June 6, 2014 5:22 pm

Why the world’s favourite topiary plant Buxus is under threat

New techniques are being developed to protect the box hedge in the fight against blight
The overhanging gardens of Marqueyssac in Dordogne, France©Alamy

The overhanging gardens of Marqueyssac in Dordogne, France

For centuries, Buxus sempervirens has been the go-to plant for anyone wanting a low- to mid-height, small-leaved, evergreen plant for topiary-making or low hedging. For an aesthetically unspectacular plant, it is definitely a case of making the most of humble attributes; Buxus doesn’t have exotic blooms or sumptuous fragrance, but it takes clipping and holds its shape like nothing else.

However, its years of domination are under threat. In the past decade an aggressive fungal disease specific to Buxus has become ever more apparent in western gardens. Popularly known as “box blight” (Cylindrocladium buxicola), the disease has attacked Buxus plantings regardless of age, location or historical significance, and the effects can be devastating. Until very recently there were no preventive measures that really worked, no chemical control to help stem the spread of the disease. Yet now, through a combination of selective breeding, chemical-based control and old-fashioned “good practice”, there may be hope on the horizon.

Buxus – boxwood to US gardeners, Buxus to continental Europeans and plain old box in the UK – is a large genus of plants of about 70 species worldwide. The majority are tropical or subtropical; Cuba is a box hotspot, with 30 species alone, and there are 17 species native to Madagascar. The remaining species are distributed across western, Central and South America, China, the Caribbean, Mexico and Europe. Only the species from Europe and a few from southeast Asia are frost hardy, and fewer still are useful garden plants. Buxus sempervirens, native to Europe and north Africa, has long been thought of as the most useful of all for western temperate gardens.

The inherent capacity of Buxus to withstand clipping into an endless variety of geometric or free-flowing shapes has been admired for thousands of years. The ancient Egyptians are believed to have used clipped box in their gardens more than 6,000 years ago, and its virtues were recognised by the Greek horticulturalist Theophrastus in 300BC. It was a key component of Roman planting schemes; Pliny the Younger made Buxus gardens, and the Romans reintroduced the plant to Britain (it was native here before the last ice age), using it in the villa gardens they created. Even in the Middle Ages, when gardening for aesthetic pleasure became an endangered pursuit, the monks that tended the monastic gardens continued to use box as an edge plant for beds of edible and medicinal herbs.

Box topiary at Tintinhull Court, Somerset, England©Howard Rice/Gap Gardens

Box topiary at Tintinhull Court, Somerset, England

Buxus had its second coming from the late 15th century, as the fashion for knot gardens and then parterres took hold. It was the ideal intersection of planting style and plant, for Buxus is ideally suited to the elaborate scrolls and swirls of the parterre, the mid-green leaf colour the perfect foil for silver-leaved lavender or buff gravel. Nathaniel Sylvester, an English royalist who fled the Cromwellian Commonwealth, reputedly planted the first Buxus in North America at his home on Shelter Island (in what is now the Hamptons) in 1652.

The ancient Egyptians are believed to have used clipped box in their gardens more than 6,000 years ago

After a rough time during the landscape era of Capability Brown and Humphry Repton – when elaborate box planting was condemned as being too fussy and contrived – Buxus made a steady comeback during the Victorian era and has been around ever since, sometimes in fashion, sometimes not. The late Christopher Lloyd’s father, Nathaniel, published a book titled Garden Craftsmanship in Yew and Box in 1925, and more recently box has featured in gardens designed by Piet Oudolf, Jacques Wirtz and Tom Stuart Smith in the voluptuous forms of topiary and crisp hedging. There were some spectacularly plump box orbs in Tommaso del Buono and Paul Gazerwitz’s gold medal-winning garden at the Chelsea Flower Show last month.

