Boxwood

History

The American Boxwood Society proclaims boxwood as man's oldest garden ornamental. Leaf and fruit fossils from boxwood have been found in more than 20 separate locations throughout Europe, some dating back 22 million years. The first known use of boxwood occurred in 4000 B.C. when the Egyptians clipped it into formal hedges for their gardens. In 1000 B.C. Homer wrote of boxwood used as yokes for the stallions driven by the King of Troy, and charcoal remains of boxwood have been recovered in England, dating back to 2,000 B.C. The furniture in Tutankhamen's tombs was of a tropical boxwood variety

The first planting of boxwood in North America is believed to have been by Nathaniel Sylvester who built a manor on a Long Island plantation in 1652. Although the dense, heavy wood has been used for many practical purposes in the past, boxwood is primarily used as an ornamental, bringing a sense of quiet simplicity, peace and stability to the garden.

Selecting the right species

While the most familiar forms of boxwood are English (Buxus sempervirens) and Dwarf English (Buxus sempervirens 'Suffruticosa'), there are about 90 species and over 150 different cultivars known, exhibiting a wide variety of forms and foliage. Boxwood can be used in many ways due to its many varied forms, such as prostrate, globe, half-erect, weeping, columnar and pyramidal. There are also interesting variations in size and foliage. Some of the ways boxwoods can be used are as foundation plantings; to separate or screen areas; to provide background for other plantings; to provide a framework for a formal garden; to outline a terrace, walkway or parking area; for planter boxes; and as topiary or specimen pieces. Boxwood is also used for bonsai in its dwarf form.

When selecting the right boxwood for the job it is important to note the particular sun and shade characteristics of a particular form as well as its maximum size, growth characteristics and hardiness. Most boxwoods are slow growing but strive to achieve their maximum height and width over time (anywhere from 2 x 2 feet to 15 x 15 feet). While trimming is recommended and greatly improves the beauty of boxwood, it's important not to fight against the natural growth habit of a variety.

Selecting a site

Boxwoods are adapted to a wide range of light conditions. They tolerate heavy shade but will grow in full sun if the roots are in a good soil environment. Boxwoods should only be planted in well-drained soils. Never plant them near downspouts or in any area that stays wet. In general, boxwoods should be planted in a somewhat protected, semi-shade location for best year round performance. The following is a list of site traits to consider:

Acidity: Boxwoods perform best at a pH of 6.5 to 7.2.

Drainage: Good drainage is very important. To determine if the soil has proper internal drainage, dig a hole one foot in diameter and a foot deep. Fill the hole with water. If the water is not gone within one hour, the site is not well drained.

Exposure: Boxwoods do best if they have partial sun during the growing season. However, during winter the site should offer protection from sunshine and wind. Plants exposed to continual, direct sun in winter will have reddish-brown or yellow leaves due to rapid temperature changes. Windy sites in winter will cause the boxwood to lose water faster than it can be absorbed. This will also cause reddish-brown discoloration of the leaves. Boxwoods planted close to the south or west sides of buildings often experience winter bronzing. English box is more resistant to this than Japanese box.

Soil Texture: A loamy soil, which has nearly equal portions of sand, silt and clay, is ideal. A sandy soil generally does not have sufficient water-holding capacity. Heavy clay soils often lack good drainage, which results in the roots being constantly wet, reducing available air. When the clay soil does dry out, it is quite hard, inhibiting root growth. To correct heavy clay soil, first add organic matter to create pore space for proper root development; then add fine pea gravel to improve the structure and plant in a slightly elevated berm.

Root Competition: Competition for water and nutrients from nearby trees, shrubs, and turfgrass should be avoided. Ideally, the site should be free of root competition. The roots of many plants will reach out as far as the drip line, some much farther. Site conditions can be modified by appropriately increasing water and fertilizer applications to compensate for the loss due to root competition. An effective long-term solution involves transplanting the boxwood or the interfering plant.

