How to Overwinter Tender Bulbs Like Dahlias and Gladiolus

Which Bulbs Need Lifting, and Where

To overwinter dahlias, dig the tubers after frost kills the tops, dry them for about a day, pack them in vermiculite, peat moss or wood shavings, and store them at 40 to 50 degrees Fahrenheit. Gladiolus corms follow a different schedule: dry them for 2 to 3 weeks, then hang them in mesh bags at 35 to 45 degrees.

“Tender bulbs” is a loose garden term. Dahlias grow from tuberous roots and gladiolus from corms, and University of Illinois Extension groups them with caladiums, cannas and elephant ears as summer plants that need to come indoors for winter in cold climates. They are the reverse of the fall-planted bulbs such as tulips and daffodils, which go into the ground at the time of year these come out.

Whether you have to dig depends on your zone. University of Georgia notes that dahlia tubers are hardy to USDA zone 8, that growers in that zone may leave plants in the ground, and that in colder areas it is best to dig. NC State Extension lists gladiolus for zones 7a through 10b and adds that hardy varieties need only a good layer of mulch, while others may need to be dug and stored, depending on the variety.

	Dahlia	Gladiolus	Canna
When to dig	After frost kills the tops	When leaves yellow or after first frost	A few days after a killing frost
Cut stems to	2 to 4 inches	1 to 2 inches	4 to 6 inches
Drying time	About one day	2 to 3 weeks	One to two days
Packing	Vermiculite, peat moss or wood shavings	Mesh bags, no packing	Newspaper or peat moss
Storage temperature	40 to 50 F	35 to 45 F	40 to 50 F

The table uses Iowa State University Extension’s figures, with the canna packing from University of Wisconsin Extension.

When to Dig Dahlias and Gladiolus

For dahlias, frost is the signal. University of New Hampshire Extension says to wait until the top growth dies back or is killed by the first hard frost. Wisconsin’s version is to dig before a hard freeze, or let the tops die back from a light frost. Do not wait long afterward: Illinois advises digging within a few days of a frost so that rot-causing organisms do not enter the bulbs.

Leave the stems alone until digging day. New Hampshire explains that dahlia stems are hollow and can collect water, which promotes crown rot and tuber decay, so cutting should be delayed until right before you dig. Iowa State’s instruction is to cut the plants back to within 2 to 4 inches of the ground several days after a hard frost.

Gladiolus does not need frost at all. Wisconsin says to lift the plants in fall either when the leaves yellow or after the first frost, and Iowa State places the job in late summer or early fall. Both jobs fit into the same weeks as the rest of fall garden bed preparation.

How to Dig and Dry Dahlia Tubers

1. Cut the stems to 2 to 4 inches right before digging.
2. Start several inches away from the plant with a garden fork or spade, loosen the soil all the way around, and lift the whole clump. Dahlia roots bruise easily, so handle them carefully.
3. Wash off the soil by swishing the clump in a tub of water or using a garden hose.
4. Let the tubers dry out of direct sunlight in a well-ventilated place at 60 to 70 degrees Fahrenheit.
5. Label each clump so you know the variety in spring.

Sources differ on how long step 4 takes. Wisconsin says several hours. Iowa State says until the roots are dry to the touch, usually about one day. New Hampshire says a few days, and Illinois gives 1 to 3 days for dahlias and cannas. The common thread is that dahlias need only a surface dry, which is much shorter than the weeks gladiolus needs.

Hold off on dividing. Wisconsin recommends waiting until spring to divide dahlia clumps, to prevent spoilage in storage.

How to Dig and Cure Gladiolus Corms

1. Dig the plants carefully with a spade and shake off the soil. Wisconsin’s advice for gladiolus is not to wash the corms.
2. Cut the foliage off 1 to 2 inches above the corm.
3. Dry the corms for 2 to 3 weeks in a warm, dry, well-ventilated location.
4. Once they are dry, remove and discard the old, dried-up mother corm at the base of each new corm.
5. Pull off the tiny cormels around the base. You can save them to grow on or discard them.

Wisconsin starts the corms with a day or two in bright sunlight and then gives the cure as about 3 weeks. Illinois puts it at up to three weeks, so plan on roughly three weeks between digging and final storage.

Storage Temperature and Packing

Dahlias go into a box with packing around them. Iowa State says to cover the tubers with vermiculite, peat moss or wood shavings and store them in a cool, dry location at 40 to 50 degrees Fahrenheit. New Hampshire describes the target as chilled, below 50 degrees but above freezing, which is 32 degrees, and suggests moistened vermiculite, wood shavings or sand. University of Georgia names 45 degrees as the best storage temperature. Illinois recommends a well-ventilated container, such as a milk crate, bread crate or cardboard box, in an unheated garage or unfinished basement. For tender plants that stay in their containers, see the guide on how to overwinter potted plants.

Gladiolus corms are stored dry and bare. Iowa State’s method is to place them in mesh bags or old nylon stockings and hang them in a cool, dry, well-ventilated place at 35 to 45 degrees Fahrenheit. NC State gives a slightly narrower 35 to 41 degrees.

Cannas sit close to dahlias: Iowa State lists 40 to 50 degrees and Wisconsin 45 to 50 degrees, wrapped in newspaper or layered in peat moss.

Checking Stored Bulbs Through Winter

Look in on the boxes about once a month. Wisconsin advises checking dahlias monthly for dehydration and misting lightly if needed. Illinois gives the same fix for bulbs that are beginning to shrink and wrinkle: moisten the packing material with a spray bottle.

Rot is the opposite problem. New Hampshire and Illinois both say to remove any tuber that has started to rot before the decay spreads to healthy ones nearby.

Spring is the time to divide. Wisconsin’s guidance for cannas is that each division should carry at least 3 eyes, and dahlia clumps held whole through winter can be split before planting.