Why dahlias are made for small gardens

Dahlias have a reputation for being big, blowsy and greedy for space, but that is only half the story. The range of compact and dwarf cultivars now available is enormous, and most of them perform beautifully in containers. A single well-grown plant in a 35 cm pot will give you flowers from July right through to the first frosts, and if you keep picking them, it will keep producing. For anyone gardening on a patio, a balcony or a small courtyard, dahlias are one of the most generous plants you can grow.

The other advantage is flexibility. Pots can be moved to follow the sun, tucked into gaps as other plants finish, or shifted under cover when autumn rain threatens to spoil the blooms. You also get total control over the compost, which matters because dahlias are hungry feeders and do best in rich, free-draining soil.

Choosing compact varieties for pots

Height is the first thing to check. A dahlia sold as a bedding plant or patio type will generally top out at 45–75 cm, which is manageable in a container. Taller decorative and cactus types will grow perfectly well in pots, but they need serious staking and are top-heavy in anything other than still air.

Reliable choices for UK containers include:

• Gallery series – short, floriferous and available in a wide range of colours, usually reaching around 50 cm.
• Melody series – bred for pots, compact and early to flower.
• Topmix and Dahlinova types – single-flowered bedding dahlias that are excellent in smaller pots and very good for pollinators.
• Pompon and ball dahlias – small blooms, sturdy stems, and they hold up well in wind.
• Dark-leaved cultivars – grown as much for their near-black foliage as their flowers, and superb against pale walls.

If you are growing from tubers rather than plug plants, buy named varieties so you know what height to expect, and choose three tubers for a large pot or one per 30 cm pot.

Pot size, compost and drainage

Dahlias are thirsty, tuberous plants and they resent being cramped. Use a pot at least 30 cm across for dwarf types and 40–45 cm for anything taller. Terracotta looks lovely but dries out fast; a good-quality plastic or glazed pot holds moisture better and is lighter to move.

For compost, mix a quality peat-free multipurpose with about a quarter by volume of well-rotted garden compost or soil-based John Innes No. 2. Add a small handful of grit for drainage and a slow-release fertiliser at planting. Always place crocks or a layer of gravel over the drainage holes, and stand pots on feet so water can escape freely. Soggy compost is the fastest way to rot a tuber.

Planting up and early pinching

Tubers can be started indoors from late March in a bright, frost-free spot, then moved outside once all risk of frost has passed — usually late May or early June in most of the UK. Plant the tuber horizontally, with the eye or old stem pointing upwards, about 5 cm below the surface.

Do not water heavily until you see growth, or the tuber may rot. Once shoots appear, water regularly and begin feeding. When plants have four or five pairs of leaves, pinch out the growing tip with your fingernails or snips. This sounds brutal but it forces branching, and you will end up with a bushier plant carrying far more flowers.

Push a sturdy cane into the pot at planting time rather than later, when you risk spearing the tuber. Tie stems in with soft twine as they grow, loosening the ties as the stems thicken.

Watering, feeding and deadheading

In high summer, a container dahlia may need watering once or even twice a day. Water in the morning where possible, aiming at the compost rather than the foliage, and give enough that it runs out of the bottom. Irregular watering leads to split tubers and weak growth.

Feed with a high-potash liquid fertiliser — tomato feed is ideal — every week from the moment buds appear until mid-September. A fortnightly dose of a general-purpose feed early in the season supports leafy growth.

Deadhead relentlessly. Cut stems back to a strong pair of leaves, and take a few longer stems for the vase; the more you cut, the more the plant produces. Remove any flowers that are going over, as they shut down further budding.

Watch for slugs when shoots are young, aphids on soft tips, and red spider mite in hot, dry greenhouses. Earwigs can nibble petals — a small upturned pot stuffed with straw on a cane makes a useful trap.

Overwintering and next season

Dahlias are not hardy in most of the UK, so plan for winter. After the first frost blackens the foliage, cut stems back to about 10 cm. You can either leave the tuber in its pot, moved to a frost-free shed or garage and kept barely moist, or lift it, brush off the compost, dry it upside down for a week and store it in a box of dry compost or vermiculite at around 5–10°C.

Check stored tubers monthly for shrivelling or mould. Come spring, divide large clumps with a sharp knife, making sure each division has an eye, and start the cycle again.