Dig up dahlias after the first frost turns the leaves black. Cut the stems down, wait about a week, then lift the tubers, dry them, and store them in a cool, frost-free spot until spring.
Most of Canada is too cold for dahlia tubers to survive winter in the ground. Lifting them saves the plants and lets you replant the same varieties next year.
Why the first frost is the signal
A light frost kills the leaves and flowers but not the tubers underground. The blackened foliage tells you the plant has stopped growing for the year. The tubers have finished storing energy.
Frost timing differs across the country. It often arrives in September on the Prairies and in northern areas. It often comes in October in southern Ontario, southern Quebec, and the Atlantic provinces. On the BC coast it can hold off until November or later. Check your own average first frost date and watch the overnight forecast.
If a hard freeze is forecast and the leaves are still green, cut the plants down and lift the tubers anyway. Frozen soil can damage them.
Dahlias you can leave in the ground
Mild coastal BC is the exception. Many gardeners there leave dahlias in the ground under a thick layer of mulch. This works only in soil that drains well. Tubers in wet winter soil rot. If your bed stays soggy from November to March, lift the tubers there too.
How to dig up dahlia tubers
Cut the stems. After frost blackens the leaves, cut each stem to about 10 to 15 cm (4 to 6 inches) above the soil. Leave the plant in the ground for about a week. This lets the tuber skins toughen, which helps them store.
Loosen the soil. Push a garden fork into the ground about 30 cm (12 inches) out from the stem. Dahlia tubers spread wide, and a fork pushed in close will spear them. Work around the clump on all sides.
Lift the clump. Pry up the whole clump with the fork. Hold it by the stem stub, not by a single tuber. Tubers snap off the crown easily, and a broken neck means that tuber will not grow.
Shake off the soil. Knock off loose soil with your hands. Rinse the clump with a hose if the soil is heavy clay, or brush it off if the soil is light. Remove any soft, mushy, or shriveled pieces.
Label the clump. Write the variety name on a tag or on masking tape. You will not remember which is which in April.
Dry the tubers before storing
Set the clumps upside down for a day or two so any water drains out of the hollow stems. Then move them to a dry, sheltered place out of the sun, such as a garage or a covered porch. Leave them for a few days to a week. The skin should feel dry, and the tubers should not be wet or sticky.
Do not dry them in direct sun or near a heater. Tubers that dry out too far shrivel over winter.
How to store dahlia tubers
Pick a storage spot that stays cool, dark, and above freezing. The target range is about 4 to 10°C (40 to 50°F). An unheated basement, a cold room, or an insulated garage that does not freeze all work.
Use a cardboard box, a wooden crate, or a paper bag. Do not use a sealed plastic container. Pack the clumps in a lightly damp, loose material:
- Peat moss
- Vermiculite
- Wood shavings
- Coarse sawdust
Layer the material between and over the clumps so the tubers do not touch each other. Keep the material only barely damp. If you squeeze a handful, no water should drip out.
Check the box once a month. Throw out any tuber with mold or rot. If the tubers look wrinkled and dry, mist the packing material lightly. If you see mold, open the box to air and add drier material.
Other tender bulbs you can lift the same way
Canna lilies, tuberous begonias, and gladiolus corms also do not survive most Canadian winters. Lift them after frost and dry them. Store canna rhizomes and begonia tubers in barely damp peat moss or wood shavings. Gladiolus corms keep best dry, in paper bags or mesh bags, in a cool spot.
Use the empty bed
Once the dahlias are out, the bed is open for fall planting. October is the main window for tulips and daffodils in much of the country. See when to plant spring bulbs in Canada by region for depths and timing. If you plan to grow garlic in the same bed, when to plant garlic in Canada by region covers planting dates and mulch.
Add a few centimetres of compost where the dahlias grew. Dahlias feed heavily, and the soil will use the extra organic matter.
Replant in spring
Plant tubers outdoors after your last spring frost, when the soil has warmed to at least 15°C (60°F). In colder regions, start tubers in pots indoors about four to six weeks before the last frost date. Plant each tuber 10 to 15 cm (4 to 6 inches) deep with the growing eye facing up. Set stakes at planting time so you do not stab a tuber later.
Large clumps can be divided before planting. Each piece needs at least one tuber and a piece of the crown with an eye. A tuber with no eye will not sprout.