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.
Dahlias are such stunning flowers and come in such a range of shapes, sizes and colours, that it is easy to overload your border or cutting bed. So, we always advise that you give a bit of thought to colour and flower size. 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, feed them well, support them, keep them free from pests and cut them often. Do these things and your reward will be found in an almost endless supply of flowers from July to November
Free from frost means just that - dahlias are tender and a light frost will at least cut down the plant while a harder freeze will kill the tuber. So pot up tubers and rooted cuttings and keep in a sunny room/greenhouse/conservatory until after the risk of frost has passed. Then plant them out. Remember to lift the tubers for storage before winter comes.
Plant in good soil, enriched with well-rotted compost, in a sunny spot with good drainage, water them well - dahlias are thirsty beasts - and feed them with diluted tomato feed from June onwards. Taller dahlias need staking well and it is best to do this at planting time. Solid stakes or stout canes and strong twine are needed as these are big, heavy plants. Especially with all those flowers.
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 where your greenfly are...
Keep cutting. For the biggest flowers - for the show bench - pinch off all buds on a stem except for the main one otherwise reduce the number of buds on a stem to three which is perfect for cutting for the house. Otherwise, leave them alone - and enjoy