Dahlias are grown for their long season of flowers, which can run from summer into autumn when many other border plants are fading. They come in everything from tidy bedding varieties to towering plants with enormous blooms. For UK gardeners the central challenge isn't persuading dahlias to flower: it's protecting these tender plants from late spring frosts and deciding what to do with their tubers in winter.
Once their growing conditions are right, dahlias are generous plants. They need sunshine, fertile soil, water during dry spells and attention to fading flowers. The first decision is whether you'll grow them from stored tubers, buy established plants or raise them from seed.
Understand the different types of dahlias
A dahlia sold as a tuber is a dormant collection of fleshy underground storage organs attached to a crown. The buds that produce stems develop at the crown, close to the remains of last year's stems. A loose tuber without any viable bud-bearing crown may never produce a shoot.
Named varieties grown from tubers normally repeat the flower characteristics of the original plant. Seed-grown dahlias can vary in colour and shape, making them useful for inexpensive displays but less predictable if you're trying to reproduce a particular cultivar.
Choose a plant for the space available. Tall decorative or cactus-flowered varieties may require strong stakes. Smaller bedding dahlias work well in containers. Consider flower size, eventual height, colour and how the flowers will fit among neighbouring plants rather than buying solely from a photograph of one perfect bloom.
When to start dahlia tubers in the UK
Dahlias are tender perennials, so frost can damage young growth. Starting tubers indoors or in a frost-free greenhouse in spring gives them a head start before they go into borders.
Set a dormant tuber cluster in a pot of peat-free multipurpose compost with the crown near the surface and the old stem pointing upwards. The long fleshy tubers sit below. You don't need to bury the whole crown deeply. Water modestly at first: a dormant tuber in cold, wet compost can rot before its shoots have started to grow.
Place the pot in good light somewhere frost-free. As shoots appear and leaves develop, increase watering gradually to match the plant's growth. Avoid forcing a young dahlia into very warm, dim conditions, which often produces stretched stems.
A tuber that seems slow to start is not necessarily dead. Check that it remains firm and that the compost isn't sodden. If it's soft and decaying, however, saving it may be impossible.
Planting dahlias outside
Choose a sunny, sheltered position with fertile soil that drains well. Dahlias appreciate moisture during active growth, but tubers suffer in persistently waterlogged ground. Work garden compost into poor soil before planting if necessary.
Do not move tender, leafy plants straight from a greenhouse into a cold, windy garden. Harden them off over several days by increasing outdoor exposure gradually. Wait until the local risk of frost has passed before planting fully grown tender dahlias outdoors.
Dig a hole large enough to accommodate the rootball without squashing it. Plant at the previous growing depth, firm the soil gently and water thoroughly. Allow enough space for the mature plant; the RHS suggests at least around 60cm between many border varieties, with more room for large ones.
Install stakes for tall dahlias at planting time. Driving a stake into the ground after a large root system has formed risks damaging underground tubers. Use soft ties, placed loosely enough that the stems can expand.
Can you plant dormant tubers directly into the ground?
Yes, provided the ground is workable and the tubers won't be exposed to damaging freezing conditions. The RHS describes direct planting in mid to late spring, with the crown positioned just below soil level. Direct-planted tubers usually flower later than ones started under cover.
This method suits gardeners who lack indoor space, although outdoor soil can stay cool and wet. A poorly drained bed is especially risky. Mark the planting position so you don't accidentally dig into a slow-emerging tuber while weeding.
Be patient when the first shoots appear. Fresh growth is attractive to slugs and snails. Check plants frequently in the evening, protect vulnerable shoots and avoid disturbing the emerging stems with a hoe.
How to grow dahlias in pots
Pots suit smaller varieties and can display flowers close to a patio or doorway. Choose a generously sized container with drainage holes. The RHS suggests a pot around 50cm wide for a standard dahlia, although compact bedding varieties can be grouped in a container of that size.
Use peat-free multipurpose compost and allow enough room above the surface for watering without spilling compost. Water well after planting. During hot summer weather, containers may dry much faster than border soil, particularly once the dahlia has become large and leafy.
