How To Start Dahlia Tubers In Spring

Starting Dahlia Tubers in Spring: Is It Worth the Effort?

Every dahlia article on the internet tells you to pot up your tubers indoors in March for a head start. Give them warmth and light, the reasoning goes, and by the time you plant them out in May they will already have roots and shoots and will hit the ground running. Flowers weeks earlier. Bigger plants. What's not to love?

The method works. We should say that upfront. You will get flowers earlier, perhaps two or three weeks earlier, if you start your dahlia tubers indoors. Whether that matters depends on what you want from your garden and how much greenhouse space you have.

We will be honest. We don't bother at Ashridge. Most gardens are at their best in June and July. By mid-July a dahlia planted directly outdoors in May will be flowering hard, and it will keep going until the frost. The three-week head start from indoor potting is nice but not transformative. And it means finding space in a greenhouse for a lot of pots at exactly the time of year when the greenhouse is already full of seedlings.

That said, there are situations where starting indoors makes genuine sense. This guide covers the method in detail, and then tells you honestly when it's worth doing and when you can skip it.

When to Start Dahlia Tubers Indoors

Start potting up tubers from mid-March to early April. This gives them four to six weeks of indoor growing time before you plant them out after the last frost, which is mid to late May in most of the UK.

Don't start too early. Tubers potted up in February produce leggy, etiolated shoots stretching for light they can't get in a British winter. They outgrow their pots before the soil outside is warm enough to receive them. March is early enough. April is fine.

We deliver your dahlia tubers from mid-March onwards. If yours arrive before you are ready to pot them up, keep them in their packaging in a cool, dark place. A shed or garage is fine. They will wait patiently for a couple of weeks.

How to Pot Up Dahlia Tubers

This is our own video showing exactly how to do it. Cover the top of the tuber with only about 1cm of compost so the eyes can reach the light immediately. If you can see the eyes bulging ready to sprout, try to get them right at soil level.

Use a 2–3 litre pot. Anything bigger is a waste of space at this stage. The tuber is small, the roots are non-existent, and it only needs to live in this pot for a few weeks before going outside.

Fill the pot with ordinary multi-purpose compost. Nothing fancy. The tuber has its own stored energy and doesn't need rich compost yet. Push the tuber into the compost with the crown facing up, covered by about 3–5cm of compost. If you are not sure which end is the crown, our tuber anatomy guide will help.

Water once, lightly. The compost should be damp, not sodden. Then put the pot somewhere frost-free, warm, and light. A heated greenhouse is ideal. An unheated greenhouse or conservatory works if nights are above freezing. A bright windowsill in the house will do.

Now wait. Don't overwater. The most common mistake at this stage is enthusiasm with the watering can. The tuber is dormant. It has no roots yet and no way to use the water. Waterlogged compost rots tubers faster than anything else. Water only when the surface of the compost is dry to the touch. A light splash, not a soak.

Within two to three weeks, you should see shoots emerging from the crown. Once the shoots are up, the tuber is rooting. You can increase watering slightly. Keep the compost evenly moist, not wet.

Growing On Your Started Tubers

Once the shoots are 10–15cm tall, you have a decision. Pinch out the growing tip to encourage bushier growth (this delays flowering by a week or so, but produces more stems and more flowers in the long run), or leave it to grow tall for the earliest possible bloom.

Keep the pot in good light and turn it regularly so the shoots grow straight rather than leaning towards the window. If the shoots are pale and stretching, they need more light. Move the pot somewhere brighter or accept that you started too early.

Don't feed at this stage. The tuber's stored energy and the nutrients in the compost are plenty. Feeding now encourages soft, lush growth that won't cope well when you plant it outside.

If your started tubers are growing well and the weather outside is still too cold for planting, pot on into a 5-litre pot if the roots are filling the original one. Don't let them become rootbound. A rootbound dahlia that then goes into open ground takes time to readjust, and you lose the head start you worked for.

