Evergreen plants retain their foliage throughout the year. This characteristic makes them popular in various landscapes, providing year-round visual interest. Understanding how temperature influences their health and survival is important for their care.
How Evergreens Endure Temperature Extremes
Evergreens possess various physiological and structural adaptations that enable them to withstand both cold and hot temperatures. During cold periods, many evergreens enter a state of winter dormancy, reducing metabolic activity. Their cells decrease water content, which helps prevent ice crystal formation that could damage tissues. Some species produce natural compounds, similar to antifreeze, such as soluble sugars and proteins, that further lower the freezing point within their cells.
The structure of evergreen needles also contributes to cold tolerance. Their small surface area and thick waxy cuticles minimize water loss through transpiration, especially during dry winter winds, preventing winter desiccation. Additionally, the conical shape of many evergreen trees allows snow to shed easily, preventing heavy loads from breaking branches.
To cope with high temperatures and drought, evergreens regulate water loss through specialized pores called stomata. They can close these stomata during the hottest parts of the day, reducing the amount of water vapor released into the atmosphere. Many evergreens also develop extensive root systems that can reach deeper into the soil, accessing moisture unavailable to shallower-rooted plants. This efficient water use helps them conserve resources during periods of limited water availability.
Recognizing Temperature Stress in Evergreens
Evergreens display distinct symptoms when experiencing temperature stress, which varies depending on whether the stress is from cold or heat. Cold stress often manifests as browning or yellowing of needles, particularly on sides exposed to wind and sun. This discoloration occurs when the plant loses water faster than its frozen roots can absorb it. In severe cases, needle drop, branch dieback, or bark splitting can be observed.
Heat and drought stress also cause visible changes in evergreens. Symptoms include wilting or drooping foliage, indicating the plant is struggling to maintain hydration. Needle scorch, where the tips or margins of needles turn brown and crispy, is another common sign. The plant’s overall appearance may become dull, and premature needle drop can occur to conserve water. Stunted growth can also indicate prolonged heat or drought conditions.
Protecting Evergreens from Temperature Challenges
Proactive measures can help evergreens withstand extreme temperatures. For winter protection:
Ensure adequate watering in the fall before the ground freezes to hydrate plants for the dormant season.
Apply a layer of mulch around the base to insulate the soil, retain moisture, and prevent fluctuating soil temperatures.
Use windbreaks, such as burlap screens, to shield evergreens from drying winter winds and harsh sun.
Apply anti-desiccant sprays to reduce water loss from needles.
Avoid de-icing salts near evergreens, as these can harm root systems.
During summer, consistent watering is important, particularly during dry spells, to ensure the plant has sufficient moisture. Young or newly planted evergreens may benefit from temporary shade to reduce heat stress. Maintaining a mulch layer helps conserve soil moisture and regulate soil temperature, keeping roots cooler. It is also advisable to avoid pruning evergreens during heatwaves, as this can add to the plant’s stress.
Choosing Evergreens for Your Climate
Selecting evergreens well-suited to the local climate is foundational for their long-term health. The USDA Plant Hardiness Zone Map provides a guide, dividing regions based on average annual minimum winter temperatures. Gardeners can use this information to choose plants known to tolerate the coldest temperatures in their specific zone.
The American Horticultural Society (AHS) Heat Zone Map indicates the average number of days per year with temperatures above 86°F (30°C), a threshold where many plants begin to experience stress. Considering both hardiness and heat zones provides a more comprehensive understanding of a plant’s temperature tolerance. Local variations, known as microclimates, can also influence a plant’s success; factors like proximity to buildings, south-facing walls, or wind exposure can create warmer or cooler pockets within a garden. Even within a specific zone, different evergreen species have varying tolerances, so researching species-specific needs is beneficial.