How Winter-Hardy Are Your Glads? Our Newsletter Readers Report

Although most references say gladiolus won’t survive winters north of zone 8, we kept hearing from customers that their glads were returning in zones 7, 6, and even 5. What’s more, these weren’t just Byzantine, ‘Boone’, or ‘Carolina Primrose’ glads — all of which we’ve found to be reliably hardy in zones 6a and warmer. These were “regular” glads they were talking about.

In zone 6a Concord, Massachusetts, for example, Jane Murphy wrote, “Some of the overlooked gladiolus bulbs I left in the garden last winter flowered this year, including a lovely ‘Spic and Span’ in October.” Kathi Frank of zone 5b Onsted, Michigan, emailed us, “I just have to tell you my joy when my ‘Atom’ glads survived the winter and came back this summer as beautiful as ever. What a bonus!” And Meredeth Allen of zone 5b Francetown, New Hampshire, called to say she “always” leaves her glads in the ground and “they come back every year.”

Then our good customer Bill Killpatrick of Lafayette, New Jersey, made a request. “I’d love it if you’d ask your newsletter readers about glads,” he wrote. “I’m just getting too old and creaky to dig ’em all up. Find it easier to just buy new every spring. But, much to my surprise, for the past four winters, a good 80% of the corms have wintered over just fine right in the garden. Officially, I’m in zone 6, but it’s a cold zone 6. We’ve NOT had reliable snow cover, I don’t mulch, nuthin’. But come spring, up pop the glads — big, double-corm, monster glads.”

So we asked our readers, and we heard from many in zones 5 through 7 whose glads over-winter in the ground and come back like perennials to bloom year after year. How can that be? Here’s what they said, along with a few possible explanations.

Warmer, Shorter Winters

The biggest factor may be that winters have been getting warmer. It’s not just that the average lows aren’t as low but also the cold isn’t lasting as long. As a result, the ground doesn’t freeze as deeply, and any corms that don’t freeze will usually sprout in the spring.

In 2012, the USDA updated its Plant Hardiness Zones Map for the first time since 1990 based on more recent and extensive data and the use of sophisticated mapping algorithms. According to the new map, many of us are now gardening in a warmer zone. Here in Ann Arbor, for example, the old map said we were in zone 5b but the new map says we’re in zone 6a — and we’ve definitely seen the truth of that in our gardens.

Spic and Span Gladiolus
‘Spic and Span’

In zone 6a Northville, Michigan, Cindy Bullington has seen it, too. She writes, “My glads not only survived without any special care over the past several winters, they became somewhat invasive. I had to substantially thin the gladiolus patch this fall. Love that global warming!”

Sabrina Sheikh of zone 6a Lexington, Kentucky, reports even broader effects in her garden. “I have a number of glads so ordinary they came from the local supermarket,” she writes. “I’ve left them in the ground for about six years now, and they always come back and bloom. I never do anything special for them, and though I have lost a few over the years, most have actually multiplied. Many other plants that are not supposed to survive here have been doing so, too. Tall snapdragons, for example, are now perennials, and when I last looked at my herb garden, I was shocked to see that, although damaged, my parsley and eucalyptus (!!!) were still alive. Climate change? That’s my guess.”

We encourage you to look up your new zone here — but warmer winters aren’t the only thing helping glads return and rebloom.

Reliable Snow Cover

Snow is a great insulator because it holds a lot of air, much like fiberglass insulation or your grandmother’s quilt. In fact, a foot of snow equals about an inch of rain — the rest of the volume is air trapped in the tiny spaces between the flakes. That means reliable snow cover will keep soil from freezing as deeply as it would otherwise, protecting corms that would have frozen to death without it.

For example, Linda Eastman writes from deep in the Adirondacks, “I have seen glads survive in zone 5a Lake Luzerne, New York, for several winters. No cover, special treatment, or anything. Of course there was lots of snow! And here in zone 5a Broadalbin I had a glad live about six years without being dug up, till an extremely cold and snowless winter finally got it.”

