Dahlia pruning in autumn: preparing for overwintering

What are dahlias?

Dahlias are stunning summer bloomers with large, sometimes enormous, double flowers in an incredible range of colours: red, pink, yellow, orange, white, purple and almost black. They grow on thick stems sometimes reaching 150 cm tall and flower from July through October. They get bigger and more beautiful as they age.

But dahlias are not winter-hardy in the UK and northern Europe. The tubers freeze to death if left in the ground. So every autumn you must dig them up, prune the plants, store the tubers and plant them again next year. This work is not difficult - you just need to know how and when.

Timing: October through late November

Prune dahlias in autumn, from October through late November. Wait until:

• The first hard frost comes (around October-November, depending on your region)
• The flowers become limp (no longer stiff)
• The stems turn black (this is the sign that frost has hit the plant)

Why wait? Because frost damage signals that the plant is sending energy back to the tubers. After a light frost the foliage turns black but the tubers are ready for harvest.

The earlier you harvest, the more risk of immature tubers. If you harvest too late (December or later), the tubers freeze in the ground.

Step-by-step pruning and lifting

Step 1: Cut the plant back

As soon as the stems are black from frost, cut the plant back to about 15-20 cm above ground. This is drastic pruning - you remove almost everything. This is correct. Remove all dead foliage and stems.

Step 2: Mark where your dahlia is

This is important. Without a marker stick you will miss the tubers. Now place a clear stick or label where the dahlia stands.

Step 3: Dig out carefully

Take a spade and dig carefully around the dahlia. You do not want to cut into the tubers. Dig about 30-40 cm around the plant and 25-30 cm deep. Dahlias grow in clusters, so you may have several tubers per plant.

Lift gently and carefully shake off most of the soil. Do not use high-pressure water - you will damage the tubers.

Step 4: Collect the tubers

Now you see the tubers - they look like small brown potatoes, sometimes very large. They hang in clusters from the old stem base. Not every piece is a tuber - they must be attached at the base.

Step 5: Clean and dry them

Let the tubers dry for about a week outside in a cold (but frost-free) place. A shed, garage or crawlspace with good ventilation is fine. The skin hardens and infections can dry out.

Storing tubers

After drying, store the tubers in a cool, dark place:

• Temperature: 5-10 degrees Celsius (not colder, not warmer)
• Dark
• Well ventilated
• Dry (not wet, not bone-dry)

Many gardeners store tubers in:

• An unheated garage
• A cold cellar
• A shed
• An unheated greenhouse

Do not place them directly on concrete floors (too cold) and not in a warm living room (too warm). They must be dry, so not directly on damp soil.

Some gardeners dust the tubers with powdered fungicide to prevent rot. This is optional.

For newcomers to dahlias

If you have dahlias from seed or you want to store tubers for the first time this winter, remember:

• Dahlias from seed do not make tubers, only small rootstocks - you cannot store those
• Cloned dahlias (from cuttings or tubers) make tubers
• Do not waste time trying to store potato-like tubers from seed

Step-by-step

Step 1: Wait for frost damage

Leave your dahlia standing until October/November, until stems are black from frost.

Step 2: Cut the plant back to 15-20 cm

Remove all dead foliage and stems. Prune drastically.

Step 3: Place a marker stick

Without a marker you cannot find your dahlias later.

Step 4: Dig out carefully

Dig 30-40 cm around and 25-30 cm deep. Lift gently.

Step 5: Clean the tubers

Let them dry for a week in a cool, well-ventilated place.

Step 6: Store at 5-10 degrees

Place them in a dark, dry location. Check several times over winter.

Frequently asked questions

Can my dahlias stay in the ground?

No, not in northern climates. They freeze to death. In very mild winters they can sometimes survive under heavy mulch, but the risk is high. It is not worth it.

My tubers are rotting in storage - why?

Probably too wet. They must be dry. Also mould can grow in warm, damp environments. Ensure cool, dry storage and good ventilation.

How long can tubers be stored?

One to two years in good conditions. After two years they lose vitality and grow more poorly.

Can I save damaged tubers?

Small damage is fine. The plant will grow around it. Large damage (more than a centimetre deep) you discard.

My dahlia has become huge - should I divide the tubers?

After a couple of years you get thick knobs with many small tubers attached. You can split them in March, before planting. Make sure each piece has some foliage and some rootstock.

Discover your own garden design

At [gardenworld.app](https://gardenworld.app) you can upload your front yard and see how your dahlias fit - with realistic growth forms and surrounding plantings. Plan and visualise before you plant your dahlias.