Causes of Freezing in Walnut Trees and Possible Protective Measures

         Late Frosts in Spring

o   After a temperature rise in March–April, walnut trees begin their vegetation period.

o   A sudden drop in temperature below 0°C can damage young buds, shoots, and blossoms.

o   Especially sensitive are the stages of bud break and the development of young green shoots and flowers. Temperatures below 0°C during flowering may cause complete yield loss.

 

           Early Budding in Unstable Weather

o   Warm periods in February–March can trigger early sap flow and bud swelling.

o   Subsequent cold snaps can severely damage tissues due to their high water content and lack of lignification.

 

           Insufficient Winter Hardiness

o   Walnut trees are warmth-loving plants and not fully adapted to very cold climates.

o   Young trees (up to 3 years) are shallow-rooted, have thin bark and underdeveloped wood, making them more prone to freezing.

o   Temperatures below –20°C, especially in the absence of snow cover, can cause damages to all  walnut varieties. Trees acclimated to gradual cooling in fall can survive lower temperatures than those exposed to sudden cold snaps.

 

            Inappropriate Agronomic Practices

o   Late nitrogen fertilization in autumn in combination with irrigation or precipitation promotes new growth that does not mature before winter.

o   Inadequate summer irrigation leads to weaker tissues that are more frost-sensitive.

o   Incorrect pruning in late autumn stimulates tender regrowth, which remains susceptible to winter damage.

 

 Low-lying Locations and Cold Air Pockets

o   In plains and valleys, cold air accumulates and lingers, increasing frost risk.

 

o   Seedlings in such areas are exposed to low temperatures for longer periods, resulting in greater damage.

 

 Selection of Suitable Varieties:

o   Choose varieties with greater cold tolerance, such as:

 

§  Chandler

§  Fernette

§  Fernor

§  Franquette

§  Lara

§  Tulare

§  Ronde de Montignac

(Note: Chandler is productive but somewhat sensitive to spring frost and may not be suitable for colder regions.)

Proper Planting Location

o   Avoid low-lying areas where cold air collects.

 

o   Choose sunny, south- or southwest-facing slopes where snow melts quickly and air circulates better.

 

o   Protect from north and east winds using hedges, windbreak nets, or buildings.

 

Adjustment of Agronomic Practices

 Irrigation:

·        Ensure moderate but regular irrigation during the growing season.

 

·        Reduce irrigation toward the end of summer to encourage hardening of plant tissues. Avoid inducing drought stress too early.

 

Fertilization:

For Conventional Growers:

·        Spring (March–April): Nitrogen fertilizer – 30–50 kg/ha in two doses:

o   First dose – early spring

o   Second dose – early June

·        Micronutrients (boron, zinc, magnesium): Apply foliar sprays of boron, zinc, and magnesium 1–2 times during May–June.

·        Summer (June): Stop nitrogen fertilization after mid-June to avoid soft late growth.

·        Autumn (Sept–Oct): Apply compound NPK fertilizer high in phosphorus and potassium, low in nitrogen. Organic fertilizer may be added every 2–3 years based on soil conditions.

For Organic Growers:

·        Spring: Apply compost or well-rotted manure (300–500 kg/ha) depending on soil fertility. Use plant extracts (e.g., nettle, legume teas) 2–3 times per season. Foliar feed with organic-certified boron and zinc products.

 

·        Summer: Apply balanced plant extracts until late June.

·        Autumn: Use wood ash (50–100 kg/ha) as a potassium source — apply with care due to its alkalinity. Apply potassium-rich compost and mulch with straw or leaves to retain soil warmth and moisture.

 Pruning:

·        Avoid pruning in late autumn, as it promotes tender regrowth.

·        Recommended timing: late winter or early spring before bud break.

Physical Protection of Seedlings in Winter

o   Mulch the root zone with a thick layer of straw, leaves, or manure to retain soil heat and moisture.

Planting at Correct Depth

o   When planting in autumn or spring, ensure the root collar is 4–5 cm below the soil surface to protect the seedling base from frost.

Adaptation to Local Climate

 o   In very cold regions, spring planting is preferable, as it reduces the risk of early-season frost injury.

Restoring frost-damaged walnut seedlings requires attention, patience, and proper care.

Diagnosing Demage

o   Buds: blackened, dry, or detached.

o   Bark: grayish-brown, cracked, or peeling.

o   Shoots: darkened, soft, or shriveled.

o   Trunk: a scratch under the bark showing brown tissue indicates death.

o   Roots: healthy roots are white/yellow and moist inside; dry or darkened roots suggest the seedling is dead.

 

Pruning Frost-Damaged Parts

When:

·        In early spring, after the risk of further frost has passed.

How:

·     Prune back to healthy tissue where the inner bark is green and moist.

•    Use clean, disinfected tools.

•    Prune conservatively if unsure how much damage has occurred — the tree may recover slowly.

 

 

Post-Frost Irrigation and Care

·        Water moderately and regularly, especially during dry periods.

•        Avoid overwatering, which can lead to root rot.

•        Apply mulch to conserve moisture and stabilize the root zone microclimate.

 

Additional Care for Recovery

·        Support: Use a stake to stabilize weakened seedlings.

·        Pest Protection: Frost-injured plants are more vulnerable to diseases and insects — monitor closely.

·        Weed Control: Eliminate weed competition for water and nutrients.