The majority of boxwood roots are near the soil surface. Because of the extensive lateral root system, boxwoods should not be planted too deeply or in locations that restrict the spread of roots. Root development will be stunted if growing too near the foundation of a house, sidewalk or other barrier.

Planting and transplanting

Planting and transplanting is best done in early fall, generally October. This allows the boxwood time to produce new roots, which will help the plant take up water during the winter. As long as the ground is not frozen, the plant spends the majority of its time in winter producing new roots.

An alternative season is early spring, before new growth starts. Late spring and summer can be a difficult time for moving boxwoods. Monitoring cultural conditions is important in this case.

When planting, dig the depth of the hole the approximate size of the root ball. It is best to place boxwoods on firm soil. One of the most common mistakes in transplanting is to plant a boxwood too deeply. The top 1/8 of the root ball should be above the existing soil level. Place the ball into the hole, carefully removing the burlap first. Fill the hole with soil and water slowly at the base of the plant. Top up the soil to just above the natural soil level, allowing the box to make new roots from the trunk.

Do not thin the foliage after planting or transplanting. It's the leaves that produce sugars the plant needs for root growth. By reducing the leaves, the plant's ability to produce new roots is proportionally reduced. The more vigorous the root growth, the quicker the plant becomes established.

Watering

The best guide to watering is watching the soil. It should be slightly moist from the surface to 12 to 18 inches below the surface all year round. As a general guide to watering, boxwoods need about 1 inch of rainfall every 10 days from spring to fall. If the 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 inch of rainfall every three weeks whenever the ground is not frozen.

Only through time and observation can an appropriate watering schedule be developed for a particular site. Preferred watering devices include soaker hoses and trickle irrigation. It is important to water to a depth of about 18 inches. This will encourage the roots to grow deep into the soil. Overhead watering only wets the foliage, which encourages Volutella leaf and twig blight. Additionally, 15 to 20 percent of the water can evaporate before it is absorbed into the soil.

Fertilizing

There is no regular schedule to guide fertilization of boxwoods, although, in general, plants in pots should be lightly fertilized every two months. The most reliable guide to applying fertilizer is by testing the soil. However, if the boxwood begins to show signs of nitrogen deficiency then it may be time to fertilize. The earliest symptom of nitrogen deficiency is yellowing of lower leaves. It will have a rather uniform yellowing that is more pronounced on the older leaves inside the plant. The leaves then become smaller and thinner and turn quite bronze in winter.

Granular, urea fertilizer with a 10-6-4 or similar normally works best, but this may need to be adjusted depending on soil type. Late fall applications of fertilizer promote root growth and provide the best results. The fertilizer should be broadcast around the base of the plant, just beyond the drip line. Surface application is the easiest and fastest technique. It is effective around the drip line because the most active roots are located there. Fertilizer particles that come into direct contact with the roots of unmulched boxwoods can cause root burn. If the fertilizer is over-applied, this will cause the foliage to brown and may even result in branches dying. This can be avoided by using broadcast fertilizers only on mulched boxwoods when the soil has adequate moisture. Use care when fertilizing, as direct contact with leaves can defoliate the plant.

Mulching

To maintain vigorous plants, add 2 to 3 inches of organic mulch over the soil surface. Mulching not only keeps the shallow roots cool, but also conserves water by minimizing evaporation of moisture from the soil. Mulch also reduces weed problems. Decomposed leaves or compost from the home is an excellent source of nutrients and organic material when applied as a mulch.

Pruning

Slow growing, broadleaved, evergreen shrubs, like boxwoods, have the majority of their growth from buds at or near the ends of new branches. Some develop a dense outer shell of foliage with only a few leaves in the center. Most require little pruning except when an occasional branch outgrows the general habit of the plant. Boxwoods develop more of a main-branch framework than do many faster growing shrubs that tend to sprout vigorously from the base.

Proper pruning can exploit the natural characteristics of a specific cultivar as well as enhance its appearance and maintain its health. There are three different methods; each may be appropriate depending on the desired results. They are thinning, shearing and pruning.

