Growing dahlias extends the season with big, bold flowers that fill the backyard with color. Most are tall, adding height at the back of the border, and the single-flowered cultivars provide nectar for bees and butterflies. On top of that, dahlias have a cut-and-come-again habit, so they supply beautiful, fresh flowers for the house. Our guide has the details on when to plant dahlia to get the most from these colorful, exotic flowers.
When to plant dahlias
Hailing from Central America, dahlias shouldn’t be planted outside until the weather warms. If you have stored or bought tubers the answer to when to plant dahlia tubers is to pot them up under cover (for example, in a greenhouse) in March or April. Rooted dahlia cuttings are also best started off under cover in mid to late spring. And potted dahlias should go in the ground after risk of frost has passed, which is May or June in most regions. Dahlias are available in such a vast range of colors and shapes that you can select the ones that best suit your style of garden. For instance, the cactus and ball varieties bring retro glamour; the single and anemone forms fit into jungle-style flower bed ideas; and the huge dinner-plate dahlias are a must-have for a cutting garden. ‘I like to intersperse dahlias among other plants,’ says Oregon-based landscape architect and garden designer Bethany Rydmark (opens in new tab). ‘For example, dahlias work well in modern meadows, as accents in a mixed border, or as infill in a parterre.’
When to plant dahlia tubers indoors
Dahlia tubers that you have stored over winter or bought can be potted up under cover (for example, in a greenhouse) in March or April, and then planted outside in May or June. ‘Planting tubers will often give you earlier flowers,’ say Dave and Sue Hall at specialist dahlia nursery Halls of Heddon (opens in new tab), near Newcastle upon Tyne, UK, ‘because more often than not they will produce multiple shoots which will not need pinching out.’ It is best to harden off the young plants to acclimatize them to outdoor conditions before planting. For example, by placing them in a cold frame or by sitting them outdoors during the day for a week covered with fleece, then gradually remove the fleece.
When to plant dahlia tubers outdoors
What about when to plant dahlia tubers outdoors? ‘Dahlia tubers can be safely planted out between April and May,’ say Dave and Sue Hall. ‘The warmer the climate, the earlier you can plant. So the further south you live in the UK, the earlier you can plant.’ In most parts of the UK and the northern US, it is safe to plant dahlia tubers direct outside from mid to late April. Usually, the tubers will set about growing their roots during late spring and begin sending up new shoots in early summer. However, sometimes they may shoot early in warm spring weather. Don’t worry because this won’t damage the plant in the long run. ‘Growth that emerges too early may be nipped back by spring frosts,’ say Dave and Sue. ‘The plants will recover and it should result in bushier plants earlier on, as they will have had a natural pinch.’
When to plant rooted dahlia cuttings
Mini plants, known as ‘rooted cuttings’, are readily available from dahlia suppliers in mid to late spring. They are often more reliable than tubers in establishing robust plants faster and can produce a better flower color. If possible, plant them up under cover (for example, in a greenhouse) during spring. ‘Dahlia rooted cuttings can be planted directly outside, but they will certainly benefit from being potted up and grown on for at least a few weeks before planting out,’ say Dave and Sue Hall. ‘In the south of the UK, planting out can be done from early to mid May. In the north and Scotland, you would not normally plant out until June. We would not advise planting out before the date of the last expected frost in your area.’ Harden off the young plants before planting out, in order to prepare them for the weather outside. For example, by placing them in a cold frame or by sitting them outdoors during the day for a week covered with fleece, then gradually remove the fleece.
When to plant potted dahlias
In late spring and early summer, ready-grown dahlias in pots can be bought from nurseries. As for when to plant dahlia like these? They can be put directly into sunny borders once all risk of frost has passed. ‘Only buy fresh, strongly growing plants that have not become pot bound,’ advise Dave and Sue Hall. ‘Being pot bound will put a check on their growth and they will be slower to get away.’ To check if a plant is pot bound, peer underneath to see what the roots look like. If they are old and in a dense knot under the plastic container or – if you are able to slide the plant out of its container – should they be growing around and around the compost, avoid purchase.
When to plant dahlias in pots
If you want to grow dahlias in pots to brighten up the patio or terrace, plant the tubers into well-drained containers of peat-free compost under cover (for example, in a greenhouse) in March or April. Then, once risk of frost has passed, move the pots outside into a sunny position. ‘Totally Tangerine’ (orange), ‘Red Fox’ (scarlet), and ‘Verrone’s Obsidian’ (claret) all reach around 2.5ft (75cm), making them ideal for containers. Water dahlias in pots regularly, as they like moist compost.
When to sow dahlia seed
Dahlias are usually grown from tubers, which is far easier and quicker. However, if you like a challenge, they can also be raised from seed. Sow the seed under cover (for example, in a greenhouse or on a bright windowsill) in early spring. Once they are large enough, harden off the young plants and then plant into borders in June.
When should I start my dahlia tubers?
Start dahlia tubers in March or April by potting them up under cover; plant them outside in May or June. If you don’t have space to start them in a greenhouse or indoors, dormant tubers can be planted directly outdoors between April and May.
How late can you plant dahlia tubers?
You can plant dahlia tubers as late as May or June in most regions. When it comes to their blooming season, they should carry on up until frost.
