Dahlias are wonderfully colourful and prolific tuberous plants that fill the late summer garden with a glorious range of flowers that continue until the first frosts. In the garden, you can have a display ranging in size from little pom-poms to huge "dinner-plates up to 25 cms (10") in diameter and in a range of shades from white and cream through yellows and oranges to pinks, reds, lilacs and purples: even near-black. About the only colour you won't find is a good blue. In the house, dahlias are a superb cut flower and the plants produce so many blooms that you can cut daily without impacting the show outdoors.
Dahlias are tender, so they should not be planted out until the frosts are over, around late May. To get a head start they can be potted up earlier and grown on in a light, frost-free place until it is a little warmer.
They are available as tubers (for potting up), as rooted cuttings for planting out in May, or as larger pot-grown plants for instant impact from late June onwards. Give them plenty of organic matter in well-drained soil and full sun.
Involute petals face inwards, to the centre of the flower, and revolute flowers are outward facing.
Single Flowered: One outer ring of florets, which may or may not overlap, and make up a disc in the middle.
Anemone Flowered: One or more outer rings of ray florets (usually flattened) around a mass of tubular florets, with no visible disc in the middle.
Collarette: One outer ring of ray florets (usually flattened) that overlap, and an inner ring, or collar, of small florets, with a visible disc in the middle.
Waterlily: Wide but shallow, fully double, open blooms with wide ray florets. The depth of the bloom is not more than one third of its diameter.
Decorative: Fully double, with no visible disc in the middle. The ray florets are normally wide, and either flat or twisted, and typically have a blunt end.
Ball: Fully double, and form either a near-perfect or flattened ball. The ray florets are rounded at the tips.
Pompon/Pompom: Fully double & spherical, with florets largely involute along their length (longitudinal axis).
Cactus: Fully double, typically with pointy ray florets, most of which are narrow and revolute for over 65% of their length (longitudinal axis) and either straight or involute for the rest.
Semi-Cactus: Fully double, typically with pointy ray florets that are revolute for between 25% and 65% of their length, wide at the base, and either straight or involute for the rest.
Star (formerly Single Orchid): One outer ring of florets around the disc. All the ray florets are either involute or revolute.
Double Orchid: Fully double, no visible disc, with triangular centres. The ray florets are narrow, and all of them are either involute or revolute.
Paeony: Several outer rings of ray florets around a central disc. The florets are flat or slightly involute at the base, and the rest is flat slightly revolute.
Dahlia petals are edible and make a lovely garnish or salad addition. However, the same is not true of the tuber: the wild varieties were a staple food source for Aztecs, but it seems that cultivation has rendered them unsuitable for snacking on.Dahlias are named after the 18th century Swedish botanist Ander Dahl, who also has a snake named after him - Dahl's whip snake, Platyceps najadum dahlii.
Our premium quality Dahlia tubers are Dutch, and then the rooted cuttings and potted plants are grown here in the UK.
Dahlias are simply stunning flowers and come in such a range of shapes, sizes, and colours that it is easy to overload your border. So, narrow your options by thinking first about their purpose. If you primarily want your dahlias for cutting, then pay attention to the stem strength and the way the flower heads are held. If you are looking for the best garden display, check out the varieties with multiple flower heads. For exhibiting, the huge decorative varieties tend to do well.
The golden rules of dahlia growing are to keep them free from frost, plant them in the sun with support, then feed them well, keep them free from pests and either cut them often, or deadhead them early. Do these six things, and your reward will be a constant supply of popping flowers from July to November or so.
Dahlias are tender Central American plants. A light frost will cut down the plant and probably damage the tuber, and a hard freeze will kill it. So, remember to lift the tubers for storage before winter comes, pot them up, and keep them in a sunny room, greenhouse or conservatory until after the last frosts. Plant them out again promptly so that their roots establish well.
Plant in good, thoroughly prepared soil, enriched with well-rotted compost, in a sunny spot with good drainage. Making a low raised bed is a good tactic on heavy clay. Fill it with a mix of compost, leafmould, worm castings and something like coconut coir to retain moisture; break up the clay a bit underneath the bed, but don't try to mix the compost blend down into it - worms will take care of this over time.
Water Dahlias well, they are thirsty beasts, and feed them with diluted tomato feed from June onwards. Taller varieties need solid stakes, bamboo canes will do for the rest, and it is best to drive these in at planting time. Strong garden twine, wrapped around several times, especially the tall, decorative "dinner plate" dahlias, will secure them nicely. Many low plants can help to cover up dahlia legs, lavender loves the same sunny, well drained conditions.
Earwigs are the dahlia grower's number one enemy but as they also eat huge numbers of greenfly, we suggest trapping them rather than using poison. Fill little pots with newspaper, dry grass or straw and put them upside down on bamboo canes amongst your dahlias. Empty the traps wherever you have greenfly, or as a public service, onto roadside Lime trees (Tilia species).
For the biggest flowers for the show bench (or just because size always matters), pinch off all buds on a stem except for the main one. Alternatively, for a balance of bigger flowers and filling up your vases around the house, reduce the number of buds on a stem to two, three or four. If your garden display is the most important thing, then leave them be and just deadhead them as soon as they begin to fade.
Cut flowers for vases in the early morning. Cut horizontally with secateurs right above a set of leaf nodes and side buds. Try to cut flowers when they are 90% open; dahlia buds don't open much after cutting.
When you've brought them inside, hold the stems underwater and make a fresh cut across the bottom, keeping the ends underwater from now on so that no air gets in, and put them directly into a container with about 6 or 7cm of really hot, not boiling, water for at least one hour. Cut off leaves that'll be underwater, as for any plant going in a vase. Keep their base in water at all times to prevent air entering the vascular systems (phloem and xylem).
Change their water every day and add solutes to it: you can buy these ready-made, or make your own florist's brew: a little squirt of lemon juice, a pinch of sugar (ideally fructose), and one drop of bleach. You're coaxing the flower into thinking it's still alive with a shot of energy, while preventing bacteria from taking advantage of the situation. By the time the flower runs its natural course, it is in a low level state of embalming, with no bacteria inside its vascular systems, preserving its appeal a little while longer.