Diseased boxwoods: should they be pulled out or not?

IN THE GARDEN THIS WEEKEND – Borer, mushrooms: perils are falling on the boxwood. And the remedies are slow to come. As a result, the presence of this small shrub in gardens is increasingly called into question.

Stop or still? Should you keep your boxwoods that are victims of moths and diseases or, on the contrary, give up and plant something else altogether instead? Large estates like Vaux-le-Vicomte took the plunge this summer. But others like Eyrignac and Marqueyssac in Dordogne, or Villandry, in Indre-et-Loire, still want to believe in the future of this small shrub with a heritage character which plays an irreplaceable geometric function, as a border plant, in a very large number of gardens. Including, perhaps yours.

“After four centuries of good and loyal service, we must save soldier Buis!” launched, for his part, Geoffroy de Longuemar, president of the Association of Parks and Gardens of Brittany, during the scientific conference organized, Tuesday and Wednesday, at the University of Tours. During these two days, the best specialists from France and Europe provided an informative overview of the health problems encountered and identified the available or future means of combating them. We might as well say it right away: the account is not there yet. Even if there are genuine glimmers of hope, particularly in terms of genetic struggle.

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Appeared in France ten years ago, the boxwood moth (Cydalima perspectalis), a nocturnal butterfly native to China, regularly hits the headlines because of the spectacular damage it causes. Its caterpillars, voracious and numerous, are capable of completely defoliating a shrub in the space of two days. Today, however, biological control methods based on organic insecticides such as Bt guarantee satisfactory control of the parasite. But they have the disadvantage of being very demanding in terms of time and monitoring, both for an individual and for a professional.

“From March to October, we do a daily inspection and treat as soon as the caterpillars appear, which has led us to intervene five times this year. This represents a very heavy workload but we were able to preserve our entire line of boxwood,” confided to Le Figaro, Laurent Portuguez, head gardener of the Château de Villandry. The important thing, in fact, is not to let yourself be overwhelmed: depending on the weather, there can be up to three generations of caterpillars per season. If an outbreak occurs during an absence, particularly in July or August during a vacation period, the boxwoods may be completely destroyed upon your return…

Entrapment and sexual confusion

Other techniques are being tested, such as trapping and sexual confusion by the diffusion of pheromones, natural substances which disorient males and prevent them from fertilizing females. But, in the field, the results are currently hardly conclusive: the levels of moth infestation are such that there are enough butterflies remaining to lay eggs and cause irreparable damage. This is how the splendid boxwood embroidery of the rose garden of Haÿ-les-Roses (Val-de-Marne) was completely destroyed during an inconclusive test recently carried out by INRA… On the other hand , trapping remains interesting for detecting thefts and intervening effectively and at the right time with a Bt insecticide.

Trichogramma, these tiny wasps which have the particularity of laying eggs in the moth eggs on which their larvae will feed, are another promising avenue of research. But this treatment is expensive and the tests carried out by Elisabeth Tabone of INRA in Antibes have not yet made it possible to find “the” strain of trichogramma sufficiently effective to control the parasite. The horizontal dispersion of insects is low, which poses a problem for the protection of borders and their activity is only optimal at temperatures between 20 and 25°C, a threshold well exceeded in summer, especially in the south of the France.

War on two fronts

A small sign of hope all the same: recent observations have shown that birds such as tits and finches are starting to feed on moth caterpillars that they previously disdained. The same goes for bats, nocturnal insectivores like the moth. The installation of nest boxes is therefore strongly recommended in addition to the system.

But if there were only the moth! Boxwood is also the victim of two formidable parasitic fungi, Volutella buxi and especially Cylindrocladium buxicola, also appeared around ten years ago. Result: in many situations, the war must be fought on two fronts, which obviously complicates the task. In addition, diseased boxwoods are more vulnerable to the moth and vice versa…

In the event of an attack of cylindrocladiosis, often devastating in spring, the only effective treatments are synthetic fungicides from the triazole family. Problem: they do not eliminate the parasite which can resurface at any time and, in the long term, the risk of resistance appearing is real. Finally, their use will be prohibited for amateur gardeners from January 1, the date of entry into application of the Labbé law…

Genetically resistant hybrids

A misfortune that never happens alone, the boxwoods sempervirens suffriticosa, traditionally used to make borders and embroidery, are by far the most susceptible to cylindrocladiasis. Their replacement by tolerant cultivars, if not resistant, is an option. But for the moment none are really satisfactory, starting with the ‘Faulkner’ variety which has raised a lot of hopes but whose results in recent trials presented in Tours have proven disappointing.

The solution could come from Belgium. After ten years of research, the Herplant company, in conjunction with various public research organizations, succeeded in creating hybrids offering a very good level of resistance to cylindrocladiasis. They will be marketed next year and are already of interest to a number of stakeholders such as the Château de Villandry. So to be continued.

Reduce hedge density

Preventive control through fertilization (phosphorus, potassium, trace elements) and tillage is also an essential prerequisite. In Lausanne, Paolo Fornara managed to restore health and vigor to the kilometers of boxwood borders that he manages in the city cemetery, by covering the ground with crushed tree pruning waste, or BRF, and by stopping the use of herbicides. “The earthworms who had nothing left to eat went back to doing their work and the addition of organic matter helped to regenerate the soil,” he explains, calling for a return to good agronomic practices. Pruning also plays an important preventive role: “We must reduce the density of hedges by shortening and thinning them,” recommends Laurent Chabane, head gardener of the Eyrignac manor, who also encourages draining the soil to prevent water to stagnate or even to blow the hedges to dry them in the event of a risk of attack.

However, so many requirements are discouraging. Especially when the result is not guaranteed. The temptation is then great to turn to similar species. But as always, these substitutes have their requirements and their faults. Holly Ilex crenatafor example, is totally allergic to limestone soils, pittosporum freezes below -5°C, Japanese spindle (Euonymus japonicus) lack of leaf density while being very sensitive to scale insect attacks, berberis have thorns, which is not comfortable for pruning them, etc. While boxwood is a rustic and robust plant, of great genetic variability, which adapts to almost all soil and climatic situations, it is clear that its replacements are much less so. Too bad it’s so parasitized…

The boxwood collection of Château de la Ballue has just been certified

The Conservatory of Specialized Plant Collections (CCVS) awarded its precious label to six new collections on Friday, during the Chantilly plant festival.

Among them, the boxwood collection of the Château de la Ballue (Ille-et-Vilaine) includes a remarkable set of 78 species for a total of around 400 boxwoods grown in the park, the fruit of 10 years of work carried out with the advice of ‘Hubert Puzenat, garden architect.

The collection is open to visitors, in addition to the visit to the historic gardens, which are absolutely remarkable. All the boxwoods are grown in natural shape and are never pruned in such a way as to allow the public to discover the great diversity of shape, color and shape. leaf shapes, growth rate, gender Buxus. The collection of boxwood from the Ballue gardens, for the moment spared from the moth and fungal diseases. fungal infections, is very healthy.

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