Maintaining proper moisture in the soil is the single greatest factor affecting the health of boxwood. An ideal soil is approximately 45% minerals, 25% air, 25% water, and 5% organic matter. The relationship between the water and air, that occupy 50% of the soil space, is an important one. Excessive water in poorly drained soil reduces available air. Conditions such as compacted soil, a high water table, or flooding, tend to reduce soil air and are damaging to boxwood roots.
Good drainage allows the proper water level to be maintained. For example, a sandy soil will permit water to move quickly through the soil. Conversely, a clay soil will hold onto the water, causing a poor drainage condition. The roots of boxwood need a continuous and adequate supply of oxygen to grow properly. To determine if the soil has proper drainage, dig a hole one foot (30 cm) in diameter and one foot (30 cm) deep. Fill the hole with water. If the water has not drained out within one hour, the site is not well drained.
During dry periods, the shallow roots of boxwood cannot extract sufficient water from the soil for transpiration the process by which water taken up by the roots is lost to the atmosphere through the pores on the underside of the leaves. For most plants, 95% to 97% of all plant loss occurs from the leaves. When dry, the plant must then draw water out of the individual plant cells, causing most plants wilt. However, boxwood leaves with their thick cuticle, do not wilt but have small wrinkles when excessively dry. The greatest water loss from a plant would be expected to occur on sunny, windy sites with low humidity.
A somewhat similar situation occurs in winter. The water in the soil has frozen and is then unavailable to the plant. If the drought or winter is too severe, the plant may lose too much water. Under this condition a permanent wilt occurs and a branch or even the entire plant dies. If these extreme conditions are temporary, the plant could re-hydrate.
Some regions have ground water with boron levels high enough to be toxic to many plants, including boxwood. Levels exceeding 0.5 ppm (parts per million) can cause injury that is similar in appearance to salt damage. Some areas experience water with a high sodium, or salt, level. Water with an electrical conductivity (EC) less than 0.75 dS/m (decisiemens per meter) is best for boxwood.
The best guide to watering is to understanding soil and water dynamics. Ideal soil should be slightly moist from the surface to a depth of 12" to 18" (30 cm to 45 cm) all year round. To monitor this, begin by digging several holes 12" (30 cm) deep. Make these holes in various spots throughout the garden at different times of the year. Then it can be determined how far down into the soil the moisture is penetrating. Only through time and observation can an appropriate watering schedule be developed for a particular site. There are many variables that will affect watering. Some of them include: the type of soil, the size of the plant, amount of rainfall, air temperature and relative humidity, slope of the soil, wind, sunlight, type and thickness of mulch, and how recently the plant was moved.
Established boxwood need about 1" (2.5 cm) of rainfall every 10 days from early spring to late fall. If fall weather is dry, water thoroughly just before the first heavy freezing weather is expected. If dry weather continues into the winter, water the equivalent of 1" (2.5 cm) of rainfall every three weeks whenever the ground is not frozen. Best results are obtained from applying water in the early morning hours. This permits adequate moisture during the day when the plant adsorbs most of its water. Morning watering also minimizes the risk of diseases, many are stimulated by water, as it has had all day to drain deeper into the soil and away from the boxwood roots.
There are numerous variables to consider when using and modifying this general watering guide. For example, automated irrigation systems can either benefit or harm boxwood. There are many irrigation systems that have been directly responsible for the death of valuable boxwood specimens. Often automatic timers are set to turn on frequently and seldom adjusted through the seasons or in response to weather conditions. Such over watering results in saturated soils that can injure or kill boxwood roots, or overly wet foliage that promotes foliar diseases.
Proper monitoring and adjusting both the duration and frequency of water applied by automatic irrigation systems cannot be over-emphasized. Preferred watering devices include soaker hoses and trickle (drip) irrigation. The all-too-common “pop-up” spray heads, designed to irrigate turfgrass areas and not ornamental plants or boxwood, are not recommended. The “pop-up” irrigation wet the boxwood foliage, promoting aggressive and serious incidents of foliar boxwood diseases. Additionally, on average, 15% to 20% of the water from the spray head is wasted, evaporating before it can be absorbed into the soil.
Best results are obtained by adjusting both the duration and frequency of irrigation watering in response to both the weather conditions and the change in seasons. Watering infrequently, with longer hours of operation in the early morning, provides the best results. Daily watering, even every two or three days, is generally not recommended. Having each zone apply water for less than 30 minutes is not recommended for most systems.
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