Thinning: Thinning is a type of pruning that reduces the number of branches at the outer edge of the shrub. For example, on a typical three foot tall B. sempervirens 'Suffruticosa' small branches 6 inches to about 8 inches are removed in a random fashion. Thinning will allow the center of the plant to receive the proper amount of sun and air. Preferably this should be done annually, but is necessary at least every two years. Thinning can be done anytime the weather is suitable for working outdoors, when the temperature is above freezing. A good time to do the thinning is late November to December. The cuttings can be saved and used for holiday decorations.

Shearing: Boxwoods are often sheared to control the size or the shape. Topiary and hedges are examples of shearing, and the effect is usually quite spectacular. Electric shears are best used to achieve straight edges for a hedge. Hand shears will give best results when used on topiary.

The best time to shear the plants is early June. A second light pruning may be desirable in July to keep the foliage neat. It is best to complete all shearing before August, or by July 15 in zones 6 and colder. Pruning after these dates can encourage late growth that may still be tender when freezing weather approaches and die back will result.

Boxwoods are highly stressed by shearing. Shearing encourages dense, multiple tip branching, leaves are cut in half, and diseases are more prevalent. However, once these concerns are understood, they can be compensated for by proper care. For example, in addition to shearing to maintain the desired shape, the boxwood must also be thinned. Thinning the plants permits the inner portion to receive light and air, which will encourage growth on the inner stems and reduce the potential for diseases.

Cutting Back: There are two reasons for cutting back boxwoods: to control growth, or to restore overgrown boxwoods. Reducing the size of overgrown boxwoods requires drastic corrective pruning. This work is best done in early winter. After a major pruning, the boxwood may appear unsightly until the foliage fills in.

If a small amount is to be cut the entire pruning can be done at one time. If a more severe pruning is needed, then a two-step approach is best. First cut large branches on just one side of the plant. The second year, the other side of the plant would be pruned in the same manner. To make the cuts on large branches, a small, sharp-toothed, curved handsaw will perform the best. The cuts should not be made flush again against the crotch, but just at the outward edge of the natural collar. This will promote the most rapid healing and compartmentalize the wound area.

There are two concerns associated with pruning cuts. Fresh pruning cuts attract female boring insects looking for a place to deposit eggs. These cuts are also open to infection if made during wet or moist weather when canker-forming fungi are sporulating. Tree coats should not be applied to pruning cuts because they may actually injure the plant and delay healing. A small saw is recommended because it produces the cleanest cut.

Pests

Insects are the most abundant and most successful form of life on earth. There are about one million species of insects that have been identified. But fortunately, only about 10 percent of these can be considered pests, and there are only five that have a serious impact on boxwood. In order of their ability to damage boxwood they are leafminer, mites, psyllid, scale, and webworm.

Leafminer: The boxwood leafminer, Monarthropalpus buxi, feeds on the tissue between the top and bottom of the leaf. The resulting damage appears as an irregular oval swelling on the leaf. There may be a slight blistering of the leaf on the lower surface with a yellowish or brown discoloration. The new foliage will not show this blistering effect until late summer. Early signs are holes on the lower leaf surface after the female deposits her eggs. Leafminers prefer the protected part of the boxwood, the lower and innermost leaves. Damage from high infestations results in premature leaf-drop.

Control is necessary when persistent or intolerable damage is observed. Control measures should be scheduled when adults are seen, usually late April to early May. Contact insecticides are effective when used against the adult. Systemic insecticides may be used against the first instar of the larvae in mid-June. While cold temperature does not cause mortality to the over-wintering larvae, they will dry up and die through the lack of moisture during hot, dry summers.

Mites: The mite is not actually an insect, but in a separate class, the Arachnida. The boxwood mite, Eurytetranychus buxi, is a rather common and widespread pest. Mites are also very inconspicuous. Early damage is not obvious and the problem is often overlooked until high populations and damage have been reached. The mite feeds on the upper and lower leaf surfaces. Depending on how large the population is, it will produce varying degrees of leaf stippling. On close examination, the damaged leaf appears as if someone took a small needle and made tiny white scratch marks.