Fruits of Buxus sempervirens©Alamy

Fruits of Buxus sempervirens

The risks posed by box blight in gardens old and new have increased due to hotter, more humid summers. An infection of Cylindrocladium buxicola is often followed by a secondary infestation from Volutella buxi, another fungal blight. C. buxicola infection starts with black spots on the leaves, which start to fall off within a few days, exposing tell-tale black stripes on the twigs. The spread can be rapid, such that by the time the symptoms are noticed the fungus has already taken hold. V. buxi causes leaf browning, followed by foliage wilt. Unlike with C. buxicola, the leaves remain on the plant, on which pale pink mould spores appear. Either one of the two blights can be hugely damaging, killing swaths of hedging and individual topiary plants large and small. Apart from the historical cost, the financial implications of replacement can be massive – assuming, once bitten, you are willing to take the risk again. Buxus is slow growing: a very large topiary ball will take about 18 years to grow and shape, and will retail for well over £1,000 for a single plant. B. sempervirens “Suffruticosa”, probably the most widely planted form for hedges and topiary, is even slower growing – and is the most prone to blight of all box cultivars.

The Gardens of Séricourt in the PasdeCalais,©Alamy

The Gardens of Séricourt in the PasdeCalais,

With their businesses at stake, European Buxus growers have been investigating ways to battle back against blight. They have developed a preventive and, to an extent, curative chemical control programme. This relies on regular monthly spraying of box plants from May to September, the key months when blights tend to strike. From May to July the chemical used is tetraconazole, and in August and September, thiophanate-methyl-prochloraz. There are snags; in many countries they are not available on the domestic market, and can only be applied by qualified professionals. Not everyone will want to use chemicals in their garden with such regularity, if at all, even if they are relatively benign. In the EU the chemicals available legally to the amateur change frequently as bans are imposed, so today’s preventive treatment may not be there to apply tomorrow.

There are also practical preventive measures that can reduce the risk of blight. Fungal diseases thrive in the fine droplet humidity created by spray irrigation, so watering once a week, direct to the root zone, is recommended over the more frequent use of pop-up sprinklers. Excessive feeding is discouraged, as soft sappy growth is more prone to infection. There are a number of slow release “tonics” available for box that seem better suited than high-nitrogen fertilisers.

©Victoria Birkinshw

Perhaps the most interesting area of research is in new, less disease-prone varieties. Didier Hermans of Herplant BVBA, a leading Belgian grower and world expert on Buxus, has been breeding new cultivars and testing existing varieties of the less disease-prone B. microphylla (Japanese boxwood). Using DNA referencing and
the large reference collection at his nursery, Hermans is now able to recommend a number of plants that, while they cannot be guaranteed as “blight free” are certainly showing good resistance. B. microphylla “Faulkner” is an excellent substitute for B. sempervirens “Suffruticosa” as a low hedging plant, while B. microphylla “National” has darker green leaves and makes a handsome topiary specimen, as does the round leaved B. microphylla “Trompenburg”. B. microphylla var. koreana is an excellent low-growing, ground-covering form. Hermans also recommends avoiding the blight-prone cultivars, such as the once ubiquitous “Suffruticosa”. If it is the end for that particular cultivar, the combination of new varieties and control techniques should mean that box will be a feature of gardens for years to come.

Matthew Wilson is managing director of Clifton Nurseries in London

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Alternatives to box

Rosmarinus officinalis©Mark Bolton/Gap

Rosmarinus officinalis

While there is no like-for-like replacement for box, there are some excellent evergreen plants that make good alternatives. Japanese holly (Ilex crenata) has been a popular topiary plant in its homeland for centuries and, when in fresh growth, is almost indistinguishable from box. There is, however, a blight that affects holly, although it isn’t quite as prolific or devastating as box blight. Myrtle (Myrtus communis) is another reasonable facsimile for box, clipping well although not holding its shape quite as long. Lonicera nitida and Lonicera pileata both share the low-growing, small-leaved characteristics of box and clip very well – but grow out quite quickly, requiring three or four cuts a year. For something a little different, a rosemary (Rosmarinus officinalis) hedge can be very effective, especially in a herb garden or potager. And in sun-drenched gardens and roof terraces the silver-grey foliage of olive (Olea europaea) looks good. If something larger is required, common yew (Taxus baccata) is unbeatable.

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NGS festival

The National Gardens Scheme (NGS), which raises millions of pounds every year for charity, will be holding its second open garden festival this weekend.

As well as hundreds of outstanding private gardens in England and Wales that will be opened to the public, there will be stalls selling plants, seeds and homemade cakes.

The gardens range from small and urban to massive and rural, and from celebrity to unknown.

ngs.org.uk

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