Cold-Climate Gardening

Frost in July! Many tough perennials, like this bee balm, can withstand an occasional unexpected frost without too much damage. Photo/Illustration: Amy Hinman

The first year I gardened in Montana, it snowed on August 21. That’s when I realized this Ohio gardener had a lot to learn about a cold and unpredictable climate. Yet my new neighbors weren’t much help. As I carted around rock and trucked in topsoil to build raised beds, all they said was, “I’ve lived here for 20 years and can’t grow a thing.” Fortunately, listening has never been one of my strong points.

Not one to be deterred, I’ve rushed outside in the middle of the night as temperatures plummeted to cover dozens of raised beds. And it’s at just such moments that I’ve questioned my own sanity. While USDA Hardiness Zone 3 (where winter temperatures can dip as low as –40°F) is not especially conducive to growing heat-loving plants, it has provided me with an opportunity to learn how to make the most of a short growing season and to extend that season as long as possible.

Start with tough perennials

The first secret to gardening in a cold climate is to grow really hardy perennials, and there are a number of plants that are troopers no matter how harsh the winter or spring. As a general rule, those that die back to the ground each fall do better than evergreen perennials, although I have successfully overwintered both lavender (Lavandula angustifolia ‘Munstead’) and sage (Salvia officinalis).

The first plant to force its way through the last of the snow is perennial cornflower (Centaurea montana). It often blooms by the beginning of May, when I am in desperate need of some color. After it flowers, I cut it back by two-thirds and it blooms again by the end of summer. Another early riser is oregano (Origanum vulgare), which I grow not only for its use in the kitchen, but also for the flowers it produces. Artemisias (Artemisia ludoviciana ‘Silver King’ and ‘Silver Queen’) poke through the soil much later, but are consistent every year, along with other herbs like valerian (Valeriana officinalis), bee balm (Monarda didyma), and anise hyssop (Agastache foeniculum). These tough plants will continue blooming through an unexpected mid-summer frost (photo, above).

Domesticated varieties of wildflowers (which I prefer for their showier blossoms) also do well. Lupines (Lupinus cvs.) are early summer bloomers that generously reseed themselves. Delphinium cultivars—6-foot giants compared to their cousin, the native larkspur—love the cool temperatures and pull through most winters without a problem (photo, above). Their biggest enemies are the wind and hail that often accompany summer thunderstorms. To keep these delphiniums from being blown over, I stake them, plant them near buildings, and grow the shorter varieties.

Peonies (Paeonia cvs.), bearded irises (Iris cvs.), sneezewort (Achillea ptarmica ‘The Pearl’), sea holly (Eryngium maritimum), and tansy (Tanacetum vulgare) are also sound choices for tough climates. I never worry about their surviving, and they produce fantastic flowers every year.

Fill in with cool-season annuals

It’s a short but colorful season in this garden just outside Glacier National Park, but self-sowers like poppies still put on a colorful display.

I love annuals for their bright color and the distinct personality they add to the garden in summer. I start thousands in my greenhouse each year, sowing them in early March for blooms by mid-July. I look mostly for fabulous color and fragrance, but I’ve also discovered many that are cold tolerant. By experimenting (and sacrificing many plants), I’ve found varieties that will even withstand light frost.

Stock (Matthiola spp.), an annual featuring spikes of flowers with a spicy, carnationlike fragrance, is one I can set out knowing it will withstand subfreezing temperatures. It might not be happy to have its leaves frostbitten, but I’ve never lost it to a late cold snap. Evening-scented stock (Matthiola longipetala ssp. bicornis), a knockout in the fragrance department, often reseeds profusely. Clarkia species and annual poppies (Papaver somniferum) also selfsow from year to year in my garden, adding drifts of color here and there.

Through the years, I’ve found that when these self-sown annuals are about 3 inches tall, it’s usually safe to transplant the annuals I’ve started in the greenhouse and hardened off. If we have a heavy frost, the leaves of transplants often turn reddish in color, but they usually survive. Those that come up on their own tend to show less stress. One thing to keep in mind when growing annuals from seed is how long it takes them to produce flowers from the time of germination. In my garden, those that take 120 days don’t bloom until the end of the season, if at all, so I look for annuals that flower sooner.

Many biennials, which have a twoyear life cycle, are also good choices for cold climates. I started parent plants of foxglove (Digitalis purpurea), sweet William (Dianthus barbatus), and hollyhock (Alcea rosea) in the greenhouse for three years in a row and then let them self-perpetuate in the garden. I’ve found that they are easy to transplant if they don’t sprout where I want them.

Raised beds and row covers extend the season

Instead of tilling the earth each spring, I garden in raised beds built from native stone. Although they are raised only 6 to 8 inches, these beds warm up earlier, and their soil dries faster than surrounding ground, which remains cold and water-logged from melting snow.

For transplants or tender annuals that could be nipped by frost, I use floating row covers for protection. The fabric is light and breathable, so it can stay on throughout June. I’ll often cover up plants again late in the season if they are still flowering when a killing frost is forecast.

A floating row cover protects the tender buds of cosmos from midsummer frosts. Row covers can also be used to get a jump on or to extend the growing season.

The greenhouse becomes a garden in summer. After seedlings are moved out, beds are planted with annuals and tomatoes.

And finally, I make the most of my greenhouse space. I plant heat-loving tomatoes, peppers, and nasturtiums (Tropaeolum majus), along with later-blooming annuals like Cosmos bipinnatus and Nicotiana sylvestris, in several indoor beds after hundreds of flats of seedlings have been moved to the garden (photo above, on right). When the weather is warm, I leave the doors open, but by August, I close the greenhouse to hold in the heat. It’s a little extra effort at the end of an intense season, but it’s worth it to enjoy the flowers just a little while longer.