When a significant temperature drop is forecast, gardeners often wonder whether to water the landscape before the cold arrives. Predicting freezing weather is a call to action, as the survival of tender and evergreen plants depends heavily on pre-emptive care. Preparing the soil and the plant’s internal systems is a proactive measure that can reduce the severity of freeze damage.
The Role of Soil Moisture in Cold Protection
Plants should be well-watered before a freeze. The primary threat to plants in freezing conditions is not just the cold, but the resulting desiccation that occurs in plant tissues. When the soil freezes solid, the water turns to ice and becomes unavailable to the roots, effectively creating a physiological drought.
Plants continue to lose water vapor through their leaves, a process called transpiration, even in winter, especially on dry, windy days. If roots cannot draw moisture from the frozen ground, the plant’s cells dehydrate, leading to damage resembling drought stress. A moist, well-hydrated root zone allows the plant to enter the cold period with maximized internal water reserves. This preparation is particularly important for evergreen species, which continuously lose moisture through their foliage throughout the winter.
The Physics of Water and Plant Tissue Defense
Water helps protect the root zone by leveraging two fundamental physical properties: its high specific heat capacity and the release of latent heat upon freezing. Specific heat capacity means water requires a large amount of energy to change temperature. Because wet soil replaces air pockets with water, it absorbs and stores significantly more heat from the daytime sun than dry soil does.
This stored heat is then slowly released back into the environment overnight, raising the ambient temperature around the plant’s root crown and lower stems. Furthermore, as water in the soil begins to freeze, it releases latent heat—the energy released when water changes phase from liquid to solid ice. This release of energy provides a temporary warming effect, which helps buffer the roots and surrounding air from dropping to low temperatures.
Proper Timing and Application for Pre-Freeze Watering
The timing and method of watering are crucial for maximizing cold protection benefits. The ideal window for deep watering is 24 to 48 hours before the expected freeze. This timing allows the water to thoroughly soak into the root zone while ensuring the soil surface has time to dry slightly, preventing an immediate surface freeze that could block deeper absorption. Watering should be performed during the morning or early afternoon when the air temperature is above 40°F (about 4°C), giving the plant time to absorb the moisture before nightfall.
The application should focus on a deep, thorough soaking of the entire root zone, rather than a light sprinkling. The goal is to saturate the soil to a depth of at least 6 to 8 inches, where the bulk of the root system resides. Avoid wetting the foliage, as water on the leaves can freeze quickly and increase the likelihood of cold damage. Focus solely on saturating the soil, as surface misting or overhead watering during the freeze can be detrimental.
Immediate Steps Following a Thaw
Once the freezing period has passed and temperatures have consistently risen above freezing, immediate steps are necessary for post-freeze care. First, inspect the plants for damage, looking for wilted, blackened, or mushy foliage, but resist the temptation to prune immediately. Damaged leaves and stems, though unsightly, provide a layer of insulation that protects the underlying healthy tissue from subsequent cold snaps.
Hard pruning of woody plants should be delayed until the threat of frost has completely passed and new growth begins to emerge, typically in late winter or early spring. Delaying pruning allows the full extent of the damage to become visible, enabling precise removal of only the dead material. If the soil is dry and has thawed completely, a moderate, deep watering will help rehydrate the plant tissues, as roots may have been unable to absorb frozen water during the cold period. For covered plants, remove the covers promptly in the morning to prevent overheating and allow the plant to adjust gradually to the returning sunlight.