Dahlias are a must-have for any summer garden with their vibrant colours, dramatic shapes, and long flowering season. Whether you’re a beginner or a green-fingered pro, these stunning blooms can bring real joy to your borders, containers, and cut flower arrangements. Plus, you invest in the tubers once; they are the gift that keeps giving year after year.
In this guide, we’ll walk you through everything you need to know to grow healthy, beautiful dahlias, from potting up your tubers to protecting your plants from pests.
Why Grow Dahlias?
Dahlias are loved for their incredible variety of colours, shapes, and sizes—from delicate pom-poms to huge dinner-plate blooms. They flower from mid-summer through to the first frost, providing long-lasting colour and impact in the garden. Plus, they’re fantastic for cut flowers, making them a favourite among florists and flower farmers.
When to Plant Dahlia Tubers in the UK
Dahlia tubers are not frost-hardy, so the best time to start them off indoors or in a polytunnel/greenhouse is from late March through April. This gives them a head start on the growing season while keeping them safe from any late cold snaps.
You can plant them directly into the ground once the risk of frost has passed—typically from mid to late May in most parts of the UK.
How to Grow Dahlias
Potting up your dahlia tubers indoors is a great way to encourage early growth and reduce the chance of slug damage later on. First of all, you’ll need pots that fit the size of your dahlias. Fill your pots two-thirds of the way up with compost and place your dahlia tuber just below the soil surface. You should place them with the ‘eyes’ facing upwards. Then, cover lightly with compost and pop them in a bright, frost-free spot. They’ll need a little watering until they start to sprout.
Pinching Out for Bushier Plants
Once your dahlias are around 20–30cm tall and have at least four sets of leaves, pinch out the growing tip just above a pair of leaves. You can do this with a sharp, clean pair of scissors. This encourages the plant to branch out and produce more flowers on stronger stems, increasing your harvest later in the year.
Hardening Off Dahlias
Before planting dahlias outside, you’ll need to harden them off, which is a process of acclimatising them to outdoor conditions to avoid shock. Start hardening off your plants about two weeks before planting out. Place them outdoors during the day and bring them in at night. Gradually increase their time outside until they’re ready to stay out permanently after the last frost.
Planting Out Dahlias
Once the danger of frost has passed (usually mid to late May in most areas), you can safely plant your dahlias in the ground or in large containers.
Top tips for planting out:
- Choose a sunny, sheltered spot with well-draining soil.
- Enrich the soil with compost or well-rotted manure.
- Space plants around 45–60cm apart, depending on the variety.
- Water thoroughly after planting.
- Add a staking system early on, this avoids damage as the plant grows.
Feeding & Watering
Dahlias are hungry plants! Once they’re established and growing well, feed them weekly with a high-potash fertiliser (like tomato feed) to encourage strong flowering. Keep them well-watered, especially during dry spells, but avoid waterlogging.
Protecting Dahlias from Slugs and Snails
One of the biggest threats to young dahlia shoots is slugs and snails, which can decimate new growth overnight. Here’s how to protect them:
- Use slug pellets (wildlife-safe options are available).
- Surround plants with grit or crushed eggshells to deter crawling pests.
- Surround plants with raw sheep’s wool.
- Place copper rings around young plants.
- Try beer traps or organic deterrents like garlic spray.
- Growing in pots or raised beds can also reduce risk.
Deadheading & Prolonging Blooms
To keep your dahlias flowering right through to autumn, deadhead regularly. This means removing blooms which are past their best by cutting back to a leaf joint or side shoot—this encourages new buds to form.
How to Lift & Store Dahlias Over Winter
Dahlias aren’t hardy, so in most parts of the UK, you’ll need to lift and store the tubers once the first frosts blacken the foliage.
How to do it:
- Cut back stems to about 10–15cm above the ground.
- Carefully lift the tubers with a fork, shake off excess soil and let them dry.
- Store in a cool, frost-free place in trays or boxes filled with dry compost or sand.
- Check occasionally for signs of rot and remove any affected tubers.
If you live in a very mild area with well-drained soil, you may choose to leave them in the ground with a thick mulch for protection—but there is always some risk.
Growing dahlias is hugely rewarding. With a little planning and care, you’ll be rewarded with months of spectacular blooms, perfect for enjoying in the garden or cutting for the vase. Whether you’re looking for a statement flower bed or simply want to add a few cheerful pots to your patio, dahlias offer variety, colour, and drama in abundance.
📍 Find a wide range of dahlia tubers, compost, pots, and expert advice in-store at St. John’s Garden Centre. Let’s get growing!