Part 1: Cold Protection of Landscape Plants

Cold Protection of Landscape Plants

Sydney Park Brown, Dewayne L. Ingram, and Thomas H. Yeager

Winter temperatures in Florida are frequently low enough to cause cold injury to tropical, subtropical, and occasionally temperate plants not adapted to Florida climatic conditions. Freezing weather normally occurs in north and central Florida, while below freezing temperatures are rare for south Florida. Tropical plants and summer annuals do not adapt or harden to withstand temperatures below freezing, and many suffer “chilling injury” at temperatures below 50°F (10°C). Subtropical plants can harden or acclimate (become accustomed to a new climate) to withstand freezing temperatures, and properly conditioned temperate plants can withstand temperatures substantially below freezing. Recently planted, unestablished plants may be more susceptible to cold injury.

Types of Freezes – Radiational and Advective

Freezes can be characterized as radiational or advective.

Radiational Freezes

Radiational freezes or frosts occur on calm, clear nights when heat radiates from the surfaces of plants and other objects into the environment. These surfaces can become colder than the air above them due to this rapid loss of heat or long-wave radiation. When the air is moist, a radiant freeze results in deposits of ice or frost on surfaces. Dry radiational freezes leave no ice deposits but can cause freeze damage. Plant damage from a radiational freeze can be minimized by reducing radiant heat loss from plant and soil surfaces, as described below.

Advective Freezes

Advective freezes occur when cold air masses move rapidly from the north, causing a sudden drop in temperature. Windy conditions are normal during advective freezes. Although radiant heat loss also occurs during an advective freeze, the conditions are quite different from a radiational freeze. Plant protection during advective freezes is more difficult.

How Cold Affects Plants

The ability of plants to withstand a freeze depends on both temperature fluctuations and day lengths prior to the event. A gradual decrease in temperature over time increases the ability of plants to acclimate to cold temperatures. That is why a sudden decrease in late fall or early winter usually results in more damage than the same low temperature in January or February. Short durations of warmer temperatures in midwinter can de-acclimate some plants, resulting in early bud break, flowering, and susceptibility to freeze injury. However, preconditioning of tropical plants to withstand chilling temperatures has not been well documented.

Cold injury can affect the entire plant or just certain plant or parts such as fruits, flowers, buds, leaves, trunks, stems, or roots. Many plant parts can adapt to tolerate cold, but flowers, fruits, and roots have little ability to adapt. Cold injury to roots of plants in containers is common but usually is not evident until higher temperatures occur. Leaf and stem tissue will not survive ice formation inside the cells (resulting from a rapid freeze), but many plants can adapt to tolerate ice formation between cells.

One type of winter injury is plant desiccation or drying out. This is characterized by marginal or leaf tip burn in mild cases and totally brown leaves in severe cases. Desiccation occurs when dry winds and solar radiation result in the loss of more water from the leaves than can be absorbed and/or transported by a cold root system. Root systems in the landscape are seldom frozen in Florida, but potting substrate in small containers in north Florida can be frozen for several consecutive hours.

via ENH1/MG025: Cold Protection of Landscape Plants.

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