Leaving dahlia bulbs in ground late into season key for success

When should I dig my dahlia bulbs, and how can they be safely stored throughout the winter?

— Ila

The longer your dahlias are in the ground curing, the more fully developed the tubers will be and the better the likelihood of bringing them through the winter successfully.

We usually will have a light frost this time of the year that starts to kill down the top of the plant followed by a hard freeze a few weeks later. Dahlias continue to grow and the roots continue to mature after light frosts, so leave them as long as possible. You also need to cut the stems back a few days before digging. Leave at least a few inches of stem to facilitate handling the clumps easily. Just be sure to dig them up before the ground starts to freeze.

When you dig up the clumps, do so with care. Your dahlias are fragile, especially right after digging. Dig around on all four sides of the plant, about a foot away from the main stalk. Once you’ve loosened the plant from the fine feeder roots, push the shovel or spading fork under the clump and lift carefully. Gently remove any large clumps of dirt and turn the clump upside down to let the plant cure a bit. After a couple of hours, you can remove the dirt with less risk of breaking those fragile tubers. When ready to clean the clump, use a garden hose to wash away as much soil as possible as it can contain microorganisms that cause rot over the winter.

At this time, you can cut the clump into smaller pieces to make more plants for next spring. When dividing clumps, each division must have a piece of the crown with an eye and a big fat root (or part of one). Always sterilize your cutting tools after each cut, and while cutting the clumps, carefully inspect the divisions to make sure that there is no rot present. Remove all feeder roots and any stalk.

After dividing your plants, wash the tubers again to remove any dirt missed in the first washing. At this time you want to treat the cut ends with a fungicide, such as Captan or sulfur to prevent rot from occurring over the winter. You should then label each division with the variety name.

After cutting, dusting and marking the tubers, let them dry for 24–36 hours. Drying time varies depending on temperature, humidity and the size of the root. Be sure to only work on them in an area that won’t freeze.

There are numerous methods to storing tubers over the winter. I don’t think the method matters as long as you keep the tubers cool — above freezing but ideally below 50 degrees — and provide for an exchange of moisture between the tubers and the storage medium. Most people use vermiculite in plastic bags to store their tubers.

Add some vermiculite to the bag, put in some tubers and then add a layer of vermiculite. You can then add another layer of tubers and then more vermiculite. Some people use a separate bag for each clump to be sure that if one clump rots, it won’t spread to the others. Make sure each bag contains at least as much vermiculite (in volume) as tubers.

Some gardeners think the plastic bags must be air-tight while others only twist them so a little air can escape. In our area with our low humidity, I think you want the plastic bags to be more on the tight end rather than open. They don’t necessarily need to be sealed shut, just loosely twisted or folded over.

After filling the bags, keep them in the coldest part of the basement or garage or another cool area that will stay around 40–45 degrees. It’s important to store the tubers at a nearly constant, cool temperature. Freezing temperatures ruin tubers, and higher temperatures encourage microorganisms and fungus to destroy them and develop sprouts and feeder roots too soon.

Check on the tubers once a month during the winter. Throw away any that show signs of rotting. If you make sure the dahlia tubers aren’t touching when you pack them in the plastic bags, it will help keep any rot from spreading.

In early spring, move your dahlia roots to a warm location (dark, but room temperature) to encourage eyes to develop. Be sure to check the moisture in the bag, and protect the tubers from drying out. You can add a teaspoon of water per quart of bag space 15 days before removing the tubers.

Dennis Hill is the nursery manager at Bookcliff Gardens, http://www.bookcliffgardens.com. Send questions to Bookcliff Gardens, 755 26 Road, Grand Junction 81506; or email .(JavaScript must be enabled to view this email address).


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