Our Whatcom County big freeze of January 2024 was tough on landscape and garden plants.
An atmospheric river first drenched the soils. Temperatures were almost balmy, and many plants (and gardeners) let down their guard. Next came very cold temperatures and even colder winds. The upper layer of soil froze, followed by more wind and snow. Tender evergreen shrubs like California lilac and California bay laurel, at the limit of their hardiness range, died back. Hardy, overwintering vegetables like kale and leeks were blasted.
The cold hardiest vegetables (Winter Vegetable Production on Small Farms and Gardens West of the Cascades, p. 38) include corn salad and spinach. (And chickweed! an edible spring-tonic.) All are cold hardy to 0 degrees Fahrenheit (-18 degrees Celsius). When the snow had melted, as I cut back the gloppy remains of the normally extremely hardy leeks, I saw small corn salad plants, bright green against the thawed ground.
Amid variable weather events and ongoing climate change in the Pacific Northwest, we might expect another freezing event this winter. You can improve winter adaptation for the plants in your garden and prepare plants for winter cold with some tools and tips.
Right plant, right place, right mulch. For trees and ornamentals, adding a thick insulating layer of mulch, such as arborist wood chips, provides substrate for fine absorbing roots and insulates the organic layer and upper mineral soil. Frozen top soils might have been the ultimate death knell this year for my showy, 8-year-old California lilac (Ceanothus), which had made it through several years of past freezes protected not far from a south-facing wall.
Increases in global average surface temperature (roughly 2 degrees F or 1 degree C) contribute more energy to the overall climate, fueling bigger swings in moisture, storms, and temperatures. Northwest gardeners are having to pay careful attention when choosing, siting, and managing their home garden plants.
USDA Hardiness Map Updated
The USDA recently updated the US plant hardiness zones (growing zones, planting zones) to reflect a changing climate. In Whatcom County, Bellingham and Ferndale’s zones warmed for the first time from zone 8a to zone 8b. Lynden’s plant hardiness zone remains unchanged at 8a. The temperature change cited by USDA is +2 degrees F from 2012 to 2023. The Sunset Western Garden Book is an example of a gardeners resource that uses these important measures.
