Dahlias are the show-offs of the late-summer garden: from midsummer until the first frost they produce enormous, flamboyant flowers in every colour except blue, from neat pompons to dinner-plate blooms a foot across. No other flower gives so much drama for so little effort once the tubers are in the ground.
Dahlias grow from tubers, not seed, and they are tender — frost kills the foliage and the tubers must be lifted and stored over winter in cold zones, or left in the ground in mild ones. That extra step puts some gardeners off, but it is simple once you have done it once, and a single tuber bought in spring can be divided into several plants within a year or two.
Dahlias at a glance
| Growing factor | What dahlias need |
|---|---|
| Sun | full sun |
| Soil pH | 6.0–7.0 |
| Days to germination | 7–14 |
| Days to maturity | 60–90 |
| Spacing | 18–24 inches apart |
| Water needs | moderate |
| Cold tolerance | tender |
| Family | Asteraceae |
Starting seeds
Dahlias need a head start indoors — sow under cover and transplant out only after the last frost. Start seeds indoors a few weeks before your last frost date: in Zones 3–5 that means sowing in late winter, in Zones 6–8 early spring, and in Zones 9–10 late winter to very early spring. Zones 11–13 are frost-free, so indoor sowing can happen almost any time, with the cooler months giving the strongest plants; confirm exact timing with your local extension service. Seeds germinate in 7–14 days in warm, moist seed mix. Use clean trays or modules, a fine seed mix, and a bright windowsill or grow lights — leggy seedlings are almost always a light problem, not a seed problem. Harden seedlings off for a week before planting out, moving them outside for lengthening spells each day. Our seed starting calculator works out your indoor sowing date, and the grow light calculator checks whether your setup is bright enough.
Transplanting and planting out
Transplant seedlings out after the last frost, on a cloudy day or in the evening so they do not wilt in the sun. Space plants 18–24 inches apart — that spacing is what keeps air moving and disease down — and water each plant in well. Our plant spacing calculator works out exactly how many plants fit your bed, and the square foot garden planner helps lay them out.
Soil and feeding
Dahlias grow best in fertile, well-drained soil with a pH of 6.0–7.0. Dig in compost before planting — it feeds the soil and helps it hold moisture without getting soggy — and mulch after planting to keep the roots cool and the weeds down. Feed during the growing season with a balanced fertiliser at the label rate, roughly once a month while plants are actively growing, and stop feeding as the season ends. Our fertiliser calculator works out the right amount for your bed, and the compost ratio calculator helps you make good compost to dig in.
Watering
Dahlias have moderate water needs: keep the soil evenly moist while plants are establishing and flowering, and water deeply rather than little and often so the roots grow down. Water at the base in the morning so the foliage dries during the day, mulch to hold moisture in, and watch for wilting in hot, dry spells. Our watering calculator gives a starting point for your bed size, and the watering schedule generator can build a routine around it.
Sunlight and temperature
Dahlias need full sun — at least six hours of direct sun a day. Dahlias are tender — frost kills them or damages them badly, so they are grown as warm-season plants in most zones and planted out only once frosts are over. In frost-free Zones 11–13 they can grow for much of the year. Use the frost date calculator to find your last frost date. Our sunlight calculator can check whether your spot gets enough sun.
Pests and diseases
The pests and diseases to watch for on dahlias are aphids, slugs, thrips. Aphids cluster on shoot tips, slugs eat young shoots as they emerge, and thrips can mark the petals in hot weather. Earwigs also hide in the flowers, which is harmless to the plant but startling in a vase. Check plants weekly, looking under leaves as well as on top, and act early. Hand-pick larger pests, knock aphids off with a strong spray of water, and remove badly affected leaves. Keep the area weed-free and avoid wetting the foliage when you water, since damp leaves invite fungal disease. Our pest identifier helper can help identify what you are seeing, and the plant disease checker helps narrow down leaf and stem problems.
Companion plants
Good companions for dahlias are lavender, marigolds, garlic, chives. Lavender, marigolds, garlic and chives are good companions, and marigolds planted nearby help keep soil pests down around the tubers. Our companion planting finder can suggest more pairings for your bed plan.
Harvesting
Cut dahlias for the vase when the flowers are fully open — unlike many flowers, dahlias do not open further once cut, so a half-open bud will stay half-open. Cut in the morning, put stems straight into deep water, and cut often: regular cutting keeps the plant producing new flowers until frost. At the end of the season, after the first frost blackens the foliage, lift the tubers, let them dry, and store them in a cool, frost-free place over winter. Our harvest date calculator can estimate your picking date from your sowing date, and the vegetable yield estimator helps you work out how much a bed will produce.
Common problems and fixes
| Problem | Likely cause | Fix |
|---|---|---|
| Tubers rot in storage | stored wet or too warm | Dry tubers before storing and keep them cool, dry and frost-free. |
| No flowers, only leaves | too much nitrogen or too much shade | Stop feeding, give full sun, and deadhead regularly. |
| Young shoots eaten | slugs | Protect emerging shoots and plant out sturdy tubers. |
Tubers rot in storage
Dahlias rot in store when they go in wet or are kept warm and damp. Let tubers dry for a day or two after lifting, shake off loose soil, and store them in a box of dry compost or vermiculite in a cool, frost-free place. Check them monthly and remove any that feel soft.
No flowers, only leaves
A dahlia in rich soil and shade grows leaves and few flowers. Dahlias flower best in full sun in ordinary soil, so stop feeding, make sure they get at least six hours of sun, and deadhead spent blooms so the plant keeps producing new ones.
Young shoots eaten
Slugs love the tender shoots that emerge from dahlia tubers in spring, and they can eat a shoot to the ground overnight. Start tubers in pots indoors so the shoots are sturdy before planting out, and protect them for the first few weeks.
Frequently asked questions
How long does it take to grow dahlias?
From planting to maturity takes 60–90 days. Seed germination takes 7–14 days, and the total time depends on whether you start from seed, a nursery plant, or a bulb or cutting.
How far apart should I plant dahlias?
Space dahlias 18–24 inches apart. That spacing gives each plant room for air movement, which keeps disease down, and room to reach its full size without crowding its neighbours.
What soil pH do dahlias need?
Dahlias prefer a soil pH of 6.0–7.0. Test your soil before amending it — most garden soils are close enough, and guessing with lime or sulphur can do more harm than good.
Do I have to lift dahlias for winter?
In cold zones, yes — frost kills the tubers if they stay in frozen ground. In mild zones where the ground does not freeze hard, you can leave them in the ground with a thick mulch. Lifted tubers can be divided in spring to make more plants.
How do I store dahlia tubers?
After the first frost, cut the stems down, lift the tubers, let them dry for a day or two, and store them in a box of dry compost or vermiculite somewhere cool, dry and frost-free. Check them monthly and remove any soft or rotting tubers.