With their bold, blowsy blooms in crimson, purple and orange, scarlet and pink, their flamboyance and strong presence are just what you need at the end of the summer, when the garden can look so dull. You'll thank them for looking so good at a time when almost every other plant is giving up the ghost and turning brown.
There are some relatively discreet, single varieties, such as the amazing velvet-petalled scarlet `Bishop of Llandaff' which has a simple arrangement of 10 or 15 petals around a central boss. And there are spectacular and chaotic forms with "cactus" flowers. This is the great thing about dahlias - you get class and brass all in one family.
Dahlias are tender tubers. Their bulb-like roots look like a bunch of salami gathered together on a string. If you plant them out before the frosts are over, or leave the tubers in the ground through the winter, they will die.
Either buy your tubers from a garden centre and plant up in pots, or put them straight into the ground when the frosts are over. But for a cheaper and wider selection, buy rooted cuttings from a specialist catalogue, as I do.
Dahlias are fast growers, so you can be sure that a rooted cutting will always give you a good-sized plant in one growing season. The nursery will send out your mini plants to coincide with the end of the frosts. But if they arrive too early, pot them on and keep them on a window sill until the weather is safe.
Which variety to choose? Cactus dahlias are the most fun, with loads of petals pointing this way and that, like brilliantly coloured giant sea anemones. `Hillcrest Royal' is a purple cactus over 1m high, whose flowers are 20cm across. Plant this next to the similar-sized orange `Classic A1' for a brilliant colour contrast.
Dahlias come in a spectrum of sizes; some reach 90cm to 120cm in height, with flowers the size of a small football, others grow to just 45cm tall, with flowers no bigger than the palm of your hand.
If you have lots of room and want a giant, go for the decorative `Zorro', with its large blooms. This looks spectacular at the centre of a table, one flower floated in a large glass bowl. The large and giantflowered varieties are mainly bred for exhibiting, and are often too top heavy to look good in the garden.
For a small garden, or for growing in pots, you need something on a smaller scale like the orchid-flowered `Jescot Julie', which has a lovely combination of colours. Its thin strappy petals are vermilion on top, with a magenta wash behind.
When you are deciding what to buy, don't order just one of any variety, apart from the giants. If you have the room, dahlias always tend to look better in clumps of three or five, so go for larger numbers of fewer varieties.
To plant them out, dig a hole 30cm wide and 30cm deep for each one, spacing them 60cm to 90cm apart. Fill the hole with compost and give it a good dousing with a full watering can. You will need a stout stake to support each plant, so knock this in now and place the plant by its side. Then give them more water to settle them in.
After about a week, scatter a couple of trowelfuls of "fish, blood and bone" fertiliser around the clump and give them another good soaking. When four pairs of leaves have formed, nip out the tip just above the third pair to give you big, busy plants.
Start tying the plants into their stakes two to three weeks after planting, and try to dead head them every once in a while to keep them looking good and prolong their flowering. You'll end up with an autumn garden that will look as gaudy and glorious as the Ascot Royal Enclosure.
Order dahlias from: Aylett, N Orbital Road (A414) London Colney, St Albans, Herts AL2 1DH Tel 01727 822255; Halls of Heddon. West Heddon Nursery Centre, Heddon-on-the-Wall, Newcastle upon Tyne NE15 OJS Tel 01661 852445Reuse content