For the top 20 dahlias to grow, click here
The dahlia is about to take centre stage in our borders and it will grab the limelight until the first frost brings the curtain down – and these days that could mean November. No other flower will carry you through from summer into late autumn with as much pizzazz and vibrance.
Among the categories, there are spiky cactus dahlias, which explode in the border like firecrackers, and prim and proper ball dahlias with comely ruffs of petals. There are gentle water lily types, impressions from a Monet painting; slightly sharper decoratives; and bee-friendly singles and semi-doubles.
The Honka dahlias, which look dismal in pictures, are highly effective in a mixed planting. Anne Barnard, of Rose Cottage Plants (see suppliers), tells me that ‘Honka Rose’ “looks as though someone has tossed a handful of pink stars into the air and they’ve landed where they fell”. The colour range is equally impressive.
Compact dahlias make great container plants. The Dark Angel Series combines small, single flowers and black, ferny foliage. Look out for ‘Star Wars’ (a yellow and orange); ‘Dracula’ (a dark-centred blood-red) and ‘Pulp Fiction’ (a neater clear-red). The Happy Series of dahlias has larger single flowers on long stems. My favourite is ‘HS Wink’, a raspberry-centred pink.
Velvety reds often have a touch of dark Guinness hues thrown in, and they won’t frazzle in hot sun as so many dark flowers do. There are soft peaches and pumpkin oranges that chime with mellow autumn days, and purples, pinks, lemons and whites. Sometimes the foliage is near-black, or military khaki.
The diversity of form, foliage and flower colour in dahlias is due to an octoploid tendency – which has nothing to do with the sea bed. In simplistic terms, the 40 or so Mexican species can have up to eight copies of the same gene. When the bees get busy you never know what’s going to pop up from the parental gene bank.
This explains how the brilliant-red, peony-flowered ‘Bishop of Llandaff’ spawned a fully-petalled, bronze-orange seedling, subsequently named ‘David Howard’.
Both are fantastic performers, with real staying power, and both have an ability to flower in poor summers. Fergus Garret, head gardener at Great Dixter in East Sussex, favours ‘David Howard’ for its “show-stopping charm” in the borders. This star performer was raised in 1958 by the late Suffolk nurseryman of the same name.
However, all that serendipity is a double-edged sword, because there are now so many dahlias on offer. New ones are constantly being raised and named by hobbyists and breeders. The cut-flower trade provides much of the impetus, for dahlias are the ultimate cut-and-come-again flower.
Show bench dahlias, symmetrical charmers disbudded to produce giant blooms, were once sized by passing the flowers through a set of rings. Many a good one landed on the cutting room floor, so to speak, but such is demand today that that’s all changed.
The wonderfully wispy whirligig christened ‘Waltzing Mathilda’, never a contender for showing, fought for and won an AGM on the RHS trial. The wild-looking, ferny foliage and ruffled, chrysanthemum-like blooms of ‘Soulman’, a rich burgundy, colour-washed in black, is another stunner that wouldn’t win a show category either.
The downsides to dahlias
There are drawbacks. You can’t get away with plonking a dahlia into a border, because these late starters only begin to get into their stride once the nights begin to get longer and cooler. This short-day tendency is shared by many late-flowering perennials – salvias, cosmos, penstemons – but their pigment-packed petals create a jewel-box quality that’s missing earlier in the year.
Garret has the solution: “We create semi-permanent gaps, or bedding pockets, in the borders and fill them with spring bulbs and bedding. We often use forget-me-nots and tulips. They’re removed in late May and then our dahlias go in.
“One of my favourites is the vivid-red ‘Witteman’s Superba’, a bold red cactus dahlia. It glows in front of a vertically striped green and cream grass, Miscanthus sinensis ‘Cosmopolitan’.”
Dedicated cutting gardens and allotments suit dahlias well; I don’t stake mine, but planting taller cosmos and Verbena bonariensis among them helps to provide support.
There’s always a debate about whether to leave them in the ground over winter, or to lift them. I’m on the side of lifting, once they’ve been frosted, because I lost the lot in 2012.
However, large tuber masses defy logic and produce less flower, not more, so regular division is needed, although there are exceptions. The man-high, rich-purple ‘Admiral Rawlings’ isn’t disturbed at Old Wollerton Hall in Shropshire – where Leslie Jenkins peps up her colour-themed borders with dahlias.
I store mine under the greenhouse bench, in large airy bread trays, and cover the muddy tubers with dry compost. They stay there, frost-free, until March, and then they’re divided (if needed) just as they start into growth.
How to grow dahlias so they come up and flower every year
- Order your tubers in the New Year. Store them in a cool, frost-free place until mid-March and pot them, preferably in a greenhouse fitted with a heater to keep frost out.
- Once temperatures approach 20C (70F) dahlias grow away enthusiastically. Pinching out the tip of the shoots encourages bushiness and you can you use the snippets for cuttings (see below).
- Harden off dahlias outside, from mid-May onwards, but cover with fleece if a frost is forecast. Don’t plant them outside until the beginning of June, as cold nights check their growth and a late frost can kill them.
- Add a small handful of blood, fish and bone to the planting hole and then apply one high-potash feed, such as Vitax Q4, on a damp day in June.
- Over-lavish feeding promotes foliage at the expense of flower.
- Once established, dahlias can survive without being watered thanks to their tuberous roots.
- Dark-leaved dahlias are more prone to slugs.
- Always deadhead to keep the flowers coming.
How to take cuttings
- Cuttings, taken in April and May, produce faster growing, stronger plants, but flower slightly later.
- Look for strong new shoots, about 2-3in in length (5-8cm), and cut the stem just above where it joins the crown – leaving a stub that will hopefully resprout.
- Trim underneath the lowest leaf joint of your cutting, using a sharp knife, and reduce any overlarge leaves by half.
- Use a mixture of compost with a little perlite and coarse grit. Water the pots and then position your cuttings round the edge of each pot, making sure that two-thirds of each cutting is submerged.
- Cover each pot with a clean plastic bag, or place in a propagator. They should root within 4-5 weeks.
- If you’re doing lots of cuttings, clean your knife between varieties to prevent the spread of viruses.