"The dahlia's first duty in life is to flaunt and to swagger and to carry gorgeous blooms well above its leaves, and on no account to hang its head," wrote British horticulturist Gertrude Jekyll in 1899, and it still holds true today.
Nature's drama queen, dahlias deliver brilliant colour and height from midsummer to late autumn – everything from neat pompoms to dinner-plate flowers. But if you want them in your garden this summer, you need to plant tubers right now in May.
Dahlia tubers are different from seeds. They're already packed with next year's growth, so they don't need to germinate – they just need warmth to wake up and start growing.
Why grow dahlias in your summer garden?
Dahlias have surged in popularity in recent years, helped by their sheer photogenic appeal – bold colour, strong shapes and flowers big enough to stop you in your tracks. They come in a huge range of sizes, from small pompom blooms to giant flowers.
Some varieties can grow up to five feet tall or more, making them ideal for adding height and drama to borders. Single-flowered dahlias are also useful for wildlife-friendly gardens, as they attract pollinators more easily than densely packed double blooms.
When to plant dahlia tubers in the UK for best results
The biggest mistake is planting too early.
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Peggy Anne Montgomery, horticulturist at Flowerbulb.eu, suggests: "Order dahlia tubers in early spring, but don't plant them outdoors until the last frost has passed." For most UK gardeners, that's mid-May, when the soil has warmed to around 10–12°C.
Dahlias are heat-loving plants and dislike cold, wet soil. If planted before the soil has warmed, the tubers can sit there doing very little, or worse, rot before they get going.
Frankie Gniwodda, an experienced dahlia farmer who grows thousands of tubers each season, sees it every year. "Planting too early into cold, wet soil — dahlias hate sitting cold. It slows growth or rots the tuber entirely. You're better off planting a bit later into warm soil than rushing and losing weeks of progress."
Gardeners who want to get ahead can start dahlia tubers indoors in pots from March or April, then move them outside once the risk of frost has passed.
Where to plant dahlias for strong growth and more flowers
Dahlias need full sun to perform well. Choose a bright, open spot that gets at least six to eight hours of sunlight a day.
They also need well-draining soil. This is non-negotiable. Dahlia tubers are prone to rotting in soggy ground, so avoid heavy, waterlogged areas. Peggy recommends slightly acidic, well-drained soil – anywhere in the pH 6.0–7.5 range works well.
Frankie keeps her priorities simple: "Warm soil, full sun, and drainage. If those three aren't right, nothing else really saves the season."
Plant them in shade, and you will get weak stems, leggy growth and fewer flowers.
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