Many species of insects and other mites feed on boxwood mites. The presence of mites can be determined by lightly 'beating' a small branch over a white sheet of paper. The mites will fall onto the card and can be seen moving around. It is important to delay control measures until mites reach a damaging level. Damage occurs at about 25 mites per 'beat'. This '25-mite' level permits predatory insects and mites to become established. Horticultural oil is effective if applied at a summer rate. It will kill eggs and adult mites. Dilute horticultural oil half strength when used this way.

Psyllid: The psyllid is a nymphal insect that feeds in the spring when boxwoods produce new leaves. It consumes leaf cells, causing the young leaves to become distorted and cupped inward. The damage is quite conspicuous. This condition is unsightly and will stop growth at the shoot apex for a 2 to 3 year period. Large populations can kill the young leaves, but do not otherwise damage the plant.

Low populations are common and injury is usually not serious enough for control measures. Control is used to reduce the population and their damage to a level that will not affect the growth and overall health of the boxwood. Correct timing in most effective in reducing populations. Good control can be obtained by using insecticidal soaps. Make an initial application as the new growth emerges in April, with a second application three weeks later.

Scale: There are a number of different scale insects that attack boxwoods. For the majority of boxwoods grown in the more temperate northerly regions, these scales are only occasional problems and are not generally destructive. The one exception is the wax scale, which attacks boxwood almost everywhere.

Correct timing of treatments will provide superior results. The eggs usually hatch in early June and the emerging crawlers are quite active. This is the best time to apply controls that should cover all of the trunk, stems and branches. Foliar sprays of horticultural oil, insecticidal soap, or residual insecticides may be used.

Webworm: Compared to all other pests, the boxwood webworm is a minor problem. However, it does have regional population outbreaks that make this insect important enough to mention. The larvae spin loose webs along the stems and twigs while feeding on the innermost leaves. Since many spiders inhabit the interior portions of boxwoods, the webbing may be a bit confusing. The real question is whether these are spider webs or webworm webs. The webworm will have fecal pellets and partially chewed brown leaves scattered through the webbing; spider webs will not.

Usually, measures to control the webworm are not necessary because population levels are seldom high enough to necessitate any action. Manually remove and destroy webs and caterpillars in small infestations.

Diseases

Fungi are responsible for more plant diseases than any other group of organisms and most of the diseases in boxwood. Fungi are small, generally microscopic in size, with a plant-like body. Since fungi lack chlorophyll, they spend part of their lives taking food from host plants. Keeping boxwood healthy with proper care greatly reduces the potential for disease. Early diagnoses and prompt corrective measures will give the most effective results when treating disease. The diseases of boxwoods can be grouped into two categories; those that affect the foliage or twig and those that affect the roots. Both produce symptoms that appear on the foliage. Listed below are the diseases that can have a major effect on boxwood and their symptoms:

Phytophthora: This soil-borne fungus damages leaves, stems and roots. Infection begins when the soil is wet and cool, about 58 to 70 degrees F. The disease will progress and cause injury only under higher soil temperatures. Phytophthora produces spores that move in a water film in the soil and is most damaging to boxwoods growing in poorly drained soils.

When infected, the leaves will first gradually turn yellow, then the edges will become wavy. Next the leaves will change to a bright straw color and remain attached to the twigs. This may happen to one branch, or several, or throughout the entire shrub simultaneously. The stem will have a dark brownish-black coloration of the vascular tissues under the bark at ground level to a few inches above. The fungus can cause a partial, or even complete, blockage of nutrient and water movement in the stem. The roots will appear dull and dark brown in texture. The 'bark' will be decayed and is easy to remove.

Avoid the initial infection by planting boxwoods in soil that has adequate internal drainage. If a boxwood is weakened by Phytophthora transplant the boxwood to a site that is well drained. If transplanting is not practical, consider drains and/or grade changes to redirect water away from the boxwood.

Macrophoma Leaf Spot: This fungus attacks weakened or decaying branches, but is only of minor concern. It is classified as a secondary invader or a weak parasite. Usually the fungus infects plants that have been weakened by poor culture, winter-burn or have overly thick foliage.

