As weather cools, it is time to plant spring flowering bulbs, but it is also time to dig up the summer flowering ones too. This includes plants with tender tubers, rhizomes and corms, such as dahlia, canna and gladiolus. The bulbs are dug after the first hard frost in the fall and stored over the winter in a place where they will stay cool without freezing.
The best time to dig is about two weeks after the plants are hit by a hard frost, giving the plant tops a chance to die. After waiting two weeks, cut the plant tops to about 6 inches from the ground and dig up the tubers, corms or rhizomes. Use a garden fork to carefully lift the bulbs out of the ground, taking care not to injure them.
Shake off as much soil as possible and then clean off the remaining soil with a gentle stream of water from the hose, again being careful not to injure the bulbs. Cut the remaining portion of the stem back to 2 inches. Next, place them in a single layer on newspaper or cardboard and allow them to dry in a protected spot, such as your garage, for a couple of days.
-- Dahlias: During the summer, the dahlia tubers will have developed into a cluster of tubers. These will be divided in the spring before replanting. Do not cut them into separate tubers before winter storage. Use a permanent ink marker to write the name of the cultivar directly on several tubers in the cluster, or write it on the bag if you store clusters in individual paper bags.
Tubers should be stored in cardboard boxes or paper grocery bags. Start with a generous layer of dry packing material, such as sawdust, peat moss or vermiculite, on the bottom of the box or bag. Place the tubers in a single layer on top of the packing material, and then top them off with another generous layer of packing material that totally covers the tubers. Store the boxes or bags in a cool location (40 to 50 degrees) where the temperature will not go below freezing during the winter. Next year's growth will sprout from the buds that are situated at the crown at the base of the old stems.
-- Canna: Cannas have rhizomes. They are treated the same as dahlia tubers for digging and storage. The clump of rhizomes can be replanted next spring or divided and replanted. New growth next year will come from buds at the base of this year's stems.
-- Gladiolus: Gladioli grow from corms that resemble bulbs. These can be dug six to eight weeks after all the flowers have faded or after frost kills the leaves.
The plants and corms are dried, or cured, for a longer period of about two to three weeks in a warm (80 degrees), dry place out of the sun. When dry enough, the old shriveled corms and tiny corms (called cormels) can be easily broken away from the base of the new corms and discarded. Do not remove the protective husks around the corm. Gladiolus corms do not need to be packed in dry packing material.
Place corms in mesh onion sacks or open paper bags that allow for good air movement. Store them under cold conditions (35 to 40 degrees) with low humidity where they will not freeze.
This may sound like considerable effort at a time of year when we are almost happy to see the end of the season. Do we really have to dig and store them each year?
Some years, dahlias and gladioli may survive if left in the ground, but it is hard to predict the severity of winters. Replacing all your summer bulbs can be costly. If you cannot bear to lose your summer bulbs or your money, it is best to dig and store them.
-- Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.