Ask the Gardener: How to care for dahlias over the winter

Ask the Expert Gardening
dahlias Tower Hill Botanic Garden
Dahlias turbocharge the fall garden. Tower Hill Botanic Garden

What to do this week: That thud your heard is the sound of the long growing season closing with a very hard frost on Nov. 9. The planting window is now shut, except for spring-flowering bulbs, which you can plant into December. But don’t dillydally. This is also the time to cut down nonwoody plants and brush and to top-dress the garden thinly with nutritious aged compost, or spread it thickly with weed-suppressing mulch. You can also do this next March or April, which has the virtue of leaving the debris as winter food and shelter for birds, so don’t feel bad if you don’t get everything done. But do store outdoor furniture, ornaments, planters, and birdbaths that are not weatherproof. Drain, coil, and store hoses on a warm day, and turn off and drain outdoor taps. Throw invasive plants like bittersweet vine out with the garbage, not the yard waste.


Q. What a wonderful dahlia year! The secret is plenty of water, especially early on. I just dug up my tubers. They are huge. Now to store them. In the past I have done it in sand, successfully, spritzing with water if they look a bit too dry, but this year it will take a lot of sand. Do you have an alternative method?


A. Congratulations! Dahlias turbocharge the fall garden. It’s been a good year for most plants because we got so much rain through July it felt like we were living in Seattle. Experiment with different storage methods, including leaving some expendables in the ground covered with 10 inches of mulch. (Two of my dahlias and many gladioli survived last winter outdoors. Another guilty pleasure from global warming.) I wait about 10 days after the killing frost before cutting blackened dahlia stalks to 6 inches and lifting the tubers gently with a garden fork. (The cold develops thicker skins before storage.) I then put them in cardboard boxes labeled by variety — without any packing at all. The tubers need to dry out, you see. But I don’t want them to lose all of their moisture, so after a month in my 55-degree basement, they get misted every few weeks. In early April I discard all shriveled, broken, or rotten tubers. I then move them to a warmer place and cut the healthy remaining tubers apart with a sharp knife so each swelling bud is attached to a piece of last year’s stalk. (They should look like potatoes with eyes getting ready to sprout.) I pot them up in the dark basement then to reduce storage time and speed sprouting before transplanting them outdoors into the light. Or you can just plant tubers directly outside in late May when you put in tomatoes. I think some dahlia varieties store better than others, including Sunshine and Chilson’s Pride. I am going to save some effort by ordering new dahlia tubers next spring. It’s fun to shop online over the winter. If you wonder whether dahlias are worth the trouble in our climate, just check out on a winter’s day and enjoy the photos.


Q. Do I need to do anything to protect my new trees before winter?

A.K., Watertown

A. If they are evergreens, make sure they get enough water before the ground freezes, and spray them with deer repellent if you live in a rural area. Put plastic coils or a metal-screen tree guard around the base of very young ones. Use a paper roll of tree wrap for thicker trunks. This is especially important for fruit trees, which are often killed by hungry rodents nibbling a circle around their tasty bark in winter. (This is called “girdling.’’) Three inches of mulch over the root area will give some protection from frost heaves, but make sure it does not touch the trunk itself.

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