Can You Leave Dahlia Bulbs in the Ground Over Winter?

Dahlias are stunning flowers that bloom from midsummer until the first frost in autumn. Their dazzling colors and lovely forms have made them garden favorites. As the end of the growing season approaches, many gardeners face the question of whether to dig up dahlia bulbs or leave them in the ground over winter.

Should You Leave Dahlia Bulbs in the Ground?

There are pros and cons to leaving dahlia bulbs in the ground versus digging them up in fall.

Pros of leaving bulbs in the ground:

• It’s less work! Digging up bulbs takes time and effort. Leaving them in place avoids this task.

• Bulbs may survive the winter if conditions are mild In zones 8-11, dahlia bulbs can often overwinter successfully in the ground.

• You’ll have flowers earlier next year Bulbs left in place can sprout and bloom up to 1 month sooner than stored bulbs planted in spring

• It eliminates the need to store bulbs over winter. Storing bulbs properly takes space and diligence to check for rotting.

Cons of leaving bulbs in the ground:

• Bulbs may rot over winter if it’s too cold and wet. In zones 5-7, bulbs left in the ground often rot.

• You risk losing varieties you want to keep. Unprotected bulbs may not survive harsh winters.

• Pests may damage the bulbs underground over winter. Voles and insects can feed on bulbs left in place.

• Flowering may be reduced next year if bulbs are weakened. Freezing and thawing cycles stress bulbs left in the ground.

Factors Determining Winter Survival

Several key factors determine whether dahlia bulbs can successfully overwinter in the garden:

• Climate zone – Dahlias survive winter best in zones 8-11. In zones 5-7, they need protection or digging up.

• Soil drainage – Well-drained soil gives bulbs the best chance. Poor drainage leads to rotting.

• Mulch layer – A thick mulch helps insulate bulbs from hard freezes and temperature swings.

• Winter temperatures – Sustained cold below 10°F often damages unprotected bulbs in the ground.

• Snow cover – A deep snow blanket helps protect bulbs from subzero air temperatures.

• Fluctuating temperatures – Repeated freeze-thaw cycles stress bulbs left in the ground.

• Location – Sheltered spots near foundations or walls stay warmer in winter.

• Soil composition – Sandier soils drain better than heavy clay soils.

Caring for Bulbs Over Winter

If you decide to leave dahlia bulbs in the ground, follow these tips to help them survive winter:

• Cut back stems to about 4 inches after the first light frost.

• Amend soil with compost and a balanced fertilizer in fall.

• Cover each bulb clump with 6-8 inches of mulch such as bark chips, leaves, or straw.

• Consider covering beds with a layer of row cover fabric for extra insulation.

• Ensure beds have good drainage and divert rainwater runoff if needed.

• Apply slug/snail bait around bulbs, as these pests feed on bulbs over winter.

• Check for rodent damage in winter and fill in any tunnels or holes.

• Remove mulch in spring once risk of hard frost has passed.

When to Dig Up Dahlia Bulbs

If you live where winters are harsh, digging up bulbs in fall for winter storage is recommended. Follow these guidelines:

• Wait until after the first frost blackens the foliage.

• Dig up bulbs on a dry day when soil is not muddy.

• Carefully dig bulbs using a garden fork, lifting the clumps intact.

• Allow clumps to dry in a shaded spot for a few hours before storing.

• Separate bulbs, discard any rotted ones, and dust with sulfur powder.

• Pack bulbs in slightly moist peat, vermiculite, or sawdust for winter.

• Store bulbs in a cool (40-45°F) dark place, checking periodically for rot.

• Start bulbs indoors 6-8 weeks before planting time next spring.

Overwintering Tubers in Pots

If you grew dahlias in containers, you can often overwinter the tubers in the pots:

• Move pots to a sheltered spot, ideally near a building foundation or wall.

• Wrap insulation such as bubble wrap around the pots.

• Cover the soil surface with 2-3 inches of mulch.

• Ensure pots drain freely so tubers don’t sit in water.

• Check soil moisture every few weeks and water if needed.

• Remove insulation and mulch when growth resumes in spring.

Protecting Specific Varieties

Certain types of dahlias are less hardy and may need extra protection if left in the ground:

• Waterlily dahlias have tubers closer to the surface, making them prone to heaving.

• Dark-leaved dahlias like Bishop’s Children are more tender.

• Giant exhibition types and large-flowered varieties are less cold tolerant.

• Fully double dahlias like dinnerplate types may rot if winter is too wet.

For these tender dahlias, provide extra mulch and consider covering beds with insulation layers. Or, dig tubers and store them for safest winter survival.

Let Dahlias Guide Your Winter Approach

The key to successfully overwintering dahlia tubers is matching your care steps to the conditions in your own garden. Observe how your bulbs fare each winter, and adjust your methods accordingly. With attention to location, soil, and protection, you can discover the ideal winter storage approach for your climate and favorite dahlia varieties.