When and how to grow dahlias from tubers
Dahlias are some of our favourite flowers here at Crocus. These beautiful flowers come in many colours and sizes and bloom from late summer to the first frost. It's impossible not to love these hard-working show-stoppers.
Plus, the more you cut, the more you get with dahlias, making them a fabulous cutting garden flower. For months, you can fill your home and garden with hundreds of flowers, even from a few tubers.
If you're new to dahlias, they can be confusing because they don't work like other perennials. Read on to learn how to plant, lift, and store dahlia tubers so you can replant them next year.
How to care for dahlias
When to plant dahlias
When growing Dahlia tubers in the UK, remember that these Mexican plants cannot tolerate frosts, so you have two options. In early spring, you can start them in pots in a cool, frost-free greenhouse. Dahlias respond to warmth, and shoots will grow as the temperatures rise. From around the end of May/June, when the frost has passed, you can plant them outside.
Alternatively, plant tubers directly into a border from late spring when temperatures are consistently above freezing. The ground will not likely freeze then, so it will protect the tubers. When the shoots appear in late May the risk of frost should have passed.
How to grow dahlia tubers in pots
You can start growing dahlia tubers in pots in March or April if you have space in a frost-free greenhouse. This will give your dahlias a head start and often helps them survive slugs and snails, which love to eat the tender shoots.
Fill a two or 3-litre pot with peat-free compost. Plant the tubers to lie horizontally, just below the soil surface. Ensure the old stem and eyes ( buds) are facing upwards. The tubers will look like sausages attached to a central stem.
Carefully spread the individual tubers out so that they are not touching. These tubers are fragile and work slowly, so you do not break them off the main stem. If you do break a tuber off, plant it alongside the others.
Spacing the tubers out allows the shoots space to grow and prevents them from growing into each other as they develop. Cover entirely with compost, lightly water and label your pot. Don't water them again until the first shoots appear through the soil, then water regularly, keeping the soil moist but well drained.
Place your potted tubers in a frost-free greenhouse or a cool indoor area with plenty of light. Avoid keeping them somewhere too warm, as this favours soft, leggy growth. Cooler growth will encourage sturdier plants.
If growing them in a cold frame, place a triple-layer of garden fleece over them if the temperature drops to freezing. Check these dahlias regularly to ensure the shoots don't touch the glass of the cold frame. If they do, move them indoors or to a greenhouse.
Pinching out dahlias
To encourage bushy growth, pinch out the shoots if desired. Do this three weeks after planting or when they are around 30-45 cm tall. To pinch out the tips of the main shoot, use a sharp knife or your fingers, cutting or pinching to the top pair of leaves.
Pinching out encourages shoots to emerge along the stems, creating more flowers. Removing the main bud creates two shoots below, growing into two new flowers.
Planting Greenhouse Dahlias outdoors
Plant dahlias you grew in the greenhouse outside in late May or June when there is no frost. Water the pots so that the compost isn't dry, and gently knock the plants out of the pot. Take care not to damage any of the shoots.
Dig a hole the size of the pot and gently lower the plant into the hole. Make sure that the tubers are just below the surface of the soil. Gently firm the plant in and water.
How to plant dahlia tubers in the ground
If you lack a cold, frost-free place, you can plant them directly in the ground in late spring when the likelihood of frosts has passed. Plant them just under the soil surface. You can apply a 10 cm layer of mulch, rotted manure, a cloche or fleece to protect from cold.
Place a labelled stake next to them to mark where you have planted. You can also use these stakes to support them as they grow.
Where to plant dahlias outdoors
Dahlias like a warm, sunny spot with free-draining fertile soil. Preferably enriched with well-rotted garden compost or manure. Never plant them in soils prone to waterlogging, as they will rot.
Dahlias are not great at pushing through other plants. So, plant them in a dedicated area around 60cm apart and further apart for larger cultivars.
Dedicate particular spots in the border for dahlias. Plant bedding, tulips, or wallflowers for winter and early spring colour. Then, replace them with dahlias in late spring or early summer.
Growing small dahlias in pots
If you would love to have dahlias around your garden but don't have much space, try a dwarf type grown in containers. One of our favourites is Dahlia 'Pretty Woman' with dark foliage and sugar pink flowers. Like all single flower-headed dahlias, bees and butterflies love the flowers, making them an excellent pollinator.
How to care for dahlias
Watering
Dahlia are thirsty plants and require moist soil. As soon as they start sprouting, look to water them thoroughly and regularly. They prefer a good soaking every few days rather than a daily sprinkle. If you've grown them in pots of containers, water daily to ensure they stay moist.
Feeding
Like most cut flowers, dahlias appreciate a regular feed. While it is growing, provide a high-nitrogen liquid fertiliser each week in June to ensure healthy leafy growth. Use a high-potash fertiliser each week from July to September, which will help them produce lots of flowers.
Staking
Dahlias can grow large and will require staking, especially with large-bloomed cultivars, like 'Cafe Au Lait'. Stake with canes or brushwood. We love our Crocus Collection support for dahlias. Both beautiful and functional, try our low mound support, fluted plant support or teardrop plant support.
Spacing
Allow enough space between each cultivar (at least 60cm) so the plants can grow and spread to their full size without overcrowding. If you're low on space, try smaller cultivars in pots or containers for gorgeous patio plants.
Position
As dahlias are native to Mexico, they need lots of sun to flower. Plant them in full sun where they will get ideally 6-8 hours of sunlight in the height of summer.
Deadheading
Dahlias are a fabulous plant for cutting gardens; the more you cut, the more you get. Deadhead them regularly to keep them flowering for months on end.
To deadhead, cut the faded flower stem at the first set of true leaves. Double forms last longer than singles, so there's less deadheading if you're short on time.
Overwintering dahlias
British winters can be unpredictable; some plants need lifting or mulching to survive. Dahlias fall into this category of plants that need overwintering as they are not frost hardy.
For dahlias, you have two options for overwintering. Leave tubers in the ground or lift and store them.
Overwintering in situ
If you live in a warmer area of the UK, like coastal Cornwall, you could leave the dahlias where they are for winter. Make sure the soil is well-drained. The key is to ensure the soil doesn't get waterlogged. Dahlias are most likely to rot if left in wet soil.
If you have well-draining, dry soil (like a sandy soil), then you can leave them in situ. Cover well with at least 10- 15 cm of good-quality mulch.
Lifting and storing
Most UK gardeners will have to lift and store their dahlia tubers. Wait for the first frost to blacken the leaves and lift the tubers. When lifting, use a garden fork and dig deep, far away from the base of the plant to avoid damaging the tubers. Cut the stems back to 5-15 cm.
Clean the tubers of excess soil and allow them to dry naturally indoors. Once they are completely dry, store them in a shallow tray or crate. Cover with slightly moist compost, sand or vermiculite.
Store them in a cool but frost-free location until planting out again in March under glass or May/June outdoors. Check your stored Dahlias regularly over winter to remove any rotted (diseased) tubers.
Treat Dahlias grown in pots the same. Either lifting the tubers or bringing the whole pot into the greenhouse or somewhere protected. If you store the tubers in pots in the soil, keep the pots dry.
How to protect dahlias from slugs and snails
Slugs love dahlias, especially young plants. If you have a slug and snail-prone garden, it's best to plant them in pots before planting in borders. They can reach a good height and size to avoid being completely eaten.
You can also use a range of natural repellents, such as copper rings, cloches, wool pellets and nematodes. These all work in different ways to protect or reduce the number of slugs and snails in the garden.
