Dahlias are one of the most rewarding flowers you can grow. From delicate pastels to vibrant fiesta colours, they bring life to borders, cutting gardens and pots from midsummer right through to the first frosts. This guide covers everything from potting up tubers in spring to planting out, taking cuttings, overwintering and making the most of your harvest, drawing on the experience of flower farmer Fran Phillips and gardener and author Ellen Mary alongside our own growing advice.
When to Start
Dahlia tubers should be potted up in March to early April, ideally in a frost-free greenhouse or on a bright windowsill. This gives them time to develop shoots before planting out once the risk of frost has passed, generally from late April onwards depending on your location.
Flower farmer Fran Phillips of La-Di Dardy Flowers describes it as a four-dimensional process:
“Party planning starts in January with websites, catalogues and spreadsheets full of colours and shapes. The tubers are potted up in April in the greenhouse where they are grown until hardening off in May, with planting out into good humus-rich soil in early June when the frosts have finished.”
What You Will Need
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Clean dahlia tubers (look for small shoots appearing – these will become your flower stems)
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A pot for each tuber – 2 to 3 litre pots are ideal (washed and cleaned)
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Multi-purpose compost (or a 50:50 mix of garden compost and bagged compost)
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Scissors, hand fork, trowel, gloves, labels, watering can
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A frost-free spot for your pots – a greenhouse is ideal
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A strategy for slugs and snails – dahlias are a favourite target
How to Pot Up Dahlias
Potting up is simple and satisfying. Here is the method step by step.
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Select your tuber and find a pot about twice its size. Some dahlias have elongated stems and will need a tall pot.
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Trim off any straggly roots with scissors. Take care not to damage any shoots already appearing.
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Hold the tuber in the pot so that the top, where the shoots are, sits at or just above the compost level.
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Fill around the tuber with compost, firming gently. Water, label and place in a frost-free spot.
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As shoots appear, limit each tuber to around five strong shoots. Remove the rest – you can use the best ones as cuttings.
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When the remaining shoots have two or three sets of leaves, pinch out the tips to encourage bushier growth and more flowers.
Fran Phillips uses a 50:50 combination of sieved garden compost and bagged compost, well mixed:
“Place the tuber in a pot, sized to provide some growing room and tall enough so that the compost can reach just to the tops of the tubers. Label clearly and water. Then place in a warm spot so that the tubers can get started.”
She adds a useful detail on shoot management:
“As the tuber starts to sprout, I only let five shoots grow. The others get knocked off. However, if you are planning on taking cuttings, allow a couple more to grow.”
How to Take Cuttings
Taking cuttings is straightforward and a brilliant way to multiply your dahlia collection for free. Use the healthy shoots you removed during potting up.
- When shoots are 5 to 10 cm tall, take hold of the shoot and gently pull, twist or cut so you get the very base where it joined the main plant.
- Remove all but the top pair of leaves, keeping the full length of the stem intact.
- Pop the cutting into a small pot of gritty compost, placing it around the edge of the pot. Water and label.
- Leave on a cool windowsill out of direct light. The cuttings should root easily and develop healthy tubers ready for winter storage.
Fran finds this especially useful for rescuing favourites:
“A couple of my favourites haven’t survived and a few look like they may only just survive. This is where I may take some cuttings to grow on stronger plants as it would be a shame to lose them.”
Hardening Off Before Planting Out
Before your dahlias go into the ground, they need to be hardened off. This is the same gradual process used for any tender plants being moved from greenhouse to garden.
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For a few days, take pots outside on warm or cool days. The leaves will be very tender at first.
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Over the next week, leave them out for increasingly longer periods, including windier and cooler conditions.
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If you get a warm night, start leaving them out overnight. Bring them in if temperatures drop below 4 or 5 degrees.
Fran Phillips is cautious on timing:
“Last year we waited until the end of May to be sure. Before planting out, do spend a couple of weeks hardening off plants so that they will be able to survive and thrive.”
If you have a cold frame, life will be a lot easier – see our cold frame guide for more.
