Winter Injury
Temperature Abiotic disorder
Last updated
This profile contains basic abiotic disorder data. Regional field notes and expert review are in progress.
What Causes It
Cold damages plant tissue through multiple coincident mechanisms during dormancy and the transitions into and out of it. Ice formation: when temperatures fall fast enough, water inside cells freezes into crystals that rupture membranes and cell walls. Desiccation: frozen soil prevents water uptake while wind and winter sun continue to pull moisture from broadleaf evergreens and needles, so tissue dries even though the plant is otherwise dormant. Deacclimation failure: woody plants increase cold hardiness in fall and lose it in spring; rapid temperature drops during acclimation or deacclimation can kill tissue that would survive the same temperature during midwinter hardiness. Bark and cambium damage: repeated freeze-thaw cycles on sun-exposed trunks crack bark and kill the underlying cambium.
Quick Reference
Symptoms
On broadleaf evergreens (Rhododendron, Camellia, boxwood, Pieris, Fatsia, cherry laurel): leaves turn brown or bleached tan starting at the tips and margins, most severe on exposed south and west sides and at the top of the plant. On conifers: needle tip browning, whole-branch flagging, and bleached foliage on the sun-exposed side. On deciduous woody plants: buds may fail to break open in spring or may wilt and collapse shortly after emerging. In severe cases, canes and twigs turn black or deep purple. On grafted plants, the scion may die while the rootstock survives and later sends up leggy replacement growth. On tender perennials: blackened foliage, mushy crowns, and failure to emerge in spring. Drooping and rolling leaves may occur as a protective reaction to reduce surface area exposed to cold; these leaves usually return to normal in spring. Damage often appears suddenly during a late-winter warm spell when deacclimated tissue is caught by a return freeze. (Sources: PNW Plant Disease Management Handbook Rose - Winter Injury; WSU HortSense Fatsia Cold Injury entries.)
Winter injury shows up on sun and wind exposed surfaces first: south and west sides of plants, tops of shrubs, ridge crests, bark facing the winter sun. Damage is typically uniform over the exposed surface rather than spreading from a focal point. Symptoms appear in late winter or early spring as temperatures warm; they do not progress through summer the way active pathogens do. The same plants tend to suffer damage in the same places year after year on the same site.
Timeline: Acute cold events cause damage within hours to days that becomes visible as tissue warms in spring. Chronic winter desiccation accumulates through dormancy and shows up at bud break. Deacclimation failure shows up as dead tissue on plants that looked fine at midwinter but collapsed after a warm late-winter week ended in a hard freeze.
Triggers & Conditions
Cold temperatures below the plant's hardened tolerance; rapid temperature drops during acclimation (fall) or deacclimation (late winter/early spring); frozen soil combined with wind and sun exposure on broadleaf evergreens; late-season fertilization that prevents full dormancy onset; marginal species at the edge of their hardiness range; newly planted stock not yet acclimated to the site. (Sources: PNW Plant Disease Management Handbook Rose - Winter Injury; WSU HortSense Fatsia Cold Injury.)
Acclimation phase (late autumn), midwinter, and deacclimation phase (late winter through bud break). PNW Handbook Rose entry: 'Rapid temperature drops during acclimation or deacclimation can injure roses where the same drop might not injure roses during mid-winter hardiness.
Puget Sound lowlands rarely see the sustained deep cold that drives winter injury in colder zones, but the region has its own distinct pattern. Warm rainy winters can soften hardiness on borderline plants (Fatsia, Pittosporum, Dicksonia, cold-borderline camellias and hebes), then a brief arctic outflow through the Fraser River valley drops temperatures into the teens and catches deacclimated tissue. The 2022 arctic outflow and the late-winter events in 2024 produced visible winter injury on broadleaf evergreens throughout the Kent valley and Seattle metro. Zone-8b reliable plants can take 15F once hardened, but when the preceding two weeks were in the 50s the same plants burn at 25F. South and southwest-facing walls and exposures are the worst for desiccation. Newly planted stock and pot-up transfers from zone 9 or 10 nurseries are the most vulnerable. Early-fall fertilization of marginal plants compounds the problem because it delays dormancy onset.
Management
Prevention
- Choose cultivars adapted to your growth zone and microsite
- Do not fertilize late in the growing season
- Mulch over the root zone before winter
- Water deeply before forecast freeze events
- Protect broadleaf evergreens from winter sun and wind on exposed sites
- Provide supplemental irrigation during dry fall and winter periods
- Plant graft unions below the soil surface in colder areas
Mitigation
- Rinse salt spray off foliage after storms
- Assess and prune dead tissue after spring bud break
Place broadleaf evergreens and marginal-hardiness species on protected microsites: north and east exposures, sheltered courtyards, near larger plantings that break the wind. Avoid south and southwest walls where reflected heat drives deacclimation in late winter. Use windbreaks of hardy conifers to shelter beds of sensitive plants. Install irrigation that can deliver deep winter watering when needed.
Plant Tolerance
All landscape plants have a hardiness limit; injury occurs when conditions cross that limit. Borderline species at the edge of their hardiness zone, newly planted stock, and plants in exposed microsites are the most vulnerable. Broadleaf evergreens are more susceptible to desiccation injury than conifers or deciduous plants.
More Tolerant
- Native Puget Sound conifers (Pseudotsuga, Thuja plicata, Tsuga heterophylla) at their native range
- Native deciduous hardwoods adapted to the region
- Cold-hardy rose cultivars bred from R. rugosa, R. arkansana, R. wichurana
- Most zone 6 and colder hardy landscape plants in zone 8b
More Sensitive
- Marginal zone 9 and zone 10 species grown at the edge of their hardiness
- Newly planted stock of any species
- Broadleaf evergreens on exposed south and west sites
- Plants fertilized late in the growing season
- Grafted roses with tender scions
- Tropical and subtropical species used as annual or temporary landscape plants
Species hardiness rating relative to site zone, acclimation state at the time of a cold event, age of the planting (first-year plants are more vulnerable), microsite exposure to wind and winter sun, soil moisture at the time of freeze (dry soil holds less heat), snow cover (insulates and protects), and mulch depth over the root zone.
Secondary Effects
Bark splits and frost cracks from freeze-thaw cycling create entry points for opportunistic canker pathogens including Nectria, Botryosphaeria, and Cytospora on stressed hardwoods. [VERIFY] Specific pathogen predisposition is widely cited in horticultural literature but not directly documented in the HFG library for winter-injured hosts.
Winter-damaged canes and shoots can die back well below visibly damaged tissue, requiring hard spring pruning to live wood.
Chronic winter injury on the same plants year after year indicates species-site mismatch and progressive decline.