The following comments are from scientists, nurserymen and private growers. There are many differences in opinion even amongst the leading experts. Everyone has a story to tell, perhaps a treatment to recommend…
A problem with blight seems to be the variability and the source of the disease. The designer, Julian Treyer-Evans points out that the potency can differ from one garden to the next, even from one side of a garden to another, that ‘it may depend for example on the soil itself, the airflow, the sun, the shade, it’s care‘. The way the disease spreads, if it does – because it doesn’t always – can depend on so many shades of circumstance.
Karel Goossens of Buxus & Taxuskwekerij Goossens, a leading box and yew nursery in Belgium, points out that the source of the disease is unknown. It may not just be in the soil but could be introduced in various other ways including by birds and animals, possibly even by the rain.
GOOD CULTURE FOR PREVENTION
This is undoubtedly the best and most effective course.
Widely recommended good practice is to quarantine new plants before they come into contact with your existing stock. Provenance is important as plants from warm countries may be ‘soft’ and lacking resistance. Too much nitrogen in feed should be avoided as the resulting sappy growth is more susceptible to disease. Grow your box to be tough.
Buy new box plants from a trusted source or grow your own from cuttings. Plants should be mulched well to prevent any spores splashing up and ideally be watered at the base. Pruning should be done as little as possible, only in dry weather. Tools should be disinfected frequently – diluted bleach does the trick.
Air flow is vital. Don’t plant too close. When visiting Covertside in Gloucestershire last summer we noticed that John Sales, formerly Head of Gardens for the National Trust, keeps his boxwood hedges very slim for this reason.
As plants mature they are inclined to thicken. Lynn Batdorf, formerly in charge of the boxwood collection of the National Arboretum in Washington, recommends plunging your hands into mature and congested specimens to thin them out from the inside and so improve the air circulation.
Effective fungicides for blight are only available to professionals with a spraying license – an expensive business and not one that many gardeners would wish to undertake. Dr. Henricot, concludes that there are no effective fungicides once the disease has reached the adult leaves – a view shared by Professor Kelly Ivors from North Carolina University. She has conducted trials into the comparative effectiveness of fungicides as used for prevention versus cure. The results, she says, were clear cut. Fungicides are highly effective in prevention but not as a cure.
Not so, says Karel Goossens. While he agrees that the adult leaves cannot be cured once infected, he says that the plants can be saved, even at this point, by preventative spraying. New growth will spring from underneath and alongside the old affected leaves leading to a full recovery of the plant.
BIOLOGICAL TREATMENTS & MINERAL FERTILIZERS
Over the fourteen years since blight first hit Belgium in 2000, he, along with other Belgian growers, have developed a two-pronged method of fighting the disease by alternating fungicides and ‘plant fortifiers’. From mid-May to the end of October, Karel has a three week turnaround with fungicides alternated with EM and Oenosan EM and Oenosan, sprayed on the leaves in the morning when the leaves are covered in dew and the stomata are open.
EM (Effective Micro-organisms) are naturally occurring bacteria which are scientifically cultured to contain useful bio-active substances including enzymes and fungi that will enhance plant growth and protect against disease.
The mineral fertilizer Oenosan, is a ‘calcareous nutritional supplement’, primarily composed of calcium, magnesium carbonate, silica and iron, micronised Calcite (CaCO3), supplemented by small amounts of volcanic dust and traces’. The manufacturers claim that this will boost efficiency by at least 300%, providing a more powerful and resistant crop.
Karel also uses these products to ferment organic material to what he calls ‘ Bokashi’ or ‘Japanese for good fermented organic material’ which is applied to the soil.
The result has been startling. Since using EM and Oenosan regularly he has reduced the use of chemicals at his nursery by a 75% over a two to three year period.
He believes that clipping makes boxwood more prone to disease. Unclipped plants in his nursery are only sprayed once a year whereas clipped ones are sprayed up to seven times. He is considering trialling winter clipping when the plants are dormant.
The latest idea in Belgium (and possibly in the UK soon) is to set up spraying teams that will assess the situation in individual gardens and prescribe the best mix of biological and chemical treatments to build up resistance and control the disease.
TOPBUXUS – Charles Hawes, who, with Anne Wareham, created the Veddw, Monmouthshire, (featured in Topiarius 16) a masterpiece of design with lots of box hedging, says that the arrival of blight was the most distressing infliction they had suffered in the 25 years of building the garden. Having tried everything and even attended the 2012 Box Summit, they continue to use licensed fungicides but have also introduced Topbuxus Healthmix.
Topbuxus Healthmix, new to us, is a ‘leaf fertilizer with trace elements on a natural base which, the manufacturers claim is ‘100% effective in stopping fungal diseases in box plants’. The mix contains nitrogen, magnesium, sulphur and copper which ‘burn the fungus spores and ensure healthy growth free from fungal infection’. Certainly worth a try.
Though nothing is quite like Buxus, if you do need to start afresh, there are interesting alternatives. As at Levens Hall, Teucrium x lucidrys (or wall germander) has replaced the box hedging in the Kitchen Garden at Highgrove. This can be kept as a neat hedge with frequent clipping, as at Levens Hall, or be allowed to flower, as at Highgrove, and provide a banquet for the bees. Designers are also trying Phillyrea angustifolia (sometimes described as ‘Jasmine box’) and clipping yew down to size.
Dr. David Jacques, formerly Chairman of the Garden History Society, pointed out in Topiarius (volume15) that the 1586 edition of La Maison Rustique recommended edgings were sweet smelling herbs – Hyssopus officinalis, Thymus vulgaris, Mentha pulegium, Salvia officinalis and Origanum majorana. A favoured edging in England was Armeria maritima, or thrift, and later Santolina chamaecyparissus, or Cotton Lavender. The Elizabethans, it would seem, did not care for the smell of boxwood.
This article was first published on page 50 of Topiarius Volume 18 Summer 2014.