When it comes to winter plant survival, most gardeners look to hardiness zones for guidance. But as Jason from Fraser Valley Rose Farm explains, these maps are just a starting point. The true threats to plants in winter often extend far beyond cold temperatures. In this article, we’ll explore what hardiness zones mean, how they’re used, and the key factors that determine whether your plants survive the winter.
Understanding Hardiness Zones: A Starting Point, Not a Guarantee
Hardiness zones, such as those defined by the USDA, provide a framework for understanding a plant’s cold tolerance. These zones are determined by the average annual minimum temperature in a given area. For example, Zone 6 corresponds to temperatures as low as -10°F (-23°C), while Zone 8 averages lows around 10°F (-12°C). Canada follows a similar system, adapting the USDA model for its climate.
When plants are rated for a particular zone, it means they’ve been tested to withstand the lowest temperatures typical for that zone. However, hardiness ratings don’t account for other environmental stressors like poor drainage, disease, or desiccating winds—all of which play a significant role in plant survival.
The limitations of hardiness zones have prompted alternative systems, like the Sunset Climate Zones, which factor in additional elements such as rainfall, elevation, and length of growing season. For instance, a plant suited to Zone 6 in the USDA system might fare poorly in a high-rainfall region within the same temperature range. Sunset’s system aims to address these nuances, though its 45 zones can be overwhelming to navigate without localized expertise.
Why Cold Isn’t Always the Culprit
While cold temperatures may seem like the primary threat during winter, Jason argues that other factors are often to blame when plants don’t survive. Here are four common reasons why plants struggle through winter—and how to address them:
1. Plant Health Going into Winter
The condition of your plants before winter is critical. Weak, poorly established, or struggling plants are far more likely to fail, regardless of their hardiness rating. Healthy plants store enough energy in their roots and stems to survive dormancy, while stressed plants may simply run out of reserves.
Pro Tip: If a plant is struggling in the garden, consider keeping it in a pot and providing extra care over the winter. You can bring it indoors to a cool, sheltered space like a garage to avoid additional stress.
2. Winter Wet
In regions like the Pacific Northwest, where winters are damp rather than snowy, excess moisture is a major issue. Roots sitting in waterlogged soil can suffocate due to a lack of oxygen—a condition known as anaerobic root stress. In this state, roots can no longer take up water effectively, even if moisture is abundant, often leading to plant decline.
Many catalog descriptions warn against “wet feet,” signaling that a plant requires well-draining soil to thrive through winter. Roses are a classic example, as they are particularly prone to root rot in saturated conditions. Other plants vulnerable to winter wet include lavender, rockrose (Cistus), thyme, and Mediterranean herbs. Even less obvious plants, like Japanese maples or rhododendrons, can suffer if their roots are submerged for extended periods.
Pro Tip: Evaluate drainage in your garden before winter. Plants that dislike wet conditions should be situated in well-draining soil or raised beds. Covering beds with a tarp or mulch can also help keep excess moisture at bay.
3. Desiccating Winds
Cold winds can dry out plant tissues, particularly in evergreens and semi-dormant plants like rosemary and lavender. Wind exposure accelerates moisture loss, a process known as xylem desiccation. The xylem, which transports water within the plant, can lose moisture faster than roots can replace it, especially in frozen or dry soils. This often results in blackened, dead stems above the snow line.
Jason notes that even unheated greenhouses offer significant protection by reducing wind exposure. In landscapes with no snow cover, plants like bamboo, camellias, and heathers (Erica) can be particularly susceptible.
Pro Tip: Protect vulnerable plants by wrapping them in burlap or surrounding them with a windbreak. Mulching at the base and covering stems with dry leaves can provide additional insulation.
4. Container Vulnerabilities
Plants in containers face unique challenges. Without the insulation of surrounding soil, roots are more exposed to temperature extremes and desiccation. A pot may also freeze solid, further stressing the plant. Even hardy plants may fail to survive if left in containers over winter.
Pro Tip: Move container plants to a sheltered, unheated location once they’re dormant. A garage or shed is ideal, as long as it stays above freezing. Check periodically to ensure the soil isn’t completely dry, adding water sparingly if needed.
Are Hardiness Zones Still Useful?
Hardiness zones remain a valuable tool, particularly for selecting plants suited to your climate. However, as Jason points out, they don’t account for factors like soil drainage, wind exposure, or plant health. Over the years, some systems have evolved to include additional considerations, like rainfall and growing conditions.
The Sunset Climate Zones excel in capturing these subtleties, offering a more nuanced approach. For example, Sunset zones differentiate between regions with similar temperatures but vastly different humidity or seasonal rainfall patterns, like Southern California versus the Pacific Northwest. While this system is highly informative, it requires a bit more study to navigate effectively.
Conclusion: The Real Winter Killers
While hardiness zones provide a starting point for understanding plant survival, they only tell part of the story. Healthy, well-established plants are more likely to survive winter stressors like wet conditions, wind, and container exposure. By addressing these factors proactively, gardeners can greatly improve their chances of bringing plants through even the harshest winters.
