Box & Boxwood, your questions answered

Botanical Name: Buxus sempervirens

History

Most people are surprised to discover that the low and neat, deep green Box hedging, synonymous with the formal parterres and rose gardens of elegant houses, is in fact botanically classified as a tree; reaching a mature height of 12m by 8m when left to its own devices. European Box or Buxus sempervirens in Latin, is native from southern England to northern Morocco and the Mediterranean where it can be found growing wild on scrubland, woodland, and hillsides living for several hundred years. ‘Box Hill’ in Surrey gets its name from the ancient box woodland found on the steepest west-facing chalk slopes which is one of the most established plantations in the UK believed to date back to the medieval period.

Common box timber is finely textured yet hard; perfect for wood engraving and making musical instruments. Chemicals found within the plant have been used medicinally for a variety of ailments over the centuries and in homoeopathy, the leaves are used to treat rheumatism, HIV, and fever. However, all parts of the tree are toxic and may irritate the skin or cause a stomach upset if ingested so don’t be tempted to brew up a home remedy and always wear protective gloves when pruning!

Buxus sempervirens is commonly used for topiary and hedging in gardens. It’s slow and compact growing habit (up to 10-15 cm a year) makes it the ideal candidate for formal, low hedging which can easily be maintained at between 0.5m and 1.5m, as well as being a classic topiary plant clipped into intricate shapes as well as parterre style knot gardens.

Box topiary has been used historically in many different European gardening styles, from the early Roman gardens through to modern day. From domes to animals, it can be trained into almost any shape. A variety of classic topiary forms such as balls from 30cm to 1m, cubes, spirals and cones, are widely available from specialist growers or sometimes garden centres. For an even lower growing hedge (ideal for less than 1m high) use the dwarf variety Buxus sempervirens ‘Suffriticosa’.

Identity

If you are not familiar with the appearance of Box, look out for densely packed, oval, dark evergreen leaves which feel leathery and are slightly aromatic when disturbed. The bark is smooth and silvered with green downy stems. On clipped specimens’ new growth is a vivid lime green and soft to touch, turning darker with age.

Common box is monoecious (moh-nee-shus) meaning both male and female flowers are found on the same tree. In April and May clusters of tiny, insignificant greeny yellow flowers can be found growing between the leaf axils (the angle between the upper side of a leaf and the supporting stem), each one comprising of several male flowers and a terminal female flower which on mature plants can eventually turn into a brown, woody seed case.

Being a popular garden plant, it is very likely you have grown it in your own garden or maybe   seen it used as topiary outside front doors in your neighbourhood or perhaps when visiting a country estate where it was traditionally used as edging for the old kitchen gardens or a formal knot garden. It is unusual to find it growing wild or unpruned unless in southern parts of the UK.

Pruning and Feeding

Buxus sempervirens is not too fussy about its growing conditions and will tolerate clay or chalk soil, full sun to full shade and even coastal sites. However, it does not like an overly wet or windy environment.

Wait for a dry and cloudy day in late May or early June; after all risk of frost has passed to prune Box. This ensures the best conditions to avoid spreading disease which can access the fresh wounds created by pruning and infect the plant easily. It also prevents foliage scorching as damaged leaves are more prone to drying out in the sun on a clear, hot day.

When pruning Box it is important to use freshly sharpened and sanitised hand sheers. Best practice is also to sanitise your cutting tool between pruning each separate plant or regularly along a hedge which helps to reduce the spread of infection. It may seem like a faff, but it is worth taking the extra care to maintain good health of your hedge or topiary which can often be many years old and very costly to replace. A good tip is to carry a bucket containing the necessary disinfecting equipment. There are commercial liquids specifically designed for pruning tools and rubbing alcohol (isopropyl alcohol) is also a good option. You may also want to keep a bucket of water close by, because box can secrete a sticky sap from fresh wounds which will need to be rinsed from the blades to keep them cutting cleanly. Always wear protective gloves to avoid skin irritation.

In spring, young foliage is susceptible to frost damage and if caught will turn a papery brown. This is a cosmetic defect and new growth will soon appear when the weather warms up and for this reason, it is prudent not to prune too late in the year. If required, Box can be lightly tidied up in September to ensure a crisp finish which will last through the winter months.

Feeding Box makes it more resistant to disease. Established Buxus planted in the ground should be given a general-purpose fertiliser such as Fish Blood and Bone in the spring, it is also a good time to mulch with well-rotted manure or compost to help with moisture retention in the warmer months and reduce rain splash on the foliage which can carry potentially harmful fungal spores. We also highly recommend using the foliar feed ‘Topbuxus’ throughout the growing season to maintain healthy and vigorous, disease resistant plants.

Box Hedging Problems

In recent years Box has dropped down on the popularity ratings list in some areas of the UK and is avoided for new gardens by designers due to the pest and disease double whammy of box blight and box moth caterpillar which both severely weaken and sometimes kill the plant. It is an epidemic of the plant world thought to be stimulated by changing climates. Both are causing quite a lot of problems for gardeners particularly those with mature, existing Buxus plants.