If you're growing your dahlias in pots rather than beds, there are real advantages to starting small and potting up progressively rather than planting straight into the final container. It saves space and water early on, prevents overwatering problems, and gets the best out of your compost because you're not washing nutrients through soil that has no roots in it yet.

Hardening Off Dahlia Plants

Hardening off is the process of gradually acclimatising indoor-grown plants to outdoor conditions. Skip this step and your pampered greenhouse dahlia will get a shock when it hits the cold wind and strong sun of an open border. Leaves scorch, growth stalls, and the plant sulks for a week.

Start hardening off about ten days before you plan to plant out. Move the pots outside during the day and bring them back in at night. After three or four days, leave them out overnight as well, provided there's no frost forecast. After a week to ten days, they are ready to go in the ground.

If you don't have the patience for this and the weather is settled, you can cheat. Plant out on a mild, overcast day. The worst shock comes from strong sun and wind on soft leaves, not from cool nights. An overcast, still day gives the plant time to adjust without being battered. Water in well and keep an eye on the forecast for the first week.

When Starting Dahlia Tubers Indoors Is Worth It

Starting indoors makes the most difference in specific situations. If none of these apply to you, plant directly outdoors in May and save yourself the trouble.

Short growing season. In northern England, Scotland, and colder inland areas where you can't plant out until late May or even June, starting indoors in April genuinely extends the flowering season. Without a head start, your dahlias may not flower until August, giving you just two months before frost. With one, you could be picking from mid-July.

Container growing. If you are growing dahlias in pots on a patio, starting them indoors and planting into their final containers in May means the pot is full and flowering by late June instead of late July. More value from a smaller growing space. See our dahlias in pots guide for the full container method.

Show dahlias. If you exhibit at dahlia shows, you need blooms at specific dates. Starting indoors gives you finer control over timing. Exhibition growers often start in February for early shows, but this is specialist territory.

Late delivery. If your tubers arrive in April or May and you want to give them a few weeks of root growth before planting out, a quick spell in a pot is sensible.

For everyone else, we would say this: plant your tubers directly in the ground in May, about 15cm deep with at least 15cm of soil covering the top of the crown. By the time the shoot reaches the surface, which will be late May, the risk of a late frost in the south is tiny. The dahlia gets its head start underground, and you get your greenhouse space back for tomatoes.

Frequently Asked Questions

Is it too late to start dahlia tubers in April?

Not at all. April is a perfectly good time. The tubers only need three to four weeks indoors before planting out in May. Starting in April actually avoids the leggy growth problem that comes from starting too early in February or March, when light levels are still low.

Can I start dahlia tubers on a windowsill?

Yes, a bright windowsill works fine. Turn the pot every couple of days so the shoots grow straight rather than leaning towards the light. A south-facing window is best. Avoid windowsills above radiators, which dry out the compost fast and bake the roots.

Do I need a greenhouse to start dahlias early?

No. A greenhouse is convenient but not essential. A conservatory, bright porch, cold frame, or sunny windowsill all work. The tuber needs frost-free conditions, some warmth to trigger growth, and decent light. It does not need controlled greenhouse conditions.

What if my dahlia tuber doesn't sprout?

Give it time. Some varieties are slow starters and may take three to four weeks in warmth before the eyes swell. Check that the tuber is firm. If it feels solid, be patient. If it feels soft or spongy, it may have rotted. Cut it in half. Healthy flesh is cream or white. Brown or black means the tuber is dead. See our tuber anatomy guide for help identifying healthy eyes and crowns.

Should I start all my dahlias indoors?

Only if you want to and have the space. We don't start ours indoors at Ashridge, and our dahlias flower perfectly well from July through to November. Starting indoors gives you an earlier start, perhaps two to three weeks, but it is not essential and not worth the trouble if greenhouse space is tight. For most gardeners in the south, planting tubers deep directly into the ground in late April or May works well. Full planting method in our how to grow dahlias guide.

Browse our full range of dahlia tubers, delivered from mid-March for spring planting. We stock over 80 varieties, all supplied as A-grade Dutch-grown tubers that we double-check before dispatch.