In her garden in zone 5b Galva, Illinois, Diane Gibson has seen the insulating power of snow, too: “I sometimes have glads over-winter, but it is usually when we have deep snow all winter and especially if they are planted around something that catches the snow drifts. Always a wonderful surprise.”

Airy Winter Mulch

Although snow is the best insulation, few of us can count on reliable snow cover all winter long. That’s the reason for winter mulches. Unlike common summer mulches such as pine nuggets, winter mulches are meant to capture and hold air, including the air in snow. The air does the insulating, not the mulch itself, so choose materials that won’t mat down easily, such as straw or hay (fluff it up as much as possible when spreading it), oak leaves (which remain stiff and rippled longer than most leaves), evergreen branches, and to a lesser extent shredded bark, shredded leaves, and pine needles.

Winter mulch may also help glads by shedding winter rains and keeping the soil around them drier which is what’s best for all bulbs when they’re dormant.

Linda Galante of zone 6a Livonia, Michigan, understands the value of mulch. Although she plants her glads along the warm south side of her brick house (see Microclimates, below), she also says “I mulch heavily with shredded leaves in November, and pray for snow cover as well.”

Fidelio Gladiolus

In zone 6b New Holland, Pennsylvania, Robert Hohl mulches with 2-3 inches of grass clippings. Though grass mats down and so doesn’t provide much insulation, it’s working for him. “I’m in my 80s now,” he writes, “and with each passing year digging my glads became more of a chore. So three years ago I experimented and left about a quarter of my 200 bulbs in the ground. They all came up. So two years ago I left more than half in, and again they all came up abundantly. So last fall I left them all in.”

Paul Begley of Columbia, South Carolina, reports a historic mulch for glads. “My grandmother lived in the mountains of [zone 6] eastern Kentucky,” he writes, “where it gets quite cold. She never brought her glads in for the winter, but covered them with the ashes and cinders from her coal furnace. The glads not only came through the winter, but over the years spread into a large clump. I know there aren’t many coal furnaces around any more, but maybe something similar would do the trick.”

Deeper Planting

Soil is a great insulator, too, and deep planting is another way to protect corms from freezing. If the cold can’t get down to them, they remain safe and viable.

From zone 6a Lamont, Michigan, a reader who prefers to remain anonymous writes, “I always plant my glads very deep in order to increase the support for the stem and prevent tipping. When I first planted them here, I did nothing to over-winter them because I intended to treat them like annuals. So I was surprised when they returned the next summer. Now ten years later I have come to expect it. Of course it is possible that with our recent mild winters the ground is simply not freezing as deep as I plant my glads — about 10 inches. But so far so good.”

Don’t plant them too deep, though, or your glads may never make it to the surface. Deep planting is safest in sandy soils, since they are lighter and easier for the growing foliage to penetrate.

Sandy Soil and Good Drainage

Sandy soil helps glads in many ways. Water drains through it more quickly, so the dormant corms stay drier in winter and are less prone to rot. Sand particles are larger than clay particles, so there’s more room between them for air, which means sandy soil is a better insulator. And bulbs have an easier time expanding in sandy soils, which means they’re often bigger, healthier, and better able to survive the rigors of winter.

Apricot Lustre Gladiolus
‘Apricot Lustre’

Rhea Dow writes from zone 5b Charlevoix, Michigan, “Yes, glads do survive our winters here. The key is sandy soil and snow cover.”

Slopes also provide good drainage, which may be one of the reasons why David Tomeo of hilly zone 6b Pittsburgh — as well as Linda Eastman in the Adirondacks and Paul Begley’s grandmother in the Appalachians — has had such good success over-wintering glads. (Or is it the compost?) David writes, “Not only do regular glads over-winter in my garden, they spread. Maybe 8-10 years ago when I first planted them, I dug them in fall. Not wanting to be bothered and noticing they were coming back, I quit digging, and they’ve been spreading ever since. I don’t mulch. I do dig-up my beds every 4-5 years to replenish with compost. And any bulb dug up is covered in bulblets.”

