Boxwood (Buxus) is a versatile and useful evergreen shrub that can be planted as edging, hedge, screen and specimen plant. It has been grown in the United States since the mid-17th century, when the first plants were brought to the east coast from Amsterdam. There are now approximately 150 boxwood cultivars and species that are available to home gardeners and it is the most frequently purchased woody plant in the United States.
While consistently popular with gardeners and long-considered a reliable plant, boxwood is not without its problems. Much of the recent attention to the plant has focused on those shortcomings. Boxwood can be damaged by winter wind and sun, road salt, improper planting, pruning and irrigation, and a number of pests and diseases. In the last decade, boxwood blight has become a growing concern in our area. A fungal disease caused by Calonectria pseudonaviculata, boxwood blight affects all boxwood cultivars, though some are less susceptible than others.
Winter care, proper planting and timely pruning improve your chance of success and enjoyment of this plant. Poor siting and care are, in fact, the most common problems with growing boxwood and make the plant more susceptible to disease.
Most boxwood species are native to forest understory locations or protected valleys and hillsides, where they grow on loose, relatively dry soil. While boxwood can do well in many soil types, the siting of your plants to replicate the protection of its natural habitat is important.
Hardy in USDA zones 6 to 9 (occasionally colder), boxwood will grow in full sun but prefers a slightly shaded location. Keep in mind that the shrubs will also need some shade in winter, when the leaves have fallen off deciduous trees and taller shrubs. Choosing a position with some shade, up to about 20 percent, will reduce both summer scald and winter injury risk. Shade also reduces the likelihood of mite damage. Exposure to strong winter winds should be avoided but air circulation around the plant is important in reducing the incidence of disease.
Soil should drain well, as boxwood won't tolerate waterlogged conditions. Additions of organic matter may be necessary and proper systems to enhance drainage may be needed. In areas with acid soil, lime should be added to raise the pH to 6.5 - 7.5. Dolomitic lime (calcium carbonate or calcium magnesium carbonate), with its low oxide content, will maintain an elevated soil pH for longer period of time than quicklime (calcium and magnesium oxides) or slaked lime (calcium and magnesium hydroxides). If you don't know the pH of your soil, you can have it tested in the lab of your county's cooperative extension office.
Boxwood is an evergreen shrub and will continue to take up nourishment from the soil through the colder months; it should be planted at a time of year that is less challenging for it to establish its root system, neither too hot nor too cold. Early fall (September) or spring (April) planting are best as long as the weather is cooperating and the warmest and coldest temperatures have passed.
To plant, dig a hole 2 to 3 times as wide as the shrub's root ball and the same depth. When planting, give each plant some room for sunlight and good air circulation. Do not plant where other root systems will be in competition.
If the shrub is "balled and burlapped", place the root ball in the hole and cut the twine wrapped around the trunk. Roll the burlap off the root ball so that it lies flat in the hole and remove the burlap without damaging the root ball. The top of the root ball should be slightly higher than the ground level. Heavy shrubs usually settle once they are planted and watered. It's better to plant your boxwood too high than too low.
Shrubs purchased in containers should be removed from their pots. Roots should be inspected. If the plant is pot-bound, tease the roots free of the compacted shape. Plant container shrubs at the same height they have been planted in their pot.
Boxwood plants have shallow and delicate roots. Do not locate other plants too close to the shrubs or disturb the roots with foot-traffic or edging of beds.
After planting, apply an inch of composted bark chips or any suitable, permeable, organic mulch. The mulch will help retain moisture in the ground and maintain the cool, moist conditions needed at the roots. Remember not to mulch up to the base of the shrub or you'll create a damp environment that is an open invitation to pests and disease and a growing space for above-ground roots.
Remove the layer of mulch in the spring and replace it with fresh mulch, over moistened soil, once the earth has warmed up. Gently tidy and re-mulch in the fall after the soil has completely frozen through, as a layer of insulation.
Boxwood plants can have an extended life and it's easy to fall into the habit of neglecting their growing environment and soil condition. Amending the soil lightly with compost in the spring will condition it and enables the plant to take up the nutrients it requires.
