This is my essential guide to planting dahlias. Dahlias are among the lowest maintenance, highest production cut flowers and garden plants you can grow. In a good year, they'll flower from late June to early December (particularly in a sheltered spot). They come in all shapes and sizes, and are available in most of the best flower colours. They are some my favourite ever garden plants.
Soil and Site
Dahlias thrive in most sunny situations and do best in a fertile soil, with moisture and good drainage.
Dig a hole at least 30cm square and 30cm deep for each one, spacing each dahlia tuber 75cm apart (depending on expected final size of variety).
What to do with your bulbs when they arrive
Dahlias are tender tubers. Their root structures look like a bunch of salamis gathered together on a stem. If you plant them out before the frosts are over, they may get frosted and die, so pot them up in March or early April, in a generous pot (at least 2 litre – I use 3 litre pots) filled with multi-purpose potting compost.
Place them in a light, frost-free place and keep the compost moist. They will have formed bushy plants by the time the frosts have ended and will be in flower by the beginning of July. Watch Sarah pot up her dahlia tubers in our quick video.
In the garden
If you don’t have anywhere to grow the potted tubers, you can put them straight into the ground when the frosts are nearly over, mulching them or protecting with a cloche or protective horticultural fleece if the foliage appears before the frosts are over. This system involves less work, but you’ll have plants several weeks behind those brought on inside.
To do this, cover the base of the hole with compost or manure and give it a good dousing with a full watering can, then plant the dahlia. Add grit to the planting hole on heavy clay. You will need a stout stake, not just a bamboo cane, to support each plant and it is a good idea to knock this in first and then place the plant by its side.
After about a week in the ground, scatter a couple of trowelfuls of Fish, Blood and Bone around the clump and give them another good soaking. Once a fortnight, feed them with a liquid balanced feed like Seasol. In a drought, it’s a good idea to water them once a week, with a good flood not a gentle sprinkle. With the stake in place at planting, tie them in every couple of weeks. Dahlias grow very quickly once they get going and can easily break off right at the base in wind or rain if they are not securely staked.
Choose a container which is at least 30cm (12") in diameter and depth for optimum growth. Use multi-purpose compost and add a slow-release fertiliser for strong growth. Plant tubers as deep as you would when planting in the ground.
Keep in mind that all dahlias – even very healthy and long-standing old ones – grow at hugely different rates. Not all dahlias grow quick and fast and often more interesting varieties are slower, more delicate growers. Watch our quick video on growing dahlias in pots.
Whether you have raised your dahlias outside in the garden or under cover, you need to pinch out the tips of the main shoot as they grow. Either with a sharp knife, or squeezed between your thumb and forefinger, remove the main shoot down to the top pair of leaves.
You also need to remove all but five shoots sprouting from the tuber. There may be several more shoots, some of them weedy, but all but five must go. It feels brutal, but pinching out encourages bushy plants and with only five stems allowed to develop, you will get strong, vigorous growth that will produce lots of flowers.
You could choose to grow some of your tubers to take cuttings, and turn one tuber into ten more tubers. Watch and read more about how to take dahlia cuttings...
In recent years, our winters in the South of England have been so mild that dahlias left in the ground, mulched deeply to protect them from the frost, have re-emerged fine, bulking up and flowering well before the other plants grown on in pots. You could opt for this low-maintenance regime, but you risk losing your plants if we are hit by a hard winter. To replace them is cheap and easy, so this is what we do at Perch Hill, saving lots of time and hassle in the autumn and again in the spring. Find out more about overwintering your dahlia tubers in our handy article.
Earwigs can be a problem with dahlias, eating the flowers and the leaves. The organic way of control is to position pots filled with straw upside down raised on canes dotted throughout your dahlias. The earwigs crawl into the straw in the heat of the day. At the end of the day you can bag them, burn them or release them somewhere else far from your dahlias. Slugs also love dahlias, especially when they first shoot, so protect them from the word go.
Only pick dahlias in full flower. Recut the hollow stem ends under water to avoid airlocks.
If you don’t pick every flower for the house, it’s a good idea to have an occasional blitz of deadheading. This will make them look much better and will prolong flowering. Cut heads off, removing the whole dead flowering stem.
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