Dahlia (Dahlia spp.) bulbs actually are tubers and have eyes like potatoes. They can remain in the ground during the winter in U.S. Department of Agriculture plant hardiness zones 8 through 11 if properly protected. Elsewhere, you need to dig those tubers in autumn and store them in a chilly indoor location until late spring.
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USDA Zones 8 Through 11
In the zones where dahlias are hardy, you need only take a few simple steps to protect your dahlias over winter:
- Snip the foliage off at
ground level about one week after it is killed by frost or in mid-November in frost-free climates.
- Lay a sheet of plastic over the dahlia bed.
- Pile about 6 inches
of mulch, such as straw or dead leaves, on top of the plastic.
- Remove the mulch and plastic in
early spring to allow the soil to warm faster.
Spade up and divide your dahlia clumps in spring at least once every two to three years.
USDA Zones Lower Than 8
Dig Dahlia Tubers
In colder zones, wait to dig your dahlia tubers until about two weeks after a hard frost has shriveled their foliage. First, cut the stems back to about 6 inches above the ground, discarding the dead foliage. Then dig the clumps of tubers, hose the soil off of them and remove any damaged ones. Attach aluminum labels to the cropped stems with wires or write the dahlias' names directly on their cleaned tubers with an indelible marker.
Afterward, upend the tubers in a protected location and allow them to dry for one or two days. You can divide individual tubers from the clump at this time, if you prefer, or wait until spring when the eyes are more visible.
Select a Storage Site
The tubers' storage site must be dark and cool, such as a chilly basement, root cellar or garage. Ideally, the temperature in that site should remain between 40 and 50 degrees Fahrenheit over the winter, with a humidity level near 50 percent.
Store Dahlia Tubers
Place the tubers in large paper bags, cardboard boxes or crates lined with newspaper. Cover those tubers with a light and barely damp medium, such as peat moss, sand, sawdust or wood shavings, with the crown of each clump protruding. Close the lids on the boxes or fold over the tops of the paper bags and clip them shut with clothespins.
Prevent Dahlia Dehydration
Inspect the tubers about once a month and sprinkle them lightly with water if they appear to be drying up. In that case, you also may want to replace your porous containers with nonporous ones -- such as plastic garbage or grocery bags, plastic or plastic-lined boxes, or plastic buckets – filled with one of the lightweight mediums already mentioned.
If you divide your dahlia clumps in autumn, try encasing individual tubers in plastic wrap for storage.