Boxwoods, belonging to the Buxus species, are highly valued evergreen shrubs used widely in landscapes for their dense, year-round foliage. As broadleaf evergreens, these plants retain their leaves and continue a low level of biological activity throughout the winter months. Even when the plants appear dormant, they still require access to moisture to maintain their internal structure. Boxwoods must be monitored and often require supplemental hydration to prevent significant damage. Proper winter moisture maintenance is necessary to ensure these shrubs remain healthy and vibrant for the following spring.
Understanding Winter Desiccation
Boxwoods are particularly vulnerable to a condition called winter desiccation, or winter burn, because their leaves are always present and actively losing moisture. The process of transpiration, where water vapor is released through the foliage, continues even in cold temperatures, especially on sunny or windy days. This moisture loss from the leaves creates a demand for water that the roots must meet.
The danger arises when the ground freezes solid, locking the available moisture in the soil as ice. When the roots cannot absorb water from the frozen soil, the plant essentially experiences a physiological drought. The shallow root systems of boxwoods make them especially susceptible to this problem.
Alternating periods of freezing and thawing compound the risk, leading to root heave. This process lifts the plant slightly out of the ground, exposing the root crown and roots to drying air and freezing temperatures. The combination of moisture loss and the inability of the roots to replace it results in the scorched, bronzed, or brown foliage characteristic of winter burn.
Indicators for Determining Water Needs
Determining when to water a boxwood in winter should be based on soil conditions and temperature, not a fixed calendar schedule. A good guideline is to check the soil moisture every four to six weeks during dry, winter periods. The frequency of watering is determined by the environmental conditions and the plant’s immediate surroundings.
To check the soil, insert a trowel or your finger two to three inches deep near the base of the plant. If the soil feels dry and crumbly at this depth, the plant needs water. If the soil is still cool and slightly moist, hold off on watering.
Watering must only occur when the air temperature is consistently above freezing, ideally 40°F or higher, for two to three consecutive days. This temperature threshold is necessary to ensure the soil is thawed enough to absorb the water and allow the roots to take it up. Watering frozen ground is ineffective, as the water will run off or freeze on the surface, potentially causing damage.
Newly planted boxwoods, those exposed to high winds, or shrubs located near reflective surfaces need more frequent checks. These factors accelerate moisture loss from the foliage and the soil. Established plants protected by a layer of mulch can go longer between supplemental waterings.
Practical Application and Volume
Once the need for water has been established and the temperature is favorable, the method of application must ensure the water deeply penetrates the root zone. Slow, deep watering is preferred over a quick surface spray to prevent runoff and encourage the water to soak in. Using a soaker hose or allowing a garden hose to trickle slowly at the base of the shrub is the most effective approach.
The best time of day for winter watering is midday. This allows the water maximum time to penetrate the soil before evening temperatures drop. Watering later in the day increases the risk of the water freezing around the root crown, which can cause damage.
The volume of water needed depends on the size of the boxwood. A good rule of thumb for established plants is to apply approximately five gallons of water per foot of plant height or spread. This deep soak encourages roots to grow downward, making them more resilient to future dry conditions.
A two to three-inch layer of organic mulch, applied before the ground freezes, is invaluable for conserving moisture. Mulch acts as an insulator, moderating soil temperature and significantly reducing the rate at which water evaporates from the soil.