Keeping box healthy in Ham’s historic garden

Box hedging and topiary in the Cherry Garden at Ham House, Richmond-upon-Thames

Ham House’s Cherry Garden is a spectacular box parterre, filled with lavender and hundreds of metres of clipped topiary. Using a design inspired by the seventeenth century, we care for 80 large box cones and 250 meters of dwarf box hedging. So when an eagle-eyed visitor spotted an unusual caterpillar on one of the box hedges, Head Gardener Rosie Fyles was naturally concerned.

‘We identified the specimen as a box tree caterpillar, the greenish yellow larvae of a moth that feeds on box (Buxus) plants, said Rosie. 'A year before I had seen the same insect cause similar devastation in gardens in northern Italy, so I knew what was coming.’

Native to Asia, the Box Tree Moth (Cydalima perspectalis) was first reported in the UK in 2008 and has since become widespread in London.

Formal garden under threat

‘With box hedging creating much of the formality and impact in Ham’s Cherry Garden, and a further 700m of box along borders and in the kitchen garden the threat of damage was huge,’ explained Rosie.

Box tree caterpillars can turn a plant completely leafless, before damaging the wood to cause it to die. The plants in the Cherry Garden were fast showing patches of dieback, networks of webbing where the insects feed and frass (droppings) near the damaged areas.

Ham has been gardening using organic principles for over a decade, so needed to find a way to restore balance to the garden working with nature.

Bumblebee on lavender in the garden at Ham House, London
Bumblebee on lavender in the garden at Ham House, London
Bumblebee on lavender in the garden at Ham House, London

Expert advice on organic controls

The garden team talked to a range of UK experts in organic and biological pest control. They recommended pheromone traps to catch male moths, monitor their numbers and lifecycle and help reduce the numbers reproducing.

They also suggested spraying with Dipel, a biological control using a naturally occurring soil bacteria (Bacillus thuringiensis var kurstaki) to control the population. Nematodes were introduced on a trial basis - lacewing and parasitic wasp larvae to reduce numbers.

The European Box and Topiary Society proved really helpful. They included Ham House Garden in a UK trial for two new deterrents: one uses synthetic thyme extract and the other uses caterpillar frass (droppings) to deter the moth. The results are expected in the next few years.

With such a big threat to key areas of the garden Rosie has had to re-think priorities without compromising on the quality of the garden. The team has constantly monitored the pest to control it at key times.

'Sometimes we have had to be pragmatic where we want to be perfectionist,' she adds. 'The situation has also raised questions about why we have box in productive areas like the Kitchen Garden, and whether we could look at other hedging that is edible and attracts pollinators.’

Together the team has been able to minimise damage to the Cherry Garden and other hedging.

‘Thanks to the right advice, discipline and patience, we have kept the box in the historic garden as healthy as possible so it’s there for everyone to enjoy for years to come.’

A view into the historic formal gardens of Ham House in Richmond
A view into the historic formal gardens of Ham House in Richmond
A view into the historic formal gardens of Ham House in Richmond

Garden team tips

What can help with Box Tree Caterpillar?

*Try at home* Look for overwintering caterpillars

The first sign of damage will be from caterpillars that have overwintered any time from mid-March when there are warmer, brighter days. They seem to target the newest foliage on the outside of the plants first and are relatively easy to spot at about 10mm.
 

*Try at home* Pick caterpillars off by hand

Removing caterpillars from small areas or plants in containers is possible if you are vigilant for signs of damage. In a garden like Ham it takes large numbers of people to methodically and frequently inspect plants to concentrate on the worst areas.
 

*Try at home* Pruning out damage

If caught early enough, the damage caused by the caterpillar can be removed during pruning. However as the caterpillar damage can sit below fresh regrowth, pruning may reveal the extent of the defoliation caused. This has happened on some of Ham’s low hedges.

*Try at home* Pheromone traps

Use a trap with a pheromone lure to see if the box moth is flying and to trap male moths to reduce numbers breeding. They will alert you to the adult moth being active and the need to visually inspect your plants more frequently.

*For professionals* Biological spray

Professional gardeners can use Dipel a biological spray control. Getting the timing right makes a huge difference. An autumn spray helps to counteract overwintering caterpillars which can lessen numbers in spring. A really warm year like 2018 may mean four generations of caterpillar and Dipel can be used at least twice for each.

*Try at home* Nematodes

We have used Lacewing / wasp larvae (Chrysoperia carnea / Trichogramma evanescens) as biological controls in the garden. We believe that they have helped to reduce numbers. They have also delayed the impact of the caterpillar, allowing gardeners more time to spray or physically remove.