Box clever: The scientific research being done to save our buxus hedges
The future of treasured box hedges and parterres at historic gardens such as those of the National Trust in Britain and at the likes of Versailles may be assured thanks in part to the work of a New Zealand scientist.
Dr Matthew Cromey, formerly of Plant & Food Research and now principal plant pathologist at Britain's Royal Horticultural Society at Wisley, has been working on mitigating the effects of box blight, which is affecting – and at times killing – the plant around much of the globe.
The blight, caused by a fungus, first came to the attention of horticulturists in Britain and New Zealand in the mid-1990s, and in the United States in 2011.
Until then Buxus sempervirens was considered an easy plant to grow. "All you had to do was trim it," Matthew says. Pretty much the only problem it suffered was caused by volutella blight, which was relatively easy to cure.
Classified as a new disease in that it was new to science, it was unlikely it was a new fungus, Matthew explains. He believes it was probably a minor pathogen on some other plant, such as sarcococca, a genus of flowering plants in the buxus family often known as sweet box, and hailing from eastern Asia and the Himalayas. In sarcococca, it may have been a minor problem, but not so in box, where its spores land on a leaf, and, if there is enough moisture, germinate and produce a structure (appressorium) that sticks onto the leaf and penetrates it.
This is more serious than volutella which enters only via wounds, Matthew explains. "Cylindrocladium buxicola grows in the leaf, killing the tissue. It gets its nutrients from dead tissue," he says. "Classic fungi behaviour."
The dead or dying leaf then drops off and, especially in high humidity, produces another batch of spores which move onto another leaf.
Unlike, say the rust fungus which is dispersed by wind, the spores of the blight fungus are sticky and spread by what is known as rain-splash, bouncing back off the ground to reinfect the plant. This is why overhead irrigation is best avoided on box.
The very reason that box makes such a good subject for topiary and hedging makes it such an easy target for the fungus. "Buxus makes a good hedge plant because you trim it and it sprouts back beautifully," Matthew explains. "After the leaves are infected and fall, and the subsequent stem dieback, the plant grows new leaves and shoots, onto which spores from the fallen infected leaves 'splash', thus continuing the infection."
And while the gardener might think a plant has recovered, spores can live in the soil for up to seven years – as long as a leaf can survive. "Box leaves are hard leaves and take quite a long time to break down, but once a leaf has broken down, the spores go too."
Logic suggests that destroying all fallen leaves and infected tissues would solve the blight problem. Not so! "If you removed 99.9 per cent of the infected matter, it just means that it would take longer for reinfection to occur, but it will happen. That's why it is important to keep on top of the disease," Matthew warns.
Unlike say, botrytis, which has a huge range of host plants, the box blight fungus is specific to buxus and (to a limited extent) other members of the buxaceae family. It is not a threat to any other plants.
This is why prevention is, as always, better than cure. Matthew – whose work on fighting the disease involves collating research from around the world, experimentation on managing it through plant architecture, blight-resistant cultivars and combining control strategies – advocates vigilance and garden hygiene in avoiding its arrival and stopping its spread.
In New Zealand, the disease is thought to be limited to the North Island, and possibly Dunedin, but a ready and free movement of plants makes it spread relatively easily.
Box blight does well in enclosed, moist or humid environments, so take care where you plant box and how you manage it, says Matthew. He also does not recommend growing too much of the plant, such as surrounding box topiary with box hedges. "Only plant as much as you can manage. The less box you grow, the more time you can spend on looking after it."
Box leaves should be kept as dry as possible – no overhead irrigation. Mulching around the base of plants will help reduce splash – a major cause of reinfection.
Pruning and plant architecture also play a part in managing the disease. Less frequent clipping will reduce the density of the plant and allow increased airflow, he says. Box hedges should be pruned with a convex rather than flat top, and pruning should only be carried out in dry conditions.
Gardening tools and clothing – secateurs, gloves and the like – should always be cleaned between gardening stints, preferably with a garden disinfectant or a mild bleach solution.
Matthew also recommends putting new box plants in quarantine, ideally for up to six weeks, and for several months should there be lots of other box already in the garden. A new plant might appear fine but fungicides can temporarily mask infection, he says. A better bet than buying in would be taking cuttings from healthy box plants in your own garden.
The earlier the fungus is diagnosed, the easier it may be to manage. Symptoms include leaves turning brown and dropping, causing bare patches; black streaks and dieback on young stems, and in wet conditions the white spore masses of the fungus may be seen on the undersurfaces of infected leaves.
To help identify the disease, Matthew suggests placing leaves in a plastic bag with moist tissue for a few days. If white spores form, it is box blight. Spore masses of volutella will look pink.
Unfortunately, most people only notice the blight when their plants have been severely infected, says Matthew, who likens this to the tip of an iceberg. Infection will have spread further than it shows, he adds, making it wise to cut out more than you think you need to, no matter the time of year.
As the blight does not kill roots, it is not necessary to cut out the whole plant, but clippings and fallen leaves should be removed from the centre of the plant, around its base and the surface of surrounding topsoil. Then bag and put out with the rubbish rather than compost. Fungicides are only effective when used in conjunction with cutting back.
The disease is a major problem in Britain and Europe, not least because many of their historic and grand gardens have used buxus extensively and for decades – if not centuries – as an integral part of their design. Chopping it out was not feasible, for example in British National Trust gardens, as an essential part of the trust's work is preserving heritage, Matthew says.
And while the gardeners at Versailles may have taken such extreme measures in years past, work by Matthew and his colleagues around the world is showing that it is no longer necessary.
He likens it to growing roses. "They suffer diseases and pests, which are always going to be with us. You just have to manage the problems."
- NZ Gardener