Do not over-fertilize. If older, lower leaves begin to yellow at the tips, the plant may need additional magnesium or nitrogen. Other nutrient deficiencies and problems can also produce pale or discolored leaves and tips, including plants grown in a low pH soil by the grower. Keeping the pH near the correct range is important because nutrients in the soil are only available to the plant under those conditions. The best way to determine whether the soil needs additional nutrients or pH adjustment is by testing it. If soil testing indicates supplementing soil nutrition, add a granular, urea-based fertilizer in the fall.
Continue to give the plants room for sun and air circulation. An annual tidying that removes dead, crossing and damaged branches is recommended. Thinning to remove some of the branches at the outer edge of the plant is necessary and particularly important for the dense common (American) boxwood and English boxwood types (see "Prune, shear and thin" below). The annual tidy up should include shaking branches to remove dead leaves and raking under the plant. Good plant hygiene is essential to reducing the likelihood of disease.
The preferred time to prune boxwood is late winter or early spring in the New York area, once the coldest plunges of temperature have passed, around March 15. Boxwood takes to light cutting and shaping quite well but little is required other than to correct the shape by reducing a branch here and there. If you respect the natural shape of the plant you have chosen and prune simply to maintain tidiness, you will limit the potential for plant stress and die-back. Do not prune when the temperature is projected to be below 40°F for several weeks.
If you are regularly cutting most of the branch tips of your shrub and altering its natural shape significantly, that is considered shearing. June is best; do not risk die-back by shearing after mid-August. This type of reduction is the most stressful to the shrub and, for dense plant types, should be accompanied by thinning. Boxwoods do not reliably regenerate leaves from bare wood, so you need to restrict shearing to no more than ¾ of the way into the current exterior layer of leaves. The top of the plant should remain less wide than the lower branches so that sunlight can reach all parts.
Thinning to remove some of the branches at the outer edge of the plant is different from pruning or shearing, and particularly important for the dense common (American) and English boxwood types. Thick growth reduces sunlight into the center of the shrub, which causes foliage drop, weakening the plant. The resulting leaf debris accumulates in the lower branches, causing abundant and vigorous aerial roots to grow in this moist and dark environment. Eventually, the exposed aerial roots will die, shocking the weakened plant. Lack of air circulation will also promote disease if thinning is ignored.
Thin dense boxwoods from early spring to late fall if the foliage completely hides the view of the interior branches. Do not thin when extreme temperatures are predicted. To thin, using sharp bypass pruners, reach inside about 6 inches, and remove a twig. Continue to prune until the small interior twigs become intermittently visible, along with the older, light green interior leaves. When pruning is completed, the shape and size of the boxwood should look unchanged. Use clippings for propagation or holiday decoration if desired.
Boxwoods may need to be be protected from wind and salt spray during the coldest months depending upon the degree of shade and protection afforded by their site. These evergreen plants will continue to photosynthesize for an extended period, and even on occasion during the winter, but will be challenged to take up the necessary moisture from the dry or frozen soil. Wind blowing past the leaves will speed up evaporation creating even dryer conditions and a great stress on the plants. The use of burlap fastened to stakes to create a wind break for the location is an excellent idea. You may not want to wrap the shrubs too closely with burlap or their access to sunlight will be restricted. (Wrapping completely should be considered if your shrub is in a position that is likely to be subject to forceful deposits of snow from a roof ledge, snow plow or other source. Otherwise, brush off heavy snow that settles on the plant to avoid crushing injuries.)
If the boxwoods are near a path or roadway that is treated with salt in the winter, positioning the windbreak to also prevent salt spray on the plants will increase their chance of making it through the winter in good health. Use plant-friendly ice-melting products on your own property.
Take measures to see that boxwoods are watered until the ground freezes and during any warm breaks in the winter weather.
Check that mulch has not drifted up to the leafy base of the shrub where, along with the dense leaf structure, it can make a cozy nest for rodents that will strip the bark from the shrub and may kill it.
Boxwoods that are unprotected from winter wind are likely to experience dehydration, like many other broadleaf evergreen plants. The wind can remove moisture from leaves at a time when the ground is frozen and the plant cannot bring enough water up from its roots to make up for the loss from its leaves. The result is brown, dry leaves.
