Leave Dahlias In Ground, Or Lift Them Out For Winter

Q: What should I do with gladioluses and dahlias that are turning yellow and have fading flowers? When should I cut them?

A: Several readers have asked about dahlias, wondering when to plant them to get the spectacular late-summer bloom they offer. Plant in spring after danger of frost has passed. Tubers are then available from specialists and in nurseries.

Opinions differ about whether dahlias need to be lifted and stored, or whether they can be left in the ground to winter over and grow again in spring after temperatures warm up. If you want to leave the plant in the ground, cut off the yellow foliage when it has died and put about 2 inches of any organic mulch over the plant.

Dahlias left in place eventually become large tuber masses with shrubby growth that may not be desirable, although some gardeners have mulched their plants and had dahlias reappear satisfactorily for years. A hard, deep frost such as the one here in 1990 can kill dahlia tubers outright, so leaving them in the ground is a calculated risk.

Local dahlia growers advise waiting to dig dahlias until about Nov. 1 or just after the first frost. Cut off the foliage, leaving about 8 inches of stalk at the top, about one week before digging them up. Lift carefully with a spade or fork, beginning to dig at least 12 inches away from the center of the plant. Dahlias grow from small individual tubers into masses of underground tuber in summer, and it's easy to break them when digging.

Lift the entire plant and take care to keep the tubers attached to the "neck," which is last year's center stalk. New plants will develop only from tuberstthat were previously attached to that stalk.

Shake off the soil, and let the clump sit in a cool, dry place until the surface is dry to the touch. Some growers immerse the tubers in a 10 percent household-bleach solution. Tie on a label. Store the dahlias in a dry, dark, cool, frost-free area in boxes of dampened sawdust, peat moss or vermiculite.

Check them during the winter for signs of rot. Redampen the storage materials if the dahlias are drying up. The object is to keep them alive, but not to get them growing.

Check in late February for the signs of "eyes," little pinkish knobs on the ends of tubers indicating that growth points for new stalks are present. Divide the tuber mass into growing tubers. Plant in sunny areas after frost danger is over.

Let gladiolus foliage get completely brown, and then dig out the bulbs; each one will probably be surrounded by dozens of "cormels," tiny grown miniature corms that, if planted out in warm weather, will produce flowers in subsequent years.

Store them in brown paper bags or spread out in flats. Air circulation is necessary. They can be stored dry; the moisture essential to dahlia tubers is not needed for gladioluses. In some milder areas, gladioluses left into the ground will winter over.

Gardening runs Friday in Scene and Sunday in Home/Real Estate. It is prepared by Mary Robson, area horticulture agent; Holly Kennell, Washington State University/King County Cooperative Extension agent, Susan Miller, integrated-pest-management specialist, and volunteer Master Gardeners.