James Whitaker
Why Dahlias Deserve a Place in Every British Garden
There is a moment in late August when the herbaceous borders begin to flag. The lupins have long gone over, the roses are between flushes, and everything looks a little tired after weeks of heat and unpredictable rain. Then the dahlias step forward. From midsummer until the first proper cold nights of October or even November, they produce flower after flower in colours that range from softest cream to deepest burgundy, often on the same plant. Few garden plants offer such generous returns for such modest effort.
Dahlias originate in the cool mountain regions of Mexico and Central America, which explains why they thrive in our long, mild British summers but sulk at the first hint of frost. Give them a sunny spot, decent soil and consistent moisture, and they will reward you for months. The key is understanding a few simple timings: when to plant, when to pinch, and when to stop.
Choosing Tubers and Getting the Timing Right
Dahlias grow from tuberous roots that look a little like bunches of small, fleshy potatoes. Buy firm tubers with at least one visible "eye" — the small growing shoot that emerges from the crown where the stems meet the tubers. A tuber without an eye will simply rot in the ground, so check carefully before planting.
Timing matters enormously. Dahlias are tender, and a late frost will blacken emerging shoots overnight. In most of the UK, wait until the risk of frost has passed — typically late May in the south, early to mid-June in northern England and Scotland. If you want to steal a march, start tubers in pots of moist compost in a frost-free greenhouse or on a bright windowsill from late March, then plant them out once the soil has warmed.
- Planting depth: about 10–15cm deep, with the eye facing upwards.
- Spacing: 45–60cm apart for bedding varieties, up to 90cm for the big exhibition types.
- Soil: fertile, free-draining, improved with well-rotted manure or garden compost the previous autumn.
- Support: insert a sturdy stake at planting time, so you do not spear the tubers later.
Pinching Out for Bushier, Better Plants
This is the single step that separates a spindly dahlia from a magnificent one. When the main stem has produced three or four pairs of leaves — usually around three to four weeks after planting — pinch out the growing tip with your thumb and forefinger, just above a leaf joint. This sounds brutal, but it prompts the plant to branch from below, giving you a fuller, sturdier specimen with far more flowering stems.
For the very largest blooms, such as the giant decorative and cactus types grown for shows, growers go further: they disbud, removing the smaller side buds to channel the plant's energy into one or two spectacular flowers per stem. For most of us, though, the pleasure is in abundance rather than perfection. Pinch once, feed regularly, and let the plant do the rest.
Repeat the pinching process on the new side shoots if you want an even bushier plant, though a single pinch is usually enough. Tall varieties will need tying in to their stakes every week or two as they grow.
Watering and Feeding Through the Season
Dahlias are thirsty plants with hollow stems that can collapse dramatically if they dry out. Water deeply and consistently — a thorough soak two or three times a week is far better than a daily sprinkle. During hot spells, check container-grown dahlias daily, as they dry out alarmingly fast.
Feed every fortnight from mid-July with a high-potash liquid fertiliser, the sort sold for tomatoes. Too much nitrogen gives you masses of leaf and precious few flowers, so avoid general-purpose feeds once buds appear. A mulch of garden compost around the base helps conserve moisture and keeps the roots cool.
- Deadhead relentlessly. Cut or snap off spent flowers to keep new buds coming.
- Cut for the vase in the cool of the morning, and sear the stem ends in hot water for a few seconds to stop them wilting.
- Watch for slugs when shoots are young, and for earwigs hiding in the petals later on.
Overwintering and Planning for Next Year
The first sharp frost will turn the foliage black almost overnight. This is your signal to act. Cut the stems back to about 15cm, then lift the tubers carefully with a fork, brushing off excess soil without washing it. Leave them upside down for a week or so to drain, then store them in trays of slightly damp compost, vermiculite or even newspaper in a cool, frost-free place such as a shed or garage.
Check them periodically through the winter and remove any that have turned soft or mouldy. In mild areas, you can risk leaving tubers in the ground under a thick mulch of bark or straw, but lifting is far more reliable, particularly on heavy clay soils where winter wet is more dangerous than cold.
Come spring, the cycle begins again: divide the clumps, pot them up, and wait for those long, warm evenings when the dahlias take over the garden. Once you have grown them, it is very hard to imagine a late summer without them.