Exposure to strong sun can make matters worse. It may lead to cracking bark when temperatures fluctuate too rapidly from warm, daytime sun to frosty night. It also leads to a condition called "bronzing" in boxwoods, which is the exposure of the red pigments in the leaves as chlorophyll is lost in cold, sunny conditions. In the cold, chlorophyll cannot be manufactured by the plant as quickly as it is lost as a result of sun exposure. Most boxwoods bronze to some extent during the winter.
These problems can be reduced by selecting cultivars that are noted for their resistance to bronzing, by keeping the plants as healthy as possible and by planting the shrubs in a protected and partly shaded sites. Screening the shrubs is another option.
Complete desiccation of leaves will turn them a pale, straw color and, along with winter sun scald, can lead to branch die-back,. Bronzing alone does not cause permanent harm to the shrubs; the leaves return to the familiar green color in the spring when chlorophyll production picks up.
Finally, clear any snow or ice accumulation off the plant directly so that the weight does not cause limb breakage.
Boxwood browning and die-back is more likely to be related to environmental conditions than to disease or pests. Stress issues resulting from cold (described above) or water are the most common.
Boxwoods have wide and shallow root systems and need regular water as well as soil that drains effectively. About one inch of water, every 10 days, from spring until ground freeze, is a good rule of thumb. Do not water from above the leaves as splashing water is a significant vector of disease.
If the proper site preparation and planting is not carried out, drainage can be an issue. Roots must be handled carefully during planting and not disturbed later by digging roughly around an established plant or edging too frequently. Compaction of soil around boxwood, due to foot traffic or construction, will disrupt the permeability of the soil and the plants will suffer due to loss of water, oxygen and root growth. Similarly, boxwood planted too close to a wall or foundation will lack the space needed for root growth.
The use of boxwoods as border plants leads many plantings to be exposed to the salts used to melt snow and ice on roads and pathways. Boxwoods are salt-sensitive and the shallow, wide roots system increases their vulnerability. Much like winter wind, salt reduces the moisture in the leaf and leads to browning at the tips. Salts in abundance can significantly damage a plant.
Do not allow layers of mulch to build up and create a damp environment that welcomes disease, vermin and the growth of delicate roots above the soil level, while restricting air and water available to the root system. Over-edging the plant and heaping excess soil at its base can create a similar problem. The plant can actually die of root suffocation. Above-ground roots are even more exposed to winter salting and cold, and weaken the plant. Organic mulch also takes up nitrogen from the soil as it decomposes and over-mulching will deplete nitrogen rapidly. Use of inappropriate mulches that do not add to the soil nutrition or restrict the passage of air and water are also not helpful.
Boxwood shrubs are vulnerable in times of temperature extremes and more so if they are in containers. The container means that the roots of the plant are closer to the surface and less insulated than they would be in the ground. In winter, even plants that would be hardy in our area may find the additional cold they experience at the root too challenging. They will also warm more quickly in a period of mild winter temperature, so the whipsaw is greater. Summer heat will more directly affect the plant's roots.
Begin by choosing a container that allows for the shallow and wide root system of the plant. Remember that dark containers will heat up and expose your plant's roots to greater heat fluctuations in winter sun and heat extremes in the summer. Boxwood roots will die in excessive heat.
In all other matters, care for boxwoods in containers as you would those planted in the ground. Water them well in the late fall so that they are in top condition when the temperature drops. Once the soil has frozen, mulch the top of the container, but do not allow the mulch to creep up the stem of the plant or clump under the leaves where it can harbor disease. Do not mulch deeply or the roots can be starved of air and water. Be mindful of water needs in the summer.
If the boxwood you have chosen is very hardy in our area, well planted in a protected space, and its care requirements met, it should survive in a container. If it is likely to be exposed to harsh winds or strong sun over the winter, it may need some additional protection. You will have to judge whether a burlap screen is necessary. The more protected the location of the container (protected by building structure, awning, other plants), the less likely you are to need the extra barriers to protect container-planted boxwoods.