James R. Freed, Washington State University Natural Resources Extension Professor
Frost pockets are low areas of land where there is little movement of air, creating an area where cool air settles. They can also form on the up-slope side of stands of established evergreens where the mass of live foliage at ground level acts as an air dam. How do you know where they are? Look for areas where the early morning fog settles during spring and fall.
These cool pockets of air create conditions that favor frost in late spring and early fall, with temperatures low enough to kill buds and new succulent growth. The coolness is easily intensified by an absence of conifer trees and an abundance of lower growing shrubs and deciduous trees that trap the air.
Frost pockets may not be good for Douglas fir, white pine, or ponderosa pine, but they are great places to establish native plants for non-timber forest products. Because these areas are often free of conifers, they have more sun and soil moisture – ideal conditions for high quality native blackberry, raspberries, evergreen huckleberry, salmonberry, thimbleberry, Indian plumb, elderberry, cloudberry, black currant, salal, and Saskatoon berry patches.
They are also great places for herbaceous plants like camas, paintbrush, pipsissewa, sunflower, strawberry, lilly, lupine, iris, horsetail, bracken fern, geraniums, foxgloves, nettles, sword fern, yarrow and native grasses that support native bees and other pollinators. These plants also provide edibles, medicinals, teas, and craft dyes for human use. Woody plants like willow, filbert, red osier dogwood, cascara, bitter cherry, Hawthorn, Oregon grape, oceanspray, ninebark, cottonwood, Garry oak, Nootka rose and snowberry are also well adapted to frost pockets and provide important supplies of browse and seeds for wildlife.
Frost pockets may not support the traditional plants needed for lumber and fiber but they are critical in the sustainable functioning of a genetically diverse forest ecosystem.