How to Overwinter Dahlias, Cannas and Tender Bulbs by Zone

Which bulbs are tender

Penn State Extension’s Chester County Master Gardeners say summer-blooming bulbs are not winter hardy: planted in spring, they bloom in summer, and you can either treat them as annuals or dig them and store them frost-free until spring. Their examples are dahlias, cannas, gladiolas and tuberous begonias. Penn State’s tropical-plants article adds elephant ears, caladium and banana to the list of tuberous plants that can be stored dormant over winter.

When to dig

• Penn State: the ideal time is after a light frost when the tops have died back, but before a hard freeze could kill the roots. Elephant ears survive a light frost but not a hard freeze, so Penn State’s author lifts them before the first frost.
• Oregon State: after a hard frost has killed the dahlia stalks. Remove the foliage and cut the stalk to 4–6 inches. If no freeze is forecast, leave the tubers in the ground three to seven days so the eyes show; if frost threatens, dig the same day. Dahlias handle rain but rot if the ground freezes.
• UC Master Gardeners, San Luis Obispo County: wait until the foliage turns yellow and brown, so nutrients move into the tubers. Where the soil freezes 4–6 inches deep, dig by November 15 or after the first killing frost. Cut the stems to 5–6 inches one to two weeks before digging.

Dig, clean and store, step by step

1. Dig wide. Oregon State digs about 18 inches from the stalk; UC San Luis Obispo encircles the plant about 12 inches out, then rocks and lifts the clump.
2. Label straight away (UC San Luis Obispo), and discard diseased plants rather than composting them.
3. Clean. For dahlias divided in fall, Oregon State and UC San Luis Obispo wash the clump gently with a hose. Penn State says do not wash elephant ears, which rot if washed.
4. Dry. UC San Luis Obispo air-dries for 24–48 hours, not on concrete: too much drying shrivels tubers, too little rots them.
5. Divide. Oregon State says discard tubers with broken necks; each piece kept needs a viable eye. It rinses divided tubers in a 10% bleach solution and lets them air-dry.
6. Pack. Oregon State uses coarse vermiculite, coconut coir or wood shavings (watch shavings, which can shrivel tubers); UC San Luis Obispo uses peat moss, vermiculite or wood shavings in layers, tubers not touching.
7. Do not seal it. Oregon State says a closed bag or air-tight tote builds up moisture and rots the tubers.
8. Check through winter. Oregon State looks for condensation and removes rotting tubers. UC San Luis Obispo moves shriveled tubers somewhere more humid and moldy ones somewhere drier.

Where to store them

The sources disagree on temperature, so each is shown as published.

UC San Luis Obispo notes that their thin skins mean losing about 20% of fall-dug dahlia tubers to rot or drying out in storage is common.

Leaving them in the ground

Oregon State’s method for dahlias in zone 7 or above:

1. Make sure the site drains well and has no standing water.
2. Cut the stems to ground level after the first frost.
3. Cover the area with a sheet of 6-mil black plastic.
4. Add at least 4 inches of organic mulch or leaves.
5. Uncover in spring when there is no threat of frost.

It says to dig and divide in-ground dahlias every three years, and warns that voles and gophers sometimes eat tubers left in the ground. UC San Luis Obispo describes the same choice as spring digging: cut stems to the crown, cover with 4 inches of mulch and do not water in winter — no storage needed, but a higher risk of rot from winter wet or freezing.

For cannas, Penn State’s author has had a bed overwinter outdoors for more than 15 years in one protected, south-facing spot in southern Allegheny County, Pennsylvania — while elsewhere in the same garden they do not survive.

Replanting in spring

Oregon State moves stored dahlias somewhere slightly warmer in March or April to wake them and bring out the eyes, and digs in-ground clumps in spring after the last frost. Penn State pots tender bulbs up four to six weeks before the last frost — about when you would start tomatoes from seed — and moves them outdoors once nights stay mild, as for tomatoes and peppers. When to start my seeds gives that tomato date for your ZIP code.

Common mistakes

• Waiting for a hard freeze before digging. Penn State says dig after a light frost but before a hard freeze can kill the roots.
• Sealing the storage box. Oregon State says trapped moisture rots tubers.
• Storing tubers where they can freeze. A frozen clump rots (Oregon State).
• Washing elephant ears. Penn State says washing causes rot.
• Keeping broken-necked tubers. Oregon State says discard them.
• Leaving dahlias in wet ground. Oregon State’s first condition is good drainage.
• Forgetting to label. UC San Luis Obispo labels each clump as it comes out of the ground.

In the South

We found no Southeast, Texas or Florida source on overwintering tender bulbs, so advice for the South here is limited. The California source writes for a mild-winter coast, and Oregon State’s in-ground rule is for Oregon. For the South, ask your county extension office.

Related

• ZIP code zone finder — your exact USDA zone and nearest NOAA station
• When to start my seeds — the spring calendar for your ZIP code
• How to Protect Plants from Frost: Covers, Timing, Temps
• When to Plant Fall Bulbs by Zone: Tulips, Daffodils, More
• When to Cut Back Perennials in Fall, and What to Leave