We seem to be living and gardening at a time of endless and seemingly unstoppable exotic pests and diseases. Lily beetle is marching inexorably across Britain, while the tiny horse chestnut leaf-mining moth fluttered over the English Channel a decade or so ago.
Sudden oak death, ash dieback and horse chestnut black canker have all become established in Britain in recent years. Our island status does not confer immunity to these pests and diseases, which are at least worrying and, at worst, capable of redefining the landscape. For many gardeners, especially those with old, established formal gardens, the most upsetting of these new arrivals has been box blight.
Cylindrocladium buxicola was first recorded in a Hampshire nursery in 1994, but it was not until a major outbreak of the disease three years later that plant pathologists and the horticultural trade began to take notice. The fungus was identified and named in 2004, by which time outbreaks were being recorded throughout the south of England and London.
By the time we’d finished, everything looked scalped, like a garden full of bald heads
The spread of box blight is often rapid; infection starts with the appearance of black spots on the leaves, followed by defoliation a few days later. As the leaves drop, the telltale symptom of box blight is revealed – black lesions on the exposed twigs. From start to finish this can all happen so rapidly that there is little or no time to react. Visual recognition of blight is not helped by the fact that foliage scorch, which while unsightly is ultimately harmless, can be easily confused with the early stages of the fungus.
Box blight doesn’t kill the plants, but it leaves them looking so unsightly, and so diminished, that it effectively renders them aesthetically worthless. Rosemary Alexander, doyenne of British garden design education and principal of the English Gardening School, discovered this in her own Hampshire garden, Sandhill Farm House. She began her garden 17 years ago and Buxus has played a significant role in the design. There are more than 60 specimen bushes, clipped into balls, clouds and multi-stemmed topiaries. Their presence in some parts of the garden is central to its success as a piece of design. The broad gravel path in the front garden (below) that Alexander refers to, tongue in cheek, as “the motorway” is both defined and beautified by voluptuous Buxus orbs.
Box balls act as markers, flanking steps up to the house, and are integral to the architecture of the garden summerhouse. But Cylindrocladium buxicola has changed all that. “The garden has been ravaged by blight,” Alexander tells me. “The first time was three years ago, when we had a specialist in to apply chemicals to stop it.” Thinking all was well, the following year she took an extended trip to China. “When I got back I almost fell over – every single Buxus bush was grey and covered in blight.” She sent samples of her plants to the RHS for testing. “They identified three different types of blight, and another fungus!”
Alexander’s experience offers a cautionary tale. The chemical control that she deployed to tackle blight is not a cure-all. While it may be effective at controlling or even eradicating outbreaks of blight, its efficacy is intrinsically linked to regular, timely, long-term application. This effectively means carrying out monthly spraying of box plants from May to September – the key period during which blights tend to strike. But the chemicals are not available to the amateur and can only be applied by a licenced professional. Regular spraying on a monthly basis by qualified operators doesn’t come cheap, and is a long-term commitment.
At RHS Wisley, research is focusing more on managing box blight rather than pursuing a cure. “Early on, the emphasis was on trying to work out exactly what it was,” says principal plant pathologist Matthew Cromey. “The advice then, in the face of an outbreak, was to remove and destroy the affected plants.” Needless to say, this resulted in some heartbreaking decisions being forced on garden owners, with topiaries, knot gardens and parterres being ripped up and burned.
The RHS doesn’t advocate such a “scorched earth” policy today. “Once blight is in a garden it’s a real headache to remove,” says Cromey, “but there are practical steps to help avoid getting it in the first place.” The spores of Cylindrocladium buxicola are sticky and don’t travel far unaided but can be easily moved from plant to plant during pruning if cutting equipment isn’t cleaned and disinfected regularly during trimming. Simply wiping down the blades of shears with diluted bleach is enough to kill off the spores.
Blight thrives in humid conditions, so overhead watering should be avoided, and a trickle irrigation system installed directly to the surface of the soil instead. Good plant husbandry is essential, such as clearing out old leaves and applying surface mulch to the soil – which can also help to reduce the spread of fungal spores. “We’re looking into the role of plant architecture,” explains Cromey, “using pruning to create a more open plant, and encourage good air circulation, and pruning the tops of hedges in different ways to see if that has an effect.” This latter point is based on the observation that box blight invariably forms in the tops of hedges, and less at the sides.
