Types of Dahlia: A Guide to the Flower Forms

There are an absurd number of registered dahlia varieties. Over 57,000. Seriously good going for a plant that only arrived in Europe in 1789. Dahlias are octoploids. They have eight sets of chromosomes where most plants have two, and all that genetic baggage means that crossing two dahlias can produce wildly unpredictable offspring. It is why breeders have been able to create such an extraordinary range of flower shapes, sizes, and colours in just over two centuries.

The National Dahlia Society (NDS), founded in 1881 and still going strong, classifies dahlias into groups based on flower form. The RHS uses a similar system with 14 groups. If you're showing dahlias competitively, the classification matters a great deal. For the rest of us, it's useful to know what you're buying, so that the tuber you plant, which looks a bit like a dirty bunch of very old carrots with no tops, ends up meeting your expectations from July into October.

All dahlias are grown in exactly the same way so for planting, feeding, and care, see our how to grow dahlias guide. This article is entirely about flower shape.

Involute and Revolute Petals

Before we get into the groups, two words come up repeatedly and they're worth understanding. Technically every "petal" on a dahlia is actually a separate flower called a ray floret (a single dahlia head can contain 200 or more individual florets), but everyone calls them petals, so we will too.

Involute petals curl inwards, closing towards the centre of the flower:

Revolute petals curl outwards, swept back and away from the centre:

The most dramatic cactus dahlias (remember those extra chromosomes) have petals doing both at once:

Once you can spot involute and revolute petals, you can identify nearly any dahlia type at a glance.

Dahlia Size Classes

The NDS also classifies dahlias by bloom diameter. This is independent of flower form, so you can have a giant decorative and a miniature decorative, a large ball and a small ball, and so on. The sizes are:

• Giant: over 25cm (10in)
• Large: 20–25cm (8–10in)
• Medium: 15–20cm (6–8in)
• Small: 11–15cm (4½–6in)
• Miniature: up to 11cm (4½in)
• Pompon: up to 5cm (2in)

You'll also see the word "dinnerplate" used freely. This is not an official NDS classification. It's gardeners' shorthand for any dahlia with blooms bigger than about 20cm (8in), roughly the size of a side plate. Most dinnerplates are decorative types, but the term is applied loosely and you will see it applied to giant cactus types as well.

Decorative Dahlias

Decorative dahlias are fully double, with no visible disc in the centre. The petals are generally wide, either flat or gently twisted, and typically have blunt or rounded tips. These form by far the largest group. Your mental picture of a dahlia is almost certainly that of a decorative.

The group includes everything from miniature varieties with flowers the size of a golf ball to the enormous dinnerplate types that need staking just to hold their heads up. The colour range is equally vast.

We sell more decorative dahlias than any other type. The Cafe au Lait family are the stars, the dahlia that launched a thousand wedding bouquets, but David Howard with his dark bronze foliage and burnt orange flowers is equally striking in a garden border. Or there is Labyrinth, a 2023 RHS AGM winner that is rapidly becoming another bestseller. For cutting, Arbatax is hard to beat.

Browse our full decorative dahlia collection.

Ball Dahlias

Ball dahlias are fully double and form a near-perfect, or sometimes slightly flattened sphere. The petals are rounded at the tips and involute for most of their length, spiralling tightly towards the centre. They're supremely satisfying flowers. There is something about a perfect sphere of colour on a long, straight stem that makes you want to pick it immediately.

If you want to learn about arranging dahlias, take a look at Common Farm Flowers where my very good and hugely talented friend Georgie Newberry does all sorts of things to dahlias. She is bright, funny and a fabulous teacher. Oh yes, and she is an RHS AGM judge.

Balls are typically medium-sized (8–15cm across) and make excellent cut flowers because their compact, tight form holds together well in the vase. They are also popular on the show bench, where judges look for uniform petal placement and a clean, symmetrical outline.

Our ball dahlia collection includes Ariston (a vivid rusty red that exhibition growers love), Wine Eyed Jill (peach with a purple centre, one of the prettiest bicolours we sell), and Boom Boom White which is exactly what the wedding florists order.

Cactus Dahlias

Cactus dahlias are fully double with narrow, pointed petals that are revolute (rolled back) for more than two-thirds of their length. The effect is spiky and dramatic. They look nothing like any other dahlia, and the story of where they came from is equally curious.

