Boxwood Questions and Answers

Boxwood Questions and Answers

Why are they called boxwood? And shouldn't you be calling them boxwoods when referring to them in the plural?

The name boxwood is derived from the young stems of some species of boxwood they are strongly four sided and are square in cross-section. Boxwood is the word to use when referring to this plant whether you are talking about one or many of them. There is no such word as "boxwoods".

Deer are a problem in my neighborhood. Will they eat boxwood if I plant it

No. Boxwood leaves contain alkaloids that are distasteful or poisonous to deer and they do not browse boxwood. It is one of the few evergreen shrubs that is usually not damaged by deer.

What conditions do I need to grow good boxwood?

You can grow boxwood in a wide variety of soil types as long as the pH is alkaline or slightly acidic and the soil is not compacted or poorly drained. Contact your local extension educator to find out where you can get your soil tested for pH. If your soil is acidic, you will need to add dolomitic lime to bring the pH up to the range of 6.5 to 7.5. Internal soil drainage is critical for boxwood. It grows poorly on heavy, compacted soil or in areas where the roots might be waterlogged for long periods of time. Boxwood can be grown in full sun or partial shade and is quite drought tolerant. Winter cold limits its survival in northern gardens and special care must be taken to select a hardy variety such as 'Vardar Valley' or Korean boxwood if you live where winters are frigid.

Should I mulch my boxwood? If so, what should I use?

Boxwood have shallow roots and benefit greatly from a layer of organic mulch at the soil surface. Shredded hardwood bark is a good mulch for boxwood since it tends to increase the pH of the soil as it decays. Voles, also known as meadow mice, enjoy the protection of a thick layer of mulch and their gnawing can devastate a boxwood planting, so limit the thickness of the mulch to one inch and keep it at least six inches away from the trunk.

My boxwood foliage turns orange every winter. How can I prevent this? Should I apply an anti-desiccant to the foliage?

Some boxwood lose much of their green color in winter if they are grown in full sun. With the loss of green color, they develop an orange cast. The green color returns rapidly when temperatures warm in the spring. There is really nothing that you can do to prevent this color change, except planting a tree nearby so your boxwood will eventually have a bit more shade.  An anti-desiccant will not help to preserve the green color, and may actually injure the boxwood.

Large branches of my boxwood turned orange and then straw-colored. What is causing this, and what should I do?

Some experts blame a disorder known as boxwood decline for these symptoms. Boxwood decline is most likely caused by a complex of diseases that appear when boxwood are suffering cultural problems such as cold injury, poor drainage, or improper pruning.

The fungus Macrophoma is a foliar pathogen that causes the oldest leaves to turn tan; small black fruiting structures can be seen on the killed leaves. Shake the major branches of your boxwood one by one to dislodge the infected leaves and then rake them from under your boxwood.  Dispose of them so the spores don't cause new infections. Another fungus, Volutella, may cause the branches to develop the color progression from orange to tan. Look for orange-pink fruiting structures along the branches in wet, warm weather and prune out infected branches.  Macrophoma and Volutella are most destructive to boxwood varieties with a tight, compact habit or to boxwood that have been sheared repeatedly. The tight foliage results in poor air circulation and slow drying after rain or dew; fungal diseases thrive in these dank conditions.

The best way to control these diseases is to improve air circulation within the plant by thinning the branches.  Cut some of the small branches back by about six inches. Thin the plant enough so you can begin to see the overall branch structure of the shrub. Avoid shearing since it promotes compact, twiggy growth and injures leaves, making them unsightly.

Root rot fungi such as Paecilomyces and Phytophthora may cause entire plants to develop the weak, stunted growth that is off-color, but the damage is usually widely spread throughout the plant. Loss of many of the fibrous roots is often caused by these fungi, and root pathogens are most problematic where soil drainage is poor. Improve soil drainage by boring deep holes in the soil around your boxwood and backfilling them with shredded hardwood bark or another loose material.

The new growth on my boxwood is curly. What causes this?

Curled leaves are the work of the boxwood psyllid, a small sucking insect that appears about the time the new foliage appears in spring. Growth is stunted and the leaves curl in response to psyllid feeding. Tolerate the damage unless most of the branch tips are infected; psyllids rarely affect the long term health of boxwood. When necessary, control psyllids by spraying plants thoroughly with horticultural oil or insecticidal soap as soon as they appear in spring.

The leaves on my boxwood look puffy and have orange spots and the foliage looks thin and unhealthy. What's the problem and what should I do?

This describes damage caused by boxwood leafminer, one of the most destructive pests of boxwood. It is difficult to control leafminer larvae because they live inside the leaves where they are sheltered from most pesticides. Adult leafminers are gnat-like, orange insects that fly around the plants in April or May when the first flush of growth is maturing. They lay several eggs inside the leaves; the larvae hatch but do not feed much until the following autumn. The young leaves look as though they were pricked with a pin. As cooler weather arrives, the larvae feed on the inside of the leaves and cause the upper leaf surface to separate from the lower leaf surface.  Damaged leaves turn orange and drop prematurely. A heavy infestation of leafminers may nearly defoliate boxwood, so you may want to treat plants if damage is significant. Use a systemic insecticide applied to the foliage in spring to kill some of the adults and the hatching larvae or apply a granular systemic insecticide to the soil around the plants in early spring. If you are planting a new boxwood and leafminers have been a problem, consider planting English boxwood, Buxus sempervirens 'Suffruticosa'. It is very resistant to leafminers.

I want to buy some specific cultivars of boxwood. Where can I find them?

Check the National Arboretum's Plant Sources Page for tips on finding a nursery that sells specific boxwood varieties that you want.

Source - The United States National Arboretum