Planting Dahlias in the Garden
Once the risk of frost has passed, your dahlias are ready for the garden.
What You Will Need
- Multi-purpose compost and gravel (if your soil is heavy clay)
- Fork, spade, gloves, labels, tape measure
- Plant food (pelletised chicken manure, blood fish and bone, or similar)
- A stake or two to three bamboo canes per plant
- Mulch
Method
- Dig a hole about 30 cm across and 30 cm deep. Add compost to the bottom and, if your soil is clay, a handful or two of grit. Water well.
- Place the tuber in the hole so that any shoots or the base of the stem sit at or just above soil level. Fill in with compost and firm.
- Position a stake next to the tuber at the same time to avoid damaging the roots later.
- Water well and wait for shoots to appear. As with potting, keep to around five strong shoots and pinch out the tips when there are two or three sets of leaves.
- Space dahlias 45 to 60 cm apart, depending on the variety.
- Feed from one to three weeks after planting, then every three weeks or so.
As the dahlia grows, tie in the stems for support. When cutting flowers for the vase, dahlias need to be in pretty much full bloom. Cut and place straight into a bucket of water, leaving in a cool spot for a couple of hours for a good drink before arranging. If you are not cutting for the vase, deadhead regularly to keep the plant producing. Dahlias are great cross-pollinators, so only leave spent flowers on if you want to collect seed.
Watering is important – dahlias need a good flood in the early morning to stop the starchy tubers drying out.
Top Tip: Try to avoid watering in the evening, as slugs love a wet night.
Growing Dahlias in Pots
Dahlias look stunning in pots and are well worth growing if you do not have garden space. Use a pot at least 30 cm wide and 30 cm deep with good compost. The same rules apply: pinch the tips, feed, water, and deadhead or cut regularly.
Overwintering Your Dahlias
There are two main approaches to overwintering dahlias, and both work well.
Option 1: Dig Up and Store
After the first frosts, cut the plants back, dig up the tubers and clean off as much soil as possible. Trim straggly roots and cut the stems down to 5 to 10 cm. Divide the clump if it comes apart easily – each section needs a piece of stem, because that is where the new shoots come from. Wrap in newspaper and store in boxes somewhere cool and frost-free.
Fran Phillips describes the process:
“For each tuber, I clean off as much soil as possible, trim off the straggly roots and cut the stems down to 5 or 10 cm. Sometimes the tubers come apart easily if I twist them. A lot of the time, it’s a wrestling match, especially with the big masses of tubers.
It’s really important to have a section of stem because that’s where the new shoots come from.”
Option 2: Leave in the Ground
Gardener and author Ellen Mary has left her dahlias in the ground for over 15 years:
“The two approaches for overwintering are to cut the plants back after the first frosts in autumn and store the tubers in a cool, dry place until the following spring, or to leave them in the ground with really good mulch. I have always done the latter, mostly due to laziness! Almost every dahlia, every year has grown back, even after long, cold winters.
They always need a thick mulch – that’s the key.”
Whichever approach you choose, the important thing is to protect the tubers from frost and excess wet over winter. For more on winter protection, see our guide on how to keep your greenhouse warm in winter.
Dahlia Varieties Worth Growing
Ellen Mary’s standout recommendation is Dahlia ‘Wine Eyed Jill’:
“She’s a baby pink, peachy perfect ball of prettiness, but the best bit is that you get three in one. As the season continues, the centre takes on a deep maroon colour just like red wine and then the flowers fade to a pale yellow. So on one plant at any one time, you can find three different-looking flowers!”
Other varieties worth trying include Arabian Nights, Boogie Woogie, Café au Lait, Bohemian Spartacus, Myrtle’s Folly, Kelvin Floodlight, Penhill Watermelon and Crème de Cognac. With so many shapes, sizes and colours available, there really is something for every garden.
As Fran Phillips puts it:
“If you grow dahlias, or would like to, there are so many to choose from. Plan what you would like and have some fun. Dahlias are great for weddings and other events, for your own house and for bouquets for friends. They are easy to grow and immensely rewarding.”
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