When a freeze warning arrives, many homeowners face a dilemma about their sprinkler systems: should they turn them on to protect their plants or turn them off to protect their pipes? The instinct to water comes from an established agricultural practice, but applying water in freezing conditions is a high-risk strategy for residential landscapes. Determining the correct action depends entirely on whether the goal is to save plant foliage or to prevent thousands of dollars in damage to the irrigation infrastructure. The complex physics involved make this method impractical for the average gardener.
Why Water Can Protect Plants During a Freeze
The use of water to guard plants against freezing is based on the principle of thermodynamics known as the latent heat of fusion. This concept explains that when water transitions from a liquid state to a solid state (ice), it releases measurable heat energy into the surrounding environment. For every pound of water that freezes, it releases approximately 144 British Thermal Units (BTUs) of heat.
This released energy is transferred directly to the plant tissue encased in the forming ice layer. As long as a continuous supply of liquid water is freezing on the plant’s surface, the heat release creates a temperature buffer. This process prevents the temperature of the ice-coated plant parts from dropping below 32°F (0°C). The goal is not for the ice to act as an insulator, but for the continuous phase change to maintain this stable, slightly warmer temperature.
Commercial growers use specialized overhead sprinklers, often in orchards, to create a thin, clear glaze of ice over buds and flowers before the temperature drops too low. They must start the system when the air temperature is still a few degrees above freezing, typically around 34°F to 36°F, to ensure the protective process is underway. The entire operation relies on precise timing and an uninterrupted flow of water until the ambient temperature rises above freezing and the ice begins to melt naturally.
When Running Sprinklers Causes Damage
For the average homeowner, attempting to use sprinklers for freeze protection carries significant hazards that often outweigh the benefits. The method relies on a continuous, uniform application of water, and if the water supply is interrupted or the pressure is too low, the technique fails immediately. The resulting ice layer then drops to the ambient air temperature, causing more damage than if no water had been applied.
A major risk is evaporative cooling, which is the exact opposite of the desired effect. If the humidity is low or the wind speed is high, some of the applied water will evaporate instead of freezing. The latent heat of vaporization, which is the energy required to turn liquid water into vapor, is roughly seven times greater than the heat released by freezing. This rapid evaporation consumes a tremendous amount of heat from the plant’s surface, super-cooling the tissue and causing quick damage.
Furthermore, the protective capability of the system is limited; the ice layer often cannot protect plants when the air temperature falls below 25°F to 20°F, depending on the application rate. The sheer weight of a thick ice accumulation can also break branches and structurally damage established shrubs and trees.
Essential Steps for Protecting Your Irrigation System
Instead of risking plant damage with a complex watering strategy, the most financially responsible action for a homeowner is to protect the irrigation system itself. Pipes and components are susceptible to damage because water expands by about nine percent when it freezes, which can burst pipes and crack components. The first step involves locating and shutting off the system’s main water supply, typically a valve situated near the backflow prevention device.
Once the water supply is cut off, the remaining water must be removed from the lines. Many systems have manual or automatic drain valves that can be opened to empty the pipes. If the area experiences prolonged deep freezes, the best practice is professional winterization, which involves using an air compressor to “blow out” the remaining water from the lines and sprinkler heads. This compressed air method ensures no residual water is left to freeze and expand.
Exposed components like backflow preventers and above-ground pipes should be insulated with foam covers or specialized insulation tape. Backflow devices are particularly vulnerable and must be protected from fluctuating temperatures to prevent costly repair or replacement.
Non-Water Methods for Saving Vulnerable Plants
Home gardeners have safer, simpler, and more effective ways to safeguard sensitive plants without relying on the risky physics of latent heat. The most common method involves creating a physical barrier to trap the heat radiating from the soil. This is best accomplished by covering vulnerable plants with frost cloth, old bed sheets, or blankets before sunset.
Covering Plants
These coverings should drape to the ground and be secured with rocks or soil to fully enclose the plant and trap the warmer air. Gardeners should avoid using plastic sheeting directly on foliage, as plastic can conduct cold and damage the tissue where it touches the leaves.
Moving Potted Plants
Potted plants that are not cold-tolerant can be moved indoors, into a shed, or into a protected area like a garage, where temperatures remain above freezing.
Another effective technique is applying a deep, two- to four-inch layer of organic mulch, such as wood chips or straw, around the base of plants. This mulch acts as an insulating blanket for the soil, moderating the ground temperature and protecting the root zone from the cold.