Friday, November 1, 2013

Nancy J. Ondra: Overwintering Tender Bulbs

This past summer marked my first as a dahlia grower. I planted six varieties in our front garden and,  apart from some struggles with goldfinches plucking the petals from unopened buds, and numerous earwigs, once the dahlias started blooming, they provided me with dramatic bouquets of cut flowers into October.

But now the colder weather’s arrived and after last weekend’s frost, the leaves on my dahlias have turned black. After reading online about different methods for storing dahlias over the winter and polling fellow Storey employees about their techniques, I decided to go to Nancy J. Ondra, author of numerous gardening books including the best-seller, The Perennial Care Manual, for advice.

Cannas, dahlias, pineapple lilies (Eucomis), and other tender bulbs bring months of beauty to your garden in the form of gorgeous flowers and often lovely leaves as well. This beauty doesn’t come cheap, though; in fact, a single bulb (or tuber or rhizome) of newer introductions can cost as much as a small shrub or tree. That makes them a potentially pricey proposition if you live where they can’t survive the winter outdoors and you treat them like annuals. But if you put a little effort into digging them up each fall for indoor storage, you can enjoy bigger and better bulbs year after year.

Dahlia clump

There are a couple of approaches to overwintering tender bulbs. Some gardeners like to dig them up, remove all the soil, and then store them in sawdust, vermiculite, or some other loose material. I tried that at first, but my bulbs ended up either rotting or shriveling by spring. Eventually, I tried leaving the soil around the bulbs, and that turned my results upside down: instead of losing 90 to 100 percent of my bulbs, I was keeping that many.

Canna clump

Once frost nips back the tops in fall, cut the stems back to 6 to 12 inches (toss the tops into your compost pile), then dig up each bulb clump. To minimize the chance of cutting through the bulbs, insert your shovel blade vertically into the soil, 6 to 8 inches deep, about 6 inches out from the outermost stems. Push down on the handle to lift the blade and pry out the whole clump, keeping as much soil as possible around it. Put the clumps into plastic shopping bags (one kind per bag), and write the name on the bag with a permanent marker. If a clump is too big to fit in a bag, either split it in two or put it in a wooden or plastic crate or heavy cardboard box lined with plastic.

Dahlia, bagged and labeled

If you grow more than one kind of each bulb (several different canna or dahlia cultivars, for example), dig and pack only one kind at a time, so you don’t get them mixed up. When you’re all done digging, fill the holes left in your garden with finished compost.

I store my tender bulbs in my unheated basement, which stays between 40° and 45°F through the winter. A cool but frost-free porch or garage would work too. Wherever they stay, the bulbs don’t need light, because they’re not actively growing. Peek into the bags every 3 to 4 weeks through the winter, and if the soil looks dry, add a few cups of water to each clump. I’ve found that it’s better to keep them more on the dry side than wet, but don’t let them dry out completely for months.

Bagged bulbs in the basement

Start watching for new growth in March and April, and try to get your tender bulbs back outside before that new growth is more than a few inches tall. If you want to expand your plantings, divide the clumps before putting them back in the ground.

Nancy J. Ondra is the author of The Perennial Care Manual, Foliage, and Grasses, and of Five-Plant Gardens (forthcoming from Storey). She blogs at

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