Raise containers off surfaces where drainage holes might become blocked and keep them in a sunny position. A heavy or wide pot can be safer than a tall, narrow one for an upright plant carrying many flowers. Stake tall varieties even in containers.
Watering and feeding dahlias
Established dahlias need water during prolonged dry spells, and plants in pots may need it very frequently in warm weather. Water the root area deeply rather than sprinkling leaves lightly. The aim is to moisten the compost or soil without leaving roots permanently saturated.
Mulch around border plants with garden compost to reduce moisture loss and weeds, but leave a little space around the stem. Where soil fertility is poor or plants are growing in containers, a suitable feed can support flowering. Follow the manufacturer's rate; excessive fertiliser is not a substitute for light, moisture and root space.
If a dahlia wilts on a very hot afternoon, check the soil before assuming it is thirsty. A pot that is already wet may have root or drainage trouble. Repeatedly flooding saturated compost will not help. Equally, a completely dry container may need watering slowly and thoroughly so water penetrates the rootball.
Pinching, staking and deadheading
For a bushier plant with more branching stems, you can pinch out the leading growing point when a young plant is sufficiently established. This encourages side shoots. Don't repeatedly pinch a plant that you want to flower as early as possible; each cut changes its growth pattern.
Tie tall stems to supports as they lengthen and check the ties after windy weather. Dahlia shoots can be surprisingly brittle. A strong stem is still vulnerable if heavy rain makes its flower heads flop.
Deadheading is one of the most valuable summer jobs. Follow a faded flower stem back to a suitable leaf joint and cut cleanly. This keeps the plant looking tidy and encourages it to put resources into more blooms rather than developing seed. Learn to distinguish a rounded flower bud from a fading flower head before snipping indiscriminately.
Growing dahlias for cutting
If you want flowers for a vase, grow several plants rather than removing nearly every opening bloom from one. Cut in the cooler part of the day, use clean secateurs and place stems into water promptly.
Choose blooms at a suitable stage for the variety. Dahlias generally don't continue opening substantially once cut from very tight buds, so pick flowers that are already open but still fresh. Remove lower foliage that would sit below the waterline and change the water regularly to slow deterioration.
Cutting flowers is effectively another form of deadheading, provided enough leaves remain for continued growth.
What to do after the first frost
A frost may blacken dahlia leaves and stems. In much of the UK, the safest way to preserve named tubers is to lift them and store them somewhere frost-free over winter.
Cut back the dead top growth, carefully loosen the soil around the root system and lift the tuber cluster without breaking its necks. Let surplus soil dry sufficiently to remove it gently. Discard damaged, rotten tissue and label the clump with its variety before the name is forgotten.
Store tubers somewhere cool, dry enough to prevent rot but not so desiccating that they shrivel completely, and protected from freezing. Check them periodically through winter. Exact packing methods vary with the storage space and local humidity; ventilated trays or suitable dry packing material may be used.
Some gardeners in mild, well-drained gardens leave tubers in the ground beneath insulating mulch. That carries a real risk in a wet or severe winter. If a variety is valuable to you, lifting it is the more cautious choice.
Troubleshooting poor dahlia growth
Leaves damaged after a cold night: Protect from frost and delay outdoor planting next season. Damaged stems may recover if the crown survives.
Lots of foliage but few flowers: Review sunlight, excessive nitrogen feeding, overcrowding and whether regular pinching has delayed flowering.
Buds collapsing or flower heads marked: Examine for slugs, earwigs and other pests. Some damage is cosmetic; remove spoiled blooms and maintain garden hygiene.
Tubers rotting in storage: Check for excessive moisture, insufficient ventilation or frost damage. Remove diseased material before it affects neighbouring clumps.
Tall plants leaning: Add or improve support, but take care not to drive stakes through the underground tubers late in the season.
A successful dahlia season begins with healthy tubers and sensible timing. Keep tender growth away from frost, give each plant light and room, water thoughtfully and remove spent flowers. You'll have a far better display than you would by chasing individual problems with more feed or repeated cutting.
Source
Royal Horticultural Society, How to Grow Dahlias.