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. This yearly removal and replanting process means that, from a garden designer's perspective, Dahlias in practice behave like annual bedding plants.
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, add some grit (not sand) if you want to improve drainage, you don't need to mix compost down into it.
Water Dahlias consistently in dry weather: the bigger the flower, the thirstier the plant. Varieties with small flowers and short stems are more drought resistant than the big showy ones.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. Earwigs also eat huge numbers of greenfly, so we suggest trapping and relocating them. 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, cut off leaves that'll be underwater, as for any plant going in a vase. Holding the stems underwater, make a fresh diagonal cut across the bottom of each stem, keeping the ends underwater - a basin is best for this. When you have trimmed a bunch, quickly put them all directly into a vase with about 6 or 7cm of hot, not boiling, water. As long as you move them quickly from water to water, the stems will not take up air into their vascular systems (phloem and xylem), which blocks them.
Change their water every day (you guessed it: quickly move them from the old vase to the new vase, or a bucket of water and then back to the vase), and add solutes to it. You can buy solutes 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 and clogging up the stem.