Why dahlias earn their place

Few plants work as hard as the dahlia. Planted out in late spring, it will pump out flowers from midsummer right through to the first proper frost of autumn, often into October and beyond in a sheltered southern garden. The more you cut, the more it blooms — a single well-grown plant can produce dozens of stems for the house, and dahlias are among the longest-lasting cut flowers you can grow.

Tubers are also wonderfully good value. Unlike seed, they give you a plant of a known colour and form, they store easily over winter, and each one can be divided into several new plants the following spring. If you only make room for one new thing this year, make it a dahlia.

Choosing healthy tubers

Tubers go on sale from late winter, and the best sell out quickly, so buy early if you have a particular variety in mind. Ordering from a specialist grower gives you more choice than the average garden centre bench, where tubers may have sat about for weeks.

What matters most is the eye — the small dormant bud at the crown, where the tuber joins the old stem. A plump tuber with no eye will never shoot, however healthy it looks. Look for firm, heavy tubers with at least one visible eye and plenty of stored energy in them.

• Decorative and cactus types for big, blousy cut flowers — 'Café au Lait' and 'Arabian Night' are deserved classics.
• Dark-leaved varieties such as 'Bishop of Llandaff' or 'Twyning's After Eight', which look good in the border before they ever flower.
• Pompon and ball dahlias for neat, long-stemmed button flowers that hold their shape in a vase for a fortnight.
• Collarette and single varieties, which are far more attractive to bees and butterflies than the fully double forms.

Anything soft, shrivelled or mouldy should go back. Firm and dry but slightly wrinkly is fine — that is just stored dormancy.

Starting tubers indoors

Dahlias are tender, so give them a head start indoors rather than planting them straight into cold, wet soil. Begin in late March to mid-April, roughly six to eight weeks before you expect to plant out.

Use a pot of two to three litres, filled with multipurpose compost lightened with a couple of handfuls of grit. Lay the tuber horizontally with the eye facing upwards and cover it with two to three centimetres of compost. Water it in once, then keep the compost barely moist until shoots appear — soggy compost is the quickest way to rot a tuber before it gets going.

Stand the pots somewhere bright at around 15–18°C: a cool windowsill, a frost-free greenhouse or a bright porch all work well. Shoots should push through in two to four weeks. From then on, water more freely and turn the pots regularly so the stems grow straight rather than leaning towards the glass.

Hardening off and planting out

Dahlias cannot go outside until all risk of frost has passed — typically late May in the south, early to mid-June further north and in exposed spots. Rushing this is the single most common reason for poor dahlias.

Harden them off properly over seven to ten days. Put the pots outside during the day and bring them back in at night for the first few days, then leave them out overnight once the forecast is reliably mild. Rushed plants sulk, scorch and stall.

Plant into a sunny spot with rich, well-drained soil. Space bedding types 45–60cm apart and larger varieties up to 75cm. Set the tuber 10–15cm deep, with the old stem just below the surface, and firm the soil around it. Push a sturdy stake in at planting time — doing it later risks spearing the tuber. Slugs adore young dahlia shoots, so set traps or barriers the day you plant out.

Pinch out for bushier plants

This is the trick that turns a spindly stem into a flowering machine. Once your plant is roughly 20–25cm tall and has three or four pairs of leaves, pinch out the growing tip just above a leaf pair with your thumb and forefinger.

The plant responds by throwing out side shoots from the leaf joints, so one stem becomes two, then four. Pinching delays flowering by a week or two at most, and you get a far sturdier, better-branched plant with many more blooms. If you are growing for enormous exhibition blooms rather than quantity, do the opposite and disbud the side shoots — but for garden colour and cutting, pinch away.

Feeding, cutting and the run to the frost

Once flower buds appear, feed every fortnight with a high-potash liquid feed — the same sort you would give tomatoes — and keep the plants well watered in dry spells. Mulch around the base to hold moisture. Deadhead and cut flowers regularly, always taking the stem back to a strong pair of leaves; cutting in the cool of the morning gives the longest vase life, and the more you pick, the more the plant produces.

When the first frost blackens the foliage, cut the stems back to about 15cm and lift the tubers. Let them dry upside down for a few days so any moisture drains from the hollow stems, then store them in barely damp compost, vermiculite or newspaper in a cool, frost-free, dark place — a garage or shed is ideal. Label them clearly, check them over once or twice in winter, and you will have your own stock ready to start again next spring.