Infected plants will have tiny, black, raised spots on the underside of either the light green or, more commonly, tan-colored foliage. These are the round and hollow fruiting bodies of the fungi and will first appear on the oldest leaves inside the center of the plant. As the infestation progresses it affects the younger leaves, which will defoliate. Heavy infestations can cause entire branches to die in only a few weeks.

Typically, the Macrophoma leaf spot can be corrected by pruning out the infected branches. In addition to pruning out the fungus, the removal of branches will improve the air circulation throughout the plant, which will help contain and eliminate the fungus. The fungus prefers a moist, cool, dark area, which can be found in the center of dense boxwood plants.

Volutella: This fungus is stem blight or canker and is the most serious disease to affect mature and overly thick 'Suffruticosa'. It can be especially severe in periods of high humidity. A heavy infestation results in defoliation and the death of entire stems.

Volutella will show up in moist weather as a mass of creamy or light pink dust or fuzz-like growth visible on the underside of the leaf. These colorful spores are very distinctive. This stage is often overlooked because it forms on the densest portions of the plant. The pink spore masses may not be visible under dry conditions. As the infection continues, the outer green leaves will become quickly and progressively discolored, changing to a dark brown and then tan color. The branch appears and smells almost as if diesel fuel has been poured on it. As the disease progresses, the entire branch will drop all of its leaves. In addition, the soft tissue of the current year's growth will turn black. If the plant is weak, the discoloration will extend well into the previous year's growth, producing a stem canker.

Avoid the initial infection. Do not water boxwood with oscillators or other methods that wet the foliage. Frequent rain or overhead watering encourages establishment of Volutella. Maintain a properly thinned boxwood to improve air circulation, which quickly dries the plant. To control established infections, prune the diseased branch 6 to 12 inches below the affected tissue. This is best done during dry weather to avoid spreading the disease. Removing the diseased branch from the site will provide nearly total control.

Nematodes: These are small eel-like worms that cause plant disease. Nematodes live as an obligate parasite, which can only grow and multiply when living on a host. However, they can live as an egg or cyst for several years without the benefit of a host.

The first visible symptoms of nematodes are wilting, stunting, yellowing and/or bronzing of the foliage. These symptoms can also indicate cultural deficiencies, but when they show up on boxwoods growing in the proper conditions nematodes are usually the culprit. Still, the only reliable method of determining the presence of nematodes is to collect soil and root samples from a suspect boxwood and have the samples examined by a plant nematologist.

Nematodes cannot be totally eliminated from the landscape. The goal is to keep the population low enough to prevent damaging symptoms that weaken the plant. Boxwood should not be grown in soils heavily infested with nematodes. In the long term, growing plants such as grasses that are not affected by nematodes will reduce nematode populations. Current control options for the lesion nematodes are either not effective or practical in the landscape. A biological control, Bacillus pentrans, has been effective when treating the soil for root knot nematodes.

Decline: Decline is thought to be the result of fungi and/or nematodes that invade the root and/or crown portions of the boxwood that are culturally weak. There does appear to be a complex of several fungi, parasitic nematodes, environmental and cultural factors associated with decline. Decline is limited to 'Suffruticosa'.

The deepest roots are affected first. As the disease progresses, the stem below the ground begins to turn brown. This will extend upward into branches, often in a random pattern. The aboveground symptoms take the form of small leaves that are brittle and yellow or red-colored as well as characteristic browning of one branch or several ranches in a random fashion. In severe cases, there is a sudden wilting of foliage or dieback of entire branches, resulting in the death of the boxwood.

Boxwoods in poor health are susceptible to decline. There is no tested, proven treatment for boxwood infected with decline. Initial infections can be avoided by keeping 'Suffruticosa' culturally healthy.

Although these are valuable culture trouble-shooters, boxwood in the Pacific Northwest is rarely stressed and consequently has little tendency to failure by disease. We find most of the plants purchased as replacements (as opposed to new installations) as a result of accident