of the world-famous long border at Inner Temple, for example, D. imperialis provides a lush green foil for surrounding vivid perennials. The flowers – elegant, simple daisies that in Mexico bloom by autumn – are therefore considered a bonus, arriving at the tail end of the year, if at all.
Last December, right at the point of threatening frosts and my annual winter mulching of the courtyard, the Garden Museum’s Dahlia tamaulipana surprised us with a sudden flush of extraordinary lavender-purple.
It happened, of course, amid lockdown without visitors to enjoy the show, though trainee Thomas Rutter and I were treated to a private view, circling the courtyard ecstatically for ever-better views of the lofty blooms and their bright yellow centres.
By early January the flowers were a froth: 20 to 25 palm-sized mauve stars that, given their height and the plant’s eventual girth, revitalised the garden at a time of year when it would usually be winding down.

I phoned Nick Macer to better understand our beast, and learned he’d only flowered it twice since bringing it back from his travels. “It’s incredible what you get away with in London!” he remarked. “Beyond the amazing, luscious foliage it is actually a plant that flowers in November or December – which is inspiring.”
One might put the emergence of flowers down to prolonged sunlight, however Macer recounted that, in the wild, tree dahlias often occupy a semi-shaded, woodland-edge environment, and suggested that perhaps the museum’s overhanging plane trees were actually a help rather than a hindrance.
“They clearly don’t enjoy long hot, dry periods,” he said. “Their leaves look happiest come early autumn, particularly if it’s been humid. They stand up proudly and look so fresh.”
With global warming lengthening our summers, species dahlias are increasingly at home in UK gardens (Macer no longer mulches his tamaulipana or imperialis for protection; nor brings them indoors); flowers or not, their freshness of leaf at the end of the season offers an attractive contrast to the fading hues of autumn.
Now is the time to pre-order dormant dahlia tubers, ready to pot up in early spring as shoots begin to emerge; perhaps of all years, this is the year to “go large”.
Matt Collins is head gardener at the Garden Museum in London.
Follow Matt on Instagram: @museum_gardener
How to grow tree dahlias
Potting up
As with all dahlias, tubers are best started off in pots in late March/early April to give them a head start. Choose a container just wider than the tuber, and half-fill with a light multipurpose compost, incorporating a little grit for drainage. Place the tuber upright (old stem remnants pointing upwards) and top-fill with compost, leaving the stem tips exposed. Stand the pot in a cool, well-lit room indoors, and water as shoots develop.
Planting out
Gradually harden off plants once new shoots have sprung and frosts are over. Plant in a sheltered site, offering a little afternoon shade, incorporating plenty of organic matter into the planting hole and some grit for heavier soils. Water well through dry spells.
Aftercare
Despite the size, staking isn’t usually required, although excessive growth can be supported by a stout central stake towards the end of the season if necessary. Feed every four weeks during the summer and provide a generous winter mulch of well-rotted manure. Stems will darken and wilt with frost, at which point cut them down to around 20cm above ground. In colder parts of the UK the tuber can be dug up and stored in a cool spot indoors in dry compost.
Species to try
Dahlia imperialis

The tallest and most imposing of species dahlias, sending up bamboo-like stems bearing lush green foliage. Buds will form as the days shorten towards late autumn, opening pink in the absence of frost.
Dahlia campanulata

An interesting dahlia to try from seed, with tall stems and weeping white petals turning pink around the central disk. For flowers it is best grown in an unheated greenhouse.
Dahlia coccinea var. palmeri

Climbing to more than 2m, this dahlia has finer, more delicate foliage and blazing orange flowers. Suitable for a smaller garden.
Dahlia tamaulipana

A robust, mound-forming dahlia enjoyed for its large glossy leaves and potential late-season, lavender-purple blooms.

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