Many summer-blooming tender perennials are great additions to the garden but are not winter-hardy in Iowa. These tender perennials must be dug up in the fall and stored indoors over winter. This is possible because these plants grow from bulbs or other geophytes like corms or tubers.
The Basic Steps
Overall, the process to save tender perennials overwinter looks the same for most species.
- Prune tender perennials back just before or just after the foliage is killed by frost in the fall.
- Carefully pull the bulbs (or tuber, rhizome, corm, etc.) out of the ground and remove any excess soil.
- Allow bulbs to dry and then store in a cool, dark, and dry location.
- Temperatures must not get below freezing.
- Cardboard boxes, plastic crates, paper bags, mesh bags, or containers of peat moss, newspaper, sand, or vermiculite are all options for winter storage.
- Bulbs must be allowed to stay dry enough to remain dormant and free from moisture-loving mold or fungus but damp enough not to dry out and shrivel up.
Check on Them Throughout the Winter
Check on bulbs periodically throughout the winter. Remove any rotting, soft, or dried-out bulbs that may develop. If conditions are too dry, lightly moisten the material they are stored in. Be careful not to overwater as it will lead to rot, mold, and fungal growth.
Divide in the Spring
The storage organs of most tender perennials multiply quite quickly in the garden. Leave them intact until spring, as any injury incurred prior to storage will increase the chances of rot. Cut the rhizomes and tubers apart in the spring, making sure at least one or two dormant buds are present on each section. Share the extras with relatives, neighbors, and friends.
Find the Right Location for Storage
Often, the most difficult part of storing tender perennials for home gardeners is finding a location with the correct temperature. Most tender perennials need to be stored in temperatures around 40-60°F. Some species prefer the cool end of that range and others prefer the warm end.
Often an unheated basement, root cellar, or cool extra bedroom to store the tender perennials adequately is unavailable. Normal interior temperatures are too warm. Most garages, even though attached, can get below freezing during extreme weather, making them too cold for survival. If this is your situation, it would be best to grow these plants as annuals instead of perennials.
Specific Recommendations for Common Tender Perennials
The storage requirements for several widely grown tender perennials are provided below.
Cut the plants back to within 4 to 6 inches of the ground a few days after a hard, killing frost. Then, carefully dig up the canna clumps with a spade or fork. Leave a small amount of soil around the cannas. Allow them to dry for one to two days. Afterward, place the canna rhizomes in large boxes, crates, or mesh bags. Store in a cool (40 to 50°F), dry location.
Carefully dig up the caladiums when the foliage droops and begins to yellow with the onset of cool fall temperatures, or wait until after the first hard frost. Gently shake the soil from the plants. Then, cure the caladiums in a warm, dry location for 1 or 2 weeks. Afterward, cut off the dry foliage and bury the tubers in dry peat moss or vermiculite. Store the tubers in a cool (60 to 65°F), dry location.
Dig up the plants after the first fall frost. Cut off the foliage. Dry the corms or tubers in a warm, dry location for 1 or 2 weeks. After drying, bury the corms or tubers in peat moss or wood shavings and store them in an area with a temperature of 55 to 60°F. This storage temperature is a little warmer than most tender perennials.
Some species and varieties do not form underground structures like corms or tubers. Overwinter these plants as houseplants in a warm, bright location. Bring them indoors before nighttime temperatures drop below 45°F
Several days after a hard frost cut the plants back to within 2 to 4 inches of the ground. Carefully dig up the tuberous roots with a spade or shovel. Gently shake off the soil and cut the stems back to the crown. Wash the tuberous roots to remove any remaining soil. Allow the tuberous roots to dry to the touch (usually about one day). When dry, place the dahlia clumps upside down in boxes or other containers and cover them with vermiculite, peat moss, or wood shavings. Store the dahlias in a cool (40 to 50°F), dry location.
Carefully dig up the plants with a spade in late summer/early fall. Gently shake off the soil from the bulb-like corms. Then, cut off the foliage 1 to 2 inches above the corms. Dry the corms for 2 to 3 weeks in a warm, dry, well-ventilated location. When thoroughly dry, remove and discard the old, dried-up mother corms located at the base of the new corms. Remove the tiny corms (cormels) found around the base of the new corms. Save the small corms for propagation purposes or discard them. Place the corms in mesh bags or old nylon stockings and hang them in a cool, dry, well-ventilated location. Storage temperatures should be 35 to 45°F.
After a killing frost, cut off the foliage 1 to 2 inches above the soil surface. Carefully dig up the rhizomes. Do not cut or injure the rhizomes. Dry the rhizomes in a warm, dry location for 1 or 2 weeks. After drying, bury the rhizomes in peat moss or vermiculite and store them in a cool (50 to 60°F) dry location.
Dig the entire plant up just before the first frost. Do not cut leaves or stems back. Leave some soil around the roots and wrap the root ball in a plastic bag. Store the uprooted plant in a cool, dark location (45 to 50°F). Allow the leaves and stems to dry down naturally. Remove the dry, brown stem before replanting outdoors in spring after the danger of frost has passed.
Carefully dig up the tuberous begonias within a few days of a killing frost. Leave a small amount of soil around each tuber. Cut off the stems about 1 inch above the tubers. Place the tubers in a cool, dry area to cure for 2 to 3 weeks. After curing, shake off the remaining soil and bury the tubers in dry peat moss, vermiculite, or sawdust. Store the tubers in an area with a temperature of 40 to 50°F.
More information about overwintering tender perennials can be found in this publication: Growing and Over-wintering Tender Perennials (pdf)