Dahlia Growing Guide

Dahlias are hard to beat for summer colour. Once they get going, they’ll flower for months, with everything from tidy little pompons to huge dinner plate blooms.

They grow from tubers rather than bulbs and are surprisingly easy once you get the basics right. Give them plenty of sun, good drainage, regular water and a bit of support, and they’ll keep producing flowers right through summer and into autumn.

When and Where to Plant Dahlias

Plant Dahlia tubers in spring once the risk of heavy frost has passed and the soil has started to warm.

In warmer areas, planting can usually begin from September. In colder areas, it is safer to wait until October or November.

Dahlias perform best in full sun, ideally with at least six hours of direct sunlight each day. Choose a spot that drains well and is reasonably sheltered from strong wind, especially for taller varieties.

If your soil stays cold and wet through spring, start the tubers in pots somewhere sheltered and plant them into the garden once conditions improve.

Preparing the Soil

Dahlias like rich soil, but good drainage is just as important.

Before planting, loosen the ground and mix through compost or well-rotted organic matter. If your soil is heavy, plant into a raised bed or improve it with plenty of compost.

Avoid planting into ground that stays waterlogged after rain. Wet soil is one of the quickest ways to lose Dahlia tubers.

If the soil is poor, add a balanced fertiliser before planting. Avoid going too hard with nitrogen, as this can produce heaps of leaves and not many flowers.

How to Plant Dahlia Tubers

1. Dig a hole wide enough to lay the tuber down without bending or damaging it.
2. Place the tuber horizontally, with the crown or old stem facing upwards.
3. Cover with around 10cm of soil.
4. Space plants around 40 to 60cm apart.
5. Mark the spot so the tuber is not accidentally disturbed before it shoots.

The crown is the swollen area where the old stem joins the tubers. New shoots grow from this area, not from the ends of the tubers.

If a tuber becomes detached from the crown and has no visible eye or growing point, it is unlikely to grow.

Watering After Planting

If the soil is already moist, there is usually no need to water heavily straight after planting.

Too much water before the shoots appear can encourage rot. Begin regular watering once the new growth is visible.

If conditions are very dry at planting time, give the soil a light watering, but avoid soaking it.

Staking, Pinching and General Care

Tall Dahlias should be staked early, preferably at planting time. Adding the stake before the tuber begins growing reduces the chance of damaging it later.

Use a sturdy stake and loosely tie the stems as the plant grows. Shorter patio and border varieties may not need support.

Once the plant reaches around 25 to 30cm tall and has three or four sets of leaves, you can pinch or snip out the growing tip just above a pair of leaves.

This encourages the plant to branch out and produce more flowering stems.

Keep an eye out for slugs and snails as the shoots emerge. Fresh Dahlia growth is one of their favourite meals.

Watering and Feeding Dahlias

Once they are actively growing, Dahlias need regular water, especially during hot and dry weather.

Water deeply at the base of the plant rather than giving them a light sprinkle each day. Try to keep the leaves dry where possible, as constantly wet foliage can encourage fungal problems.

A layer of mulch will help hold moisture in the soil and keep weeds down. Keep it slightly away from the main stems.

Feed with a balanced fertiliser early in the season. Once buds begin forming, switch to something lower in nitrogen and higher in potassium. Tomato fertiliser is often a good option during flowering.

Growing Dahlias in Pots

Dahlias grow well in pots, particularly compact, border and patio varieties.

Choose a container at least 35 to 40cm wide with plenty of drainage holes. Larger varieties need a much bigger pot and may still require staking.

Fill the container with a good-quality potting mix and plant the tuber around 10cm deep.

Potted Dahlias dry out faster than those in the ground, so check them regularly during hot weather. They will also need feeding more often, as nutrients wash out of pots more quickly.

How to Keep Dahlias Flowering

The best way to keep Dahlias flowering is simple: keep picking them.

Cut flowers regularly and remove faded blooms before they begin producing seed.

When deadheading, follow the flower stem back to a pair of leaves and make the cut there. This keeps the plant tidy and encourages fresh stems to grow.

Dahlias usually begin flowering from early to mid-summer and continue until the first heavy frost.

Cutting Dahlias for the Vase

Cut Dahlia flowers early in the morning or later in the evening, when the plants are well hydrated.

Choose blooms that are nearly or fully open, as Dahlia buds do not open much after cutting.

Use clean, sharp snips and place the stems into water straight away. Remove any leaves that would sit below the waterline and change the vase water regularly.

Common Dahlia Problems

What to Do After Flowering

Once the first heavy frost blackens the foliage, cut the stems back to around 10 to 15cm above ground level.

In warmer areas with free-draining soil, tubers can often be left in the ground over winter. Add a layer of mulch for protection and mark the spot.

In colder areas, or where the soil stays wet through winter, it is safer to lift and store the tubers.

How to Lift and Store Dahlia Tubers

1. Cut the stems back, leaving a short section attached to the crown.
2. Use a garden fork to loosen the soil well away from the plant.
3. Lift the clump gently from underneath.
4. Shake or brush off the loose soil.
5. Allow the tubers to dry somewhere sheltered before storing them.

Dahlia tubers can be brittle, so take your time. The necks joining the tubers to the crown are easily snapped.

Store lifted tubers somewhere cool, dry, frost-free and well ventilated.

Wood shavings, dry potting mix, vermiculite or loosely wrapped paper can help stop them from drying out too much.

Check them a few times over winter and remove any that become soft or rotten.

Dividing Dahlia Tubers

Dahlia clumps can be divided to produce more plants.

Each division needs a piece of the crown with at least one visible eye, along with an attached tuber.

A tuber on its own will not grow unless it has part of the crown and a growing point attached.

Dividing can be done after lifting in autumn or before planting in spring. The eyes are often easier to see once the tubers begin waking up.

Frequently Asked Questions

Are Dahlias Bulbs?

No. Dahlias grow from tubers. The tubers store energy for the plant, while new shoots grow from eyes on the crown.

Which Way Up Do You Plant a Dahlia Tuber?

Lay the tuber horizontally, with the crown or old stem facing upwards.

Do Dahlias Come Back Every Year?

Yes. In warm areas with well-drained soil, they can often be left in the ground and will return in spring.

Why Has My Dahlia Not Sprouted?

The soil may still be too cold, the tuber may have rotted, or it may not have had a viable eye. Give it a little more time before digging around.

Do Dahlias Need Full Sun?

Yes. Dahlias flower best in full sun. In too much shade, they can become leggy and produce fewer blooms.

Our Final Dahlia Growing Tip

Don’t be too kind to them.

Give Dahlias sun, good soil and regular water, then keep cutting the flowers. The more you pick and deadhead, the more they’ll produce.

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