How do I get more flowers from my dahlias, for longer?

Pinching out the growing tip, telling a spent flower from a bud so you deadhead the right thing, and the feeding and staking that carry a dahlia from July to the first frost.

Difficulty: beginner
Time: 30 min
Read: 4 min

Short answer

Pinch the growing tip out when the plant has three or four pairs of leaves, which multiplies the number of flowering stems. Then deadhead relentlessly, cutting the whole spent stem back to a leaf joint rather than snapping the head off, feed with a high-potash liquid from July, and keep it watered. It will then flower until the frost stops it.

Dahlias will flower from midsummer until the first frost blackens them, which in a mild autumn can be November. Whether they do depends almost entirely on two habits: pinching out early, and deadheading properly. The plant's aim is to set seed; every spent flower left on it is a message that the job is done.

What you'll need

Secateurs
Stakes or a ring support
Soft ties
High-potash liquid feed
Slug protection for young growth

Step by step

Pinch out the growing tip when the plant has three or four pairs of leaves.Take the central shoot out with finger and thumb, just above a leaf pair. The plant then produces side shoots from the joints below instead of one main stem, which typically doubles or trebles the number of flowering stems.
Stake before the plant needs it.Dahlias become large, heavy and brittle, and a stem snapped in a July storm cannot be repaired. Put stakes or a ring support in when the plant is small and tie in as it grows.
Learn to tell a spent flower from a bud.This trips people up constantly. A bud is round and firm. A spent flower after the petals drop is pointed and conical, and feels softer. Deadheading the wrong one removes flowers you were about to get.
Deadhead by cutting the whole stem back to a leaf joint.Do not just snap the head off, which leaves a bare stalk that dies back. Follow the flower stem down to the first pair of leaves or a side shoot and cut there. New flowering shoots come from that point.
Deadhead often — every few days in peak season.This is the single biggest factor in how long a dahlia flowers. A plant allowed to set seed slows down and stops.
Feed with high potash from July.A liquid tomato feed weekly or fortnightly from midsummer. Early in the season a balanced feed builds the plant; from July potash is what drives flowering. Too much nitrogen gives a large leafy plant with few flowers.
Water generously and consistently.Dahlias are thirsty, particularly in pots and in dry summers. Irregular watering produces smaller flowers and stress. A thorough soak two or three times a week beats a daily sprinkle.
Protect young growth from slugs and snails.Emerging dahlia shoots are among the most attractive things in a spring garden to slugs, and a tuber can be eaten to nothing before it gets going.
Deal with earwigs if the flowers are being shredded.Ragged holes in petals overnight usually means earwigs. The traditional trap — an upturned pot loosely stuffed with straw on a cane among the plants, emptied each morning — works and costs nothing. Earwigs are also useful aphid predators, so relocate rather than destroy them.
Disbud if you want fewer, larger blooms.Each flower bud usually has two smaller buds beside it. Removing those two gives a bigger flower on a longer stem, which matters for cutting and showing. Leave them for a greater number of smaller flowers.
Keep going until the frost.Do not stop deadheading and feeding in September. Dahlias give some of their best flowers in October in a mild autumn, and the plant only stops when the frost blackens it.

Tips

Dahlias are excellent cut flowers and cutting them is deadheading. The more you cut, the more the plant produces.
Single-flowered and collarette dahlias have accessible pollen and are genuinely useful to bees; the big decorative and ball types are not. Worth knowing if pollinators matter to you.
A dahlia in a pot needs a big one — at least 30 to 40 litres — plus daily watering in summer and regular feeding.
Blackened foliage after the first frost is the signal to cut the stems down and either lift the tubers or mulch them where they are.

Common mistakes

Not pinching out the tip early — You get one tall stem with a handful of flowers instead of a bushy plant with many. It costs nothing and makes a large difference.
Deadheading the buds by mistake — Spent flowers are pointed and conical; buds are round and firm. Check before cutting.
Snapping the flower head off and leaving the stalk — The bare stalk dies back and no new shoots come from it. Cut back to a leaf joint.
Feeding with a general or high-nitrogen fertiliser all season — It produces a large leafy plant with fewer flowers. Switch to high potash from July.
Stopping deadheading in September — Dahlias often produce their best flowers in October. Keep going until the frost.

Questions people ask

When should I pinch out a dahlia?

When it has three or four pairs of leaves, usually in late spring. Take out the central growing tip just above a leaf pair.

How do I tell a dead flower from a bud?

Buds are round and firm; spent flowers are pointed, conical and softer. It is the commonest deadheading mistake there is.

Why does my dahlia have lots of leaves and few flowers?

Usually too much nitrogen, sometimes too little sun. Switch to a high-potash feed from July and make sure it has at least six hours of direct sun.

Something is eating holes in my dahlia flowers overnight.

Almost certainly earwigs. Trap them in an upturned pot stuffed with straw on a cane and move them elsewhere — they eat aphids, so they are worth relocating rather than killing.