The RHS also advises that anyone buying Buxus plants should quarantine them away from existing plants before committing them to the ground. And, of course, you should only ever buy Buxus from the most reputable suppliers, who can guarantee the health of their stock. Buying a bargain Buxus plant could come at a higher price than you think.
When blight strikes to the extent that it did in Alexander’s garden, the advice now is to cut the affected plants back hard, to beyond the infected wood. “By the time we’d finished,” says Alexander, “everything looked scalped, like a garden full of bald heads.” With plenty of feed and water, the plants are recovering to the extent that they no longer detract from the overall appearance of the garden, but Alexander won’t be planting any more Buxus.
Instead, there are a great many alternatives worth considering. Alexander is experimenting with Phillyrea angustifolia ‘False Olive’, which has potential as a replacement for larger specimen topiaries but is perhaps too open and large in leaf for low hedges; and Hebe vernicosa, a naturally low-growing, compact plant with glossy leaves.
Pittosporum tenuifolium ‘Golf Ball’ also has potential for making slightly looser low hedges, while Pittosporum tobira ‘Nanum’ can be used to create a lower growing facsimile of the “cloudy” effect that has become a standard use for Buxus in recent years. Ilex crenata is probably the best like-for-like replacement for box in its most popular topiary guise, clipped into a ball, but it is intolerant of full sun and won’t cope with the dry, thin soils that Buxus seems to thrive in.
There is almost no Buxus left in the garden now. It’s been wiped out by box tree moth caterpillar.
At Chelsea Flower Show this year, there was much admiration for the tightly clipped Pinus mugo ‘Mops’ domes in Matthew Keightley’s BBC Radio 2 garden. They certainly have potential, as do other smaller pines and conifers that fell so spectacularly out of fashion from their zenith in the Sixties.
I have used clipped beech (Fagus sylvatica) domes in varying sizes as an integral part of a new rooftop garden at Rudding Park hotel spa in North Yorkshire. Beech may not be evergreen, but it has lovely autumn colour and holds its leaves through winter. To create a series of dome shapes in the shaded, north-facing lee of the spa building at Rudding Park I used Rhododendron yakushimanum ‘Percy Wiseman’. These produce a profusion of beautiful flowers but are also an excellent alternative to traditional topiary in that they hold a tight, dome-like evergreen shape, even when fully mature.
At Wisley, research into blight-resistant Buxus varieties is running in tandem with practical experimentation. Buxus microphylla hybrids, many of them raised by Didier Hermans of C Herplant BV, a leading Belgian grower and world expert on Buxus, are showing potential. Cromey and his colleagues are finding B. microphylla ‘Faulkner’, a Hermans introduction, is performing well.
Matthew Pottage, Wisley’s curator, is overseeing the planting of an “alternative parterre”, using plants that have potential to make low-growing hedges and with the capacity to take regular clipping. The lattice shaped design includes Podocarpus nivalis, a New Zealand native conifer, the sun-loving Corokia virgata ‘Silver Ghost’, and vibrant orange-flowered Berberis darwinii ‘Nana’.
The first planting was carried out last autumn. Early hopes for Lophomyrtus (a New Zealand relative of the myrtle) were dashed when the plants failed to make it through winter, but Pottage hopes that within four or five years he will be able to report on a range of plants that not only have the right aesthetic as an alternative to box but also, crucially, can take the tight clipping. For Wisley, it can’t come soon enough.
“There is almost no Buxus left in the garden now,” Pottage tells me, although not because of box blight: “It’s been wiped out by box tree moth caterpillar.” This East Asian native arrived in Europe in 2007, and an outbreak in and around London in 2015 proved devastating for many gardens. There are, however, a couple of plants of Buxus sempervirens ‘Bowles’s Blue’ at Wisley that have so far shown resistance to blight and caterpillar.
Box blight isn’t going away, and a silver-bullet cure seems unlikely. But a combination of vigilance and husbandry – selecting disease-resistant varieties from reputable sources, should ensure that Buxus remains a feature in our gardens for years to come.
Sandhill Farm House, Rogate, Hants GU31 5HU, opens for the NGS on 16&17 September 2017, 2-5pm. Entry £4.50, homemade teas (ngs.org.uk).
Matthew Wilson is a garden and landscape designer (matthewwilsongardens.com).