The entire cactus group descends from a single plant discovered in Mexico in the 1870s by J.T. Van der Berg, who named it Dahlia juarezii to honour Mexican President Benito Pablo Juárez. Juárez had died two years earlier. In France it was sold as "Les Étoiles du Diable" (Stars of the Devil) because those rolled-back petals were unlike anything European growers had ever seen. Every modern cactus and semi-cactus dahlia traces back to crosses involving that one wild plant. Prolific or what?

We don't currently stock any pure cactus types, they don't like our wet winters, but we do sell several semi-cactus varieties (see below). If you see a cactus dahlia at a show and fall in love with the spidery look, a semi-cactus is the closest thing in our range.

Semi-Cactus Dahlias

Semi-cactus dahlias sit between decoratives and true cactus types. The petals are revolute for between a quarter and two-thirds of their length, wide at the base and narrowing to pointed tips. The result is a flower with more movement and texture than a decorative but less of the full spidery drama of a cactus.

They're popular with exhibitors because a well-grown semi-cactus is genuinely spectacular, and they tend to be good in the rain. Fully revolute cactus petals can act like little funnels and channel water into the flower centre, which causes rot. The broader base of a semi-cactus petal sheds water better.

From our range: Ambition is a rain-resistant magenta-purple that also qualifies as fimbriated (split petal tips). Preference dates from 1911, making it one of the oldest dahlia varieties still in commercial cultivation, and it still has an exceptionally long vase life. Ludwig Helfert is a big tangerine orange exhibition favourite, and Rip City is deep red with a near-black heart. Moody.

Pompon Dahlias

Pompons are the smallest fully double dahlias. They must be under 5cm (2in) in diameter to qualify. The correct NDS spelling is "pompon" (one m), from the French word for the small fluffy ball on a hat, although you'll see "pompom" everywhere including, I am afraid, here.

They are fully spherical with tightly involute petals, like a smaller, even more perfectly formed ball dahlia. The flowers are produced in huge numbers on wiry stems, and because the individual blooms are small, the plants have a lighter, more relaxed look in the border than their bigger relatives. They're also extremely good for cutting. A bunch of mixed pompons in a small vase is charming in a way that a single dinnerplate can't match.

We stock a few, including Nescio (blood red, introduced in 1974, and its name means "I don't know" in Latin, which is a wonderful name for a flower), Small World (milky white flecked with burgundy, RHS AGM since 1998), and Rocco (deep claret, jewel-like).

Browse our pompon dahlia collection.

Waterlily Dahlias

Waterlily dahlias are fully double, wide and shallow, with broad petals that curve gently upwards from a relatively flat centre. The depth of the bloom must be no more than a third of their diameter. Think of the shape of a water lily, or a soup plate, and you've got it.

They have an elegance that other forms lack. Where decoratives and balls are all about abundance and volume, waterlilies are about poise. They look beautiful floating in a shallow bowl of water on a dinner table, which is probably the best possible use for them. I was very young at the time, but I remember my parents celebrating a wedding anniversary (early September) in the garden. We had a little ornamental pond and my mother had floated dozens of Glorie van Heemstede dahlia heads on the pond with lanterns around the edge. Beyond romantic.

Have a look at Karma Prospero (soft lilac-pink with dark stems) or Sam Hopkins (velvety maroon so dark it's almost black). Both are part of the waterlily dahlia collection.

Collarette Dahlias

Collarette dahlias have a single outer ring of large, flat petals surrounding an inner ring (the "collar") of shorter, often differently coloured petals, with a visible central disc. The two-tone effect is striking, and because the centre is open and accessible, collarettes are significantly better for bees and butterflies than fully double types.

We sell Pooh, introduced in 1998 and an RHS AGM holder since 2009. Red outer petals with a yellow collar. It's one of the best dahlias we stock for pollinators. Our children liked to think it was named after a certain bear who also had a close relationship with bees. Maybe.

Peony-Flowered Dahlias (and the Bishop Series)

Peony-flowered dahlias have two or more rings of outer petals around a visible central disc. They're semi-double rather than fully double, which gives them a looser, more open look. It also means pollinators can reach the centre, making them good garden plants for wildlife.

The most famous peony dahlias are the Bishop series, all descended from Bishop of Llandaff, which was bred by Fred Treseder of Cardiff and selected by Bishop Joshua Pritchard Hughes himself from Treseder's seedlings in 1924. It was originally named "Bishop Hughes", but the real Bishop objected, so it became "The Bishop", then "Bishop of Llandaff" when naming rules forbade "The". It holds an RHS AGM (1995) and is one of the most widely grown dahlias in the UK, largely because of its extraordinary near-black foliage.

