Dahlias come in a wide range of shapes, sizes and vivid colors, blooming from late spring to frost in most regions of the United States. Sprouting from bulbs, they can reach heights of 2 to 3 feet, with some varieties producing flowers as large as softballs. Dahlia bulbs cannot survive winters below U.S. Department of Agriculture hardiness zone 6, so New Mexico State University recommends digging and storing them after the first light frost of fall.
Dig dahlia bulbs before the first hard frost. A light frost will not harm them, but successive nights at temperatures lower than 32 degrees Fahrenheit will. Spade up the bulbs, trying not to pierce them with the point of the spade.
Knock the excess dirt from the bulbs, then wash them clean with a hard spray from a garden hose. Spread them on a few sheets of newspaper in your basement or shed to dry.
Examine the bulbs for damage or insects and save only the healthiest bulbs. Small areas of rot on the surface can produce fungus that will destroy the whole lot over the winter.
Spread a 2-inch layer of slightly moist peat moss in the bottom of a cooler chest. Lay the dried bulbs on the surface and cover them with another 2 inches of slightly moist peat. Continue with successive layering until all the bulbs are stored.
Close up the chest and place in an area where it will not freeze. Check it once a month to see that none of the bulbs are spoiling. Replant in spring when the last frost has passed.
If you don’t have an old cooler for storing dahlia bulbs, put them in a plastic bag with the moist peat and it will have the same effect.
In warmer climates, you may not have to dig the bulbs at all, but simply cover them with a 4-inch layer of mulch to regulate soil temperature.