Green Thumbs Up: Tips for wintering tender plants and bulbs

Suzanne Mahler

A mild and blustery All Hallows’ Eve and intermittent showers this week have undressed many of our deciduous trees and shrubs as our landscapes gradually prepare for the chilly winter season ahead.

Despite many frosty evenings during October, particularly in low-lying pockets, many communities have yet to experience a hard killing frost. While many of my tender tropical and annual plants in open areas lie limp, black, and withered, similar plants in more sheltered corners continue to linger, offering welcome reminders of summertime splendor, but with chilly nighttime temperatures predicted for the coming week, the time has come to begin putting the gardens to bed.

Once killing frost blackens the foliage of tender “summer bulbs” including cannas, calla lilies, caladiums, tuberous begonias, dahlias and elephant ears, digging and storing these summertime treasures moves to the top of my priority list since prolonged exposure to freezing temperatures can damage their fleshy storage systems. Use a digging fork when lifting these tender bulbs to reduce the risk of bruising, slicing or injuring the bulbs, which could lead to disease or rot during their winter storage.

Cannas, callas, caladiums and tuberous begonias should be carefully dug and dried in an airy, dark place without danger of freezing for a few days. Cut off dead foliage and gently brush off dried soil. Occasionally, I find it necessary to use a strong spray from a hose-end nozzle to dislodge clumps of heavy soil clinging to these bulbs prior to setting the bulbs out to dry. Take time to inspect dried bulbs for wounds, the presence of clinging soil or uninvited pests.

The showy dahlias were spectacular this season once they started to bloom during the latter half of the summer. To preserve these tubers, which resemble a small cluster of little sweet potatoes, cut the stems back to 4 to 6 inches once their foliage has been blackened by frost and cover the cut ends with aluminum foil or plastic wrap to prevent moisture filling the hollow stems.

Dahlias that have been cut back should be allowed to cure in the ground for a week or two prior to digging to help the tubers form strong “eyes” for the following growing season. Carefully dig around the perimeter of the clump with a digging fork to loosen the soil and gently lift the cluster of tuberous roots. Wash the soil from the tubers and allow them to dry completely in an airy, dark place without danger of freezing for a few days, but do not allow them to become shriveled. Cut off dead foliage and gently brush off any remaining dried soil.  

Store healthy bulbs and the tuberous clusters of dahlias in plastic produce bags that breathe, brown paper bags, or in porous containers such as bushel baskets, wooden crates or laundry baskets lined with newspaper that have been filled with peat moss, sawdust, perlite or coarse vermiculite. Keep the storage containers in the dark between 40 and 50 degrees and check periodically during the winter months for any evidence of rot and discard if spoiled.

In recent years, I have chosen to grow many of my tender summer bulbs and tropical plants in containers and have had great success over wintering these plants in my unheated garage.

Tropical plants, including the beautiful sky-blue Plumbago, Angel’s trumpet (Brugmansia), gerbera daisies, lantana, Phormium, "Black and Blue" salvia and many others are transferred to the garage prior to hard killing frosts.

Since temperatures occasionally drop into the 20s during the winter months, many of these plants drop all their leaves, but nearly all the plants revive once springtime returns. I leave the pots containing dahlias, cannas and other summer bulbs outdoors until frost blackens their tender leaves and once dormant, the foliage is cut back and the containers are moved into the garage.

Although temperatures often drop slightly below freezing, the soil in the containers offers protection for the bulbs, which begin to emerge as spring arrives; a little water, fertilizer and an occasional rejuvenation of the soil as the plants break dormancy has enabled me to carry these bulbs over for many years in the same containers. A chilly basement with temperatures in the 40 to 50 degree range could also serve as an alternative for winter storage.

As in previous years, many of my recent acquisitions have yet to find appropriate homes in the landscape necessitating quick action to ensure their survival until next spring.

Potted plants left unprotected above ground often succumb during the winter months due to severe cold or dampness accompanied by alternate freeze-thaw cycles. Some of these treasures will be heeled in, pot and all, but with limited open space in my gardens, most of these hardy orphans will join their tender counterparts in my garage after several killing frosts have caused them to go dormant but before temperatures drop consistently below freezing. The soil should be slightly moist and barring a very dry environment in your garage, they should require little or no attention for the entire winter.

They can be moved back outside in April. Avoid unheated sunrooms as warm temperatures on sunny days may cause the plants to break dormancy at inappropriate times. Sheds could be used as an alternative but may not provide sufficient protection from the cold and might be subject to rodent invasions.

While some tender plants may not survive the winter months indoors, many of these tropical treasures will recover admirably and with minimal attention during the early days of springtime, they will flourish, bigger and better next season.  

Suzanne Mahler is an avid gardener, photographer and lecturer who has been developing the 1.5-acre property surrounding her home in Hanover, Mass., for more than 30 years. She is a member of two local garden clubs, past president of the New England Daylily Society, an overseer for the Massachusetts Horticultural Society and is employed at two garden centers.