Micro-Climates Large and Small

In every hardiness zone there are warmer and colder spots, some large, some small. Urban areas, for example, are usually warmer than more rural areas, as Sherri Ribbey, assistant editor of Garden Gate magazine, recognizes. She writes from zone 5b Des Moines, Iowa, “I’ve had glads come back for maybe five years. I just happened to pick up a bag at a discount store and a few have shown up every year since. I think it helps that I’m smack-dab in the middle of the city and not out in a new suburb or the country where the wind can be pretty fierce and the temperature a little colder. Whatever the reason, it’s nice to have them.”

Every yard has warmer spots, too, where the snow melts first in spring and annual flowers keep blooming long after everything else has been blackened by frost. When you plant glads in these warm micro-climates, you give them a better chance of over-wintering.

In zone 6b Versailles, Kentucky, Sara Hellard writes, “I’ve planted glads in two places by the foundation of our home and they have wintered over quite well with minimal mulch cover.”

Linda Galante of zone 6a Livonia, Michigan, reports similar success: “I have planted all my glads along the south side of my home, and due to the southern exposure and our light colored brick wall I have yet to NOT have glads return. Love, love, love them.”

Jo Sharon of zone 6a Glastonbury, Connecticut, writes, “I’ve had glads survive the winter in both snowy years and dry years. Who knows the reason? My next door neighbor had one survive for years but it was near the heated floor slab of the house. I think the micro-climate there was at least zone 7.”

Susan Krobusek of zone 6a Farmington, NY, also understands the importance of micro-climates — and more intangible factors, too. “I’ve had glads overwinter here in western New York for several years,” she writes. “They don’t all bloom, but they’re still alive. I’m not sure why. One batch was up against the western foundation of the house, so there may have been enough reflected heat to keep them happy there. Another group is in a bed that faces south and east, and has some shelter from the north and west winds, so that could do it. Or it could be that my plants all know how much I love them, and they can’t bear to disappoint me!”

Plenty of Sun

Glads grow best with full sun all day long, and the stronger and healthier any plant is, the more likely it is to make it through the winter. Sunny sites usually don’t freeze as deeply, also, which is another good reason to plant your glads in the sunniest spot you have if you want them to return.

Are Heirlooms Hardier?

White Friendship Gladiolus
‘White Friendship’

Although that’s not a claim we’re willing to make, we will say that any glad that’s survived from the 1960s or before has to be unusually healthy and vigorous, and those qualities may very well help it over-winter more successfully.

Kerry Hoffman in zone 6b Watsontown, Pennsylvania, couldn’t agree more. “I grow cut flowers for market, specializing in heirlooms, and all of my glads are from Old House Gardens,” he writes. “Almost all of the glads I have planted in my cold zone-6b field have returned. ‘Atom’ is astounding in its hardiness, sending up baby shoots as well. I also have white glads that return strongly [‘White Friendship’ and ‘White Goddess’], and a few violet [‘Fidelio’ and ‘Violet Queen’] and apricot colors [‘Apricot Lustre’]. I honestly believe that heirloom varieties are tougher, hardier, and less disease-prone than over-hybridized, over-tweaked new varieties.”

“You Just Never Know”

So can anything guarantee that your glads will get through the winter in zone 5, 6, or 7? Probably not, but warmer winters, reliable snow cover, winter mulch, deep planting, sandy soil, good drainage, micro-climates, plenty of sun, and time-tested vigor all seem to be part of the equation. The only way to know for sure if glads will come back in your garden is to give them a try. Most are pretty inexpensive, and experimenting is one of the great pleasures of gardening.

Here’s one last report to encourage you, from Irvin Etienne, Horticulture Display Coordinator at the Indianapolis Museum of Art. He writes, “I had a few varieties of glads in our [zone 6a] cutting garden that just started looking crummy, so I decided one fall to leave them in the ground and replace them in the spring. I never ordered new ones, though, and for three years now several have come back big and healthy and blooming the way a glad should. These are in an open area, no protection. I’m not talking 100% survival but pretty darn good. We’ll see what happens this fourth year. You just never know.”