Box Blight

Box blight is a disease affecting leaves and stems of plants in the Buxaceae family, caused by the fungus Cylindrocladium buxicola. Symptoms include foliage turning brown and falling, leading to bare patches. Black streaks and dieback on young stems.

Prevention is the best strategy to fight the disease and any hedging that is specified by Bestall & Co will come with advice to feed with the foliar feed product ‘Topbuxus’ regularly throughout the growing season (March and October) which rapidly increases the health of your Box plants. We have seen great success with this, as plants in poor health and condition are far more likely to contract the disease.

We also do not use Box topiary which needs trimming more regularly or use Box in planters and avoid planting Box in areas of constant damp, shade, and poor ventilation.

Early diagnosis can make all the difference and may nip the infection in the bud. Advanced infections can be very difficult to control and often it is best to consider an alternative species, as the disease lives in the soil for up to 6 years and is unlikely to go away.

If you have not suffered with box blight, then preventative measures include:

  • Clipping less regularly to create more ventilation throughout the plants; regular pruning creates dense foliage and less air movement.
  • Cleaning pruning tools with a garden disinfectant between different areas of the garden and between gardens to minimise spread of the disease.
  • Inspecting plants for early symptoms as box blight spreads very rapidly in warm and humid conditions.
  • Watering plants at the roots, rather than from above to avoid creating a moist humid environment.
  • Using mulch under plants to reduce rain splash.
  • Feeding plants in spring and during the growing season.

If infection does occur, remove and destroy any plants that appear to be affected immediately.

Fungicidal sprays can be administered but are unlikely to be effective against box blight unless combined with the other strategies to control, as they supress the outbreak rather than kill the fungus. Consider also that there may be negative side effects to the local soil micro-biome and surrounding environment which could impact how well the plants access nutrients from the soil thus making them more susceptible to infection.

Remove all fallen leaves from the centre and around the base of affected plants and strip off the surface of the topsoil which will contain fungal spores. Infected material should be bagged to avoid dropping debris around the garden and binned – not composted.

Cut back or cut out affected parts of less severely attacked box plants or valuable specimens you are trying to save. Feed plants that have been cut back with a general-purpose fertiliser to aid recovery.

Box Rust

Box Rust is a very mild condition where some foliage can be affected by a rust-coloured blister. There is no long-term damage and treatment is to trim off the affected foliage.

Box Tree Caterpillar

Box tree caterpillars are the larvae of a moth that feeds on Buxus plants, native to East Asia and first reported in the UK in 2011. They have since become widely distributed across England particularly London and surrounding areas. This new pest is highly likely to ​reoccur repeatedly throughout the growing seasons of the future.

The adult moth usually has white wings with a faintly iridescent brown border, although the wings can be completely brown or clear. The female lays pale yellow flattish eggs on the underside of box leaves. Newly hatched caterpillars are greenish yellow, with black heads. Older caterpillars reach up to 4cm in length and have a greenish/yellow body with thick, black and thin white stripes along the length of the body. The pupae are concealed in a cocoon of white webbing spun among leaves and twigs.

The caterpillars feed on the leaves within a webbing they make over the foliage and can completely defoliate box plants in a matter of days eventually killing the plant. At the start of an infestation, plants will show patches of dieback which may be very apparent on trimmed plants and therefore can be easily be confused with the fungal disease box blight to begin with but if you see your Buxus covered in webbing and small grit-like black balls (caterpillar poo) it is a sure sign that it is box tree caterpillar.

The most successful way to eradicate an infestation is a combination of techniques used together.

  • Where practical, caterpillars should be removed by hand and destroyed (squashed or drowned).
  • Pheromone traps are available to catch the adult male moths to try and reduce the breeding population and diagnose or predict if your location is susceptible to infestation.
  • Bacteria called Bacillus thuringiensis are often sold online as a treatment for box caterpillar. However, this type of treatment should only be used by professionals who have been trained in its use. Topbuxus Xentari is one popular product but still doesn’t have a license for amateur use in UK gardens.

Chemical insecticides are not recommended due to their indiscriminate and negative impact on other beneficial pollinating insects.

Natural predators to the caterpillar are on the increase and there have been recent reports of native birds such as blue tits feeding on the caterpillars in some locations, as they become familiar with a new food source. It is not yet clear if this predation will result in a reduction of box tree moth numbers in the future but there is hope.

Alternatives to Box

If you decide for one reason or another that Box is not for you, there are a variety of compact, evergreen shrubs which can be used as alternatives however some may need more pruning to keep the formal habit. The following all have small leaves and can be clipped into neat hedging styles:

Berberis darwinii ‘Compacta’
Elaeagnus × submacrophylla ‘Compacta’
Euonymus fortunei

Euonymus ‘Jean Hugues’
Ilex crenata
Lonicera nitida ‘Maigrün’
Osmanthus delavayi AGM
Pittosporum tenuifolium ‘Golf Ball’ (PBR)
Taxus baccata ‘Repandens’ AGM