At a glance
- Summer tubers like dahlia, gladiolus and lily are frost-sensitive. They go into the ground only when no more night frosts threaten, usually from weeks 15 to 20.
- A safe marker for dahlias is the time after the Ice Saints, that is mid-May. Started in a pot beforehand, you gain a few weeks' head start.
- Set the stake right when planting, not once the plant is tall. Otherwise you later ram the stake straight through the tuber.
- A sunny spot and a free-draining, not too wet soil are decisive. In waterlogging the tubers rot before they even sprout.
Dahlias and gladioli bring colour into the bed from high summer to frost, when other perennials already fade. Their secret: they grow not from hardy roots but from tubers and corms that spend winter frost-free in the cellar. That is why they go into the ground only in spring, once the frost risk is over.
The most common mistake is impatience: planted too early, a late frost catches the young shoots. The second biggest is the forgotten stake. This article shows you how to set summer tubers correctly and stake them from the start.
Only once the frost is over
Summer tubers tolerate no frost, neither at the tuber nor at the young shoot. That is why timing is decisive. Gladioli and lilies may go into the soil from weeks 15 to 20, once it has warmed. With the especially sensitive dahlias you better wait until after the Ice Saints in mid-May.
Whoever wants a head start raises dahlias in a pot on the windowsill from April and plants the sprouted tuber out after the frosts. That way it blooms several weeks earlier. You may also set the tubers themselves earlier, as long as they lie deep enough that the shoot reaches the surface only after the frosts.
The stake comes first
The second typical mistake happens months later and is doubly annoying: the dahlia is head-high, full of flowers, a summer storm topples it, and while re-staking you ram the stake straight through the tuber. So the iron rule is: set the stake when planting, not afterwards.
Push the stake right beside the tuber into the planting hole before you fill it in. Then you know exactly where the tuber lies and do not injure it. Tie tall dahlias and gladioli loosely over the course of summer, then they withstand wind and rain too.
The most important summer tubers at a glance
- Dahlia
The star among summer tubers. Blooms from week 28 to frost in all colours and forms. Especially frost-sensitive, so plant out only in mid-May.
- Gladiolus
Brings vertical flower spikes into the bed. Whoever plants several times at two-week intervals has supply for the vase all summer long.
- Lily
Grows from a scaly bulb set a little deeper. Many lilies are moderately hardy and may stay in the mild bed.
- Begonia and canna
Tuberous begonias and canna lily love it warm and shady to part-shady. They too go outdoors only after the last frosts.
Plant in stages for a long bloom
Do not plant gladioli all at once, but in several batches at two-week intervals. That way they bloom one after another instead of all at the same time, and you have fresh flowers for the bouquet from July to September. With dahlias, regular deadheading of spent heads clearly extends the flowering time.
Into the ground only once no more frost threatens, and the stake straight into the planting hole. Whoever heeds both harvests colour for the bed and the vase from July to frost.
The core rule for summer tubers
Frequently asked questions
When do you plant dahlias and gladioli?
Only once no more night frosts threaten. Gladioli from weeks 15 to 20, once the soil is warm. The frost-sensitive dahlias better only after the Ice Saints in mid-May. Started in a pot, you gain a few weeks' head start.
How deep do you set dahlia tubers?
About five to ten centimetres deep, so the tuber is well covered with soil and the old stem base points up. Gladioli go about ten centimetres deep, lilies a little deeper.
Why do my dahlias flop over?
Because the stake is missing or came too late. Tall dahlias grow head-high and heavy and snap in wind and rain. Set the stake beside the tuber already when planting and tie the plant loosely in summer.
Can I start summer tubers in a pot?
Yes, that is especially worthwhile with dahlias. Set the tuber in a pot of soil from April, keep it bright and frost-free and plant the sprouted tuber out after the last frosts. That way it blooms several weeks earlier.
Which spot suits summer tubers?
Most want it sunny and warm with free-draining soil. Waterlogging is their greatest enemy because the tubers rot in it. Begonias and canna, by contrast, tolerate part-shade too.
Sources
- Bavarian State Institute for Viticulture and Horticulture (LWG): dahlias and summer flowering bulbs
- Julius Kühn Institute: tuber- and corm-forming summer flowers
- Related in the magazine: Overwintering dahlias and gladioli and Planting spring bulbs in autumn
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