We also stock Bishop of York (yellow and orange, burgundy stems) and Bishop of Auckland. All the Bishops share that distinctive dark foliage, which makes them look wonderful planted alongside varieties with green leaves.

Single-Flowered Dahlias

Single dahlias have just one outer ring of petals (ray florets) surrounding a flat central disc. They are the simplest flower form, closest to the wild dahlia species, and the most useful for pollinators because bees can access the pollen and nectar easily. If you want to help your local bee population, single and semi-double types are significantly better than the big fluffy doubles, which effectively lock bees out.

We don't currently stock named single dahlia varieties (the Bishops are semi-double rather than true singles), but our mixed colour collections occasionally include single-flowered forms.

Anemone-Flowered Dahlias

Anemone dahlias have one or more outer rings of flat petals surrounding a dense central mass of shorter, tubular florets. There's no visible disc. The cushion-like centre gives them a distinctive look, quite different from any other group.

We don't currently stock anemone types, but they're well worth seeking out at specialist nurseries if the form appeals to you.

Fimbriated Dahlias

Fimbriated dahlias (sometimes called laciniated) have petals with split or fringed tips. The feathery ends give the flower a softer, more intricate look. Fimbriated is technically a separate NDS group, but in practice many fimbriated varieties are also classified under another group. Ambition, for example, is both semi-cactus and fimbriated.

Star, Double Orchid, and Other Types

The NDS classification includes a few more groups that you're less likely to encounter in garden centres. Star dahlias (formerly called single orchid) have a single ring of petals that are all either involute or revolute. Double orchid types are fully double with narrow, twisted petals and no visible disc. Both are relatively uncommon in the UK trade.

New forms continue to appear. The dahlia's remarkable genetic complexity means that breeders are still producing novel flower shapes two centuries after the first European plants flowered. In 1846 the Caledonia Horticultural Society of Edinburgh offered a prize of £2,000 (over £250,000 in today's money) for the first true blue dahlia. Nobody has ever claimed it. Dahlias lack the delphinidin pigment needed for blue, and despite two centuries of trying, that particular flower remains an impossibility.

Which Type of Dahlia Should I Choose?

This depends on what you want them for.

For cutting: Decoratives and balls are the workhorses of the cutting garden. The Karma series of waterlilies were bred specifically for the vase. Pompons are wonderful for small arrangements.

For the border: Decoratives give you the biggest impact. The Bishop series peony types are stunning when massed. Mix heights and forms for the most interesting display.

For showing: Ball, semi-cactus, and decorative types dominate the show bench. Speak to your local dahlia society for current recommendations.

For wildlife: Collarettes, singles, and peony types. Anything where bees can reach the centre of the flower.

For pots: Compact varieties under 90cm tall, whatever the form. See our growing dahlias in pots guide for specific recommendations.

Frequently Asked Questions

What is a dinnerplate dahlia?

"Dinnerplate" is not an official classification. It's gardeners' shorthand for any dahlia with blooms over 20cm (8in) across. Most are decorative types, but the term is used loosely. The Cafe au Lait family, Tartan, and Fairway Spur are all dinnerplates in our range.

What is the difference between a ball and a pompon dahlia?

Size. Both are fully double, spherical, with involute petals. A pompon must be under 5cm (2in) in diameter; anything larger is a ball. Pompons are typically more prolific, producing many more blooms per plant, and the individual flowers are proportionally tighter and more uniform.

What is the difference between a cactus and a semi-cactus dahlia?

The degree of petal roll. A cactus dahlia has petals revolute for more than two-thirds of their length, producing a narrow, spiky look. Semi-cactus petals are revolute for between a quarter and two-thirds, giving them a broader base. Semi-cactus types tend to be slightly more weather-resistant because the wider petals shed rain better.

Which dahlias are best for bees?

Single, collarette, and peony types. Fully double dahlias (decoratives, balls, pompons, cactus) have so many petals that bees struggle to reach the pollen and nectar at the centre. If you want pollinators, choose varieties with an open or visible central disc. Pooh and the Bishop of Llandaff are excellent choices from our range.

Are dahlia petals edible?

Yes. Dahlia petals are edible and make a pretty garnish or salad addition. The wild tubers were a staple food for the Aztecs centuries before Europeans saw the plant, and the hollow stems were used as water pipes. The Aztec name "acocotli" literally means "water cane". Modern ornamental cultivars have been bred for bloom size rather than flavour, so don't expect much from the tubers, but the petals are perfectly safe.

Browse our full range of dahlia tubers, or go straight to a type: decorative, ball, cactus and semi-cactus, pompon, or waterlily.