Frost adds fleeting glitter to Northwest gardens

GARDENERS HAVE a flirtation with frost, especially in the Northwest where its glittering, destructive beauty is rare enough to be appreciated. Who can resist how it transforms gardens overnight? A hard frost pares down summer's denseness to reveal winter architecture. We might think our gardening tasks and their results are determined by sunshine, plant availability, rainfall. But, in fact, frost is the great decider. It neatly brackets the growing season between the first frost of winter and the last frost in springtime. This past autumn was a particularly dramatic example of killing frost. Temperatures plummeted into the teens before Thanksgiving, turning herbaceous plants into piles of black mush, and leaving procrastinating gardeners like me with bags of bulbs still needing to be planted.

According to the Old Farmer's Almanac, Seattle's frost-free growing season is, on average, from March 10 to Nov. 17. But there are frosts, and then there are frosts . . . A dip of the mercury even a degree or two below freezing is enough to kill tomatoes and other tender plants. A moderate freeze, between 25 and 28 degrees, causes widespread destruction. When the temperature drops below 24, we witness an emphatic end to the gardening season.

When you see a pine tree glistening white, or the blooms on ornamental grasses coated in glittery crystals, it's hard to remember that frost is simply a matter of temperature and water. How can something so beautiful be nothing more than the solid deposit of water vapor, formed from saturated air when surfaces are cooled to below the dew point? A frost lasting several hours or more damages plants by freezing the moisture inside leaves, which ruptures the plant's cell membranes. This causes herbaceous plants to collapse and almost melt away, freeing gardeners from routine tasks for a couple of months, anyway.

Frost has virtues besides its dazzle. Some plants need cold temperatures and frost as part of their life cycle. Lilacs and peonies require cold to set their buds, most woody plants need a couple of months of dormancy, and tulips won't bloom without a period of serious cold (which is why the tulip bulbs still in my garage should have been planted before mid-November).

Some woody plants are oblivious to frost, while others are mightily resilient. The autumn cherry, Prunus subhirtella 'Autumnalis' opens a few pale-pink flowers in between cold spells, then bursts into full bloom in early spring. Witch hazels cheer up the garden scene with orange or yellow flowers all through the winter months. The buds on sasanqua camellias wither and turn brown in a hard frost, yet as soon as it warms up this tough evergreen produces yet more buds and flowers. Viburnum tinus 'Spring Bouquet' has pale winter flowers that look ethereal dusted with frost. Sweet box (Sarcococca spp.) are glossy little evergreen shrubs that unfurl their tiny, vanilla-scented white flowers even on the coldest January mornings. Valiant hellebores are undaunted by frost, their pretty flowers untouched by winter's worst weather.

At ground level, you can rely on evergreen sedum and ornamental grasses like the sturdy carexes for texture and color. Overhead, and at every level in between, conifers and broadleaf evergreens are the sturdy stars of the winter garden. I can think of few more pleasing sights than bristly dwarf conifers frosted in white, or the jagged leaves of an Oregon grape glossed with icy crystals, especially Mahonia x media 'Arthur Menzies' that's in fragrant golden bloom right now.

If your own garden is mostly deciduous, this is the perfect moment for a trip to the Witt Winter Garden at the Washington Park Arboretum to see which stalwart plants are looking their best in the depths of winter.

Valerie Easton is a Seattle freelance writer and author of "The New Low-Maintenance Garden." Check out her blog at www.valeaston.com.