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A Christmas Spruce by Andrew Lane

Posted by sheiladearing on 21 Dec 2012 at 02:19 PM

It’s that beautiful time of year once again, where frosty mists hang in the valleys; roaring fires keep the cold out and warm our toes; and many people decorate a traditional Christmas tree with lights and baubles to brighten up the long nights.

At RHS Garden Rosemoor however, it’s a slight variation on a theme. Instead of a warm cosy fire in the grate, it is blazing bonfires and instead of decorating a quaint little spruce, we’re burning them! This is a continuation of our on-going battle against the dreaded Great Spruce Bark Beetle (Dendroctonus micans)!

A few years ago it became apparent to the team here at Rosemoor that there was mischief afoot in the Lower Woodland below the Formal Garden. The old forestry plantation of Sitka Spruce (Picea sitchensis), which provides a backdrop to the garden, was mysteriously starting to die off in patches. After an investigation and consultation with the Forestry Commission the culprit was exposed and identified as the Great Spruce Bark Beetle (Dendroctonus micans) and action was swiftly taken.

The problem with Dendroctonus micans is that its larvae feed on the cambium in the live bark (of trees such as Picea sitchensis, P. abies, P.  omorika AGM, P. pungens, and P. orientalis) and will eventually ring bark the tree and kill it. The life cycle is fairly rapid and so, without control measures, the D. micans population can multiply swiftly which increases the likelihood of damage to host trees and the spread of the pest.

The Forestry Commission's recommended control method is biological; Rhizophagus grandis; a rapidly reproducing beetle whose adult form predates on the eggs and larvae of D. micans; it was introduced to the infected sites here at Rosemoor in June 2010. Since then evidence has been found to suggest that R. grandis is reproducing and having a positive impact on the population of D. micans. The hope is that the control measure will reduce the population and spread of the pest by 80-90% which will reduce the rate of death of host trees to less than 1%.

Since the ball is rolling on the control of the pest and nature can be left to find its own equilibrium, it’s now down to the gardeners here to tidy up the fallout of the battle between spruce and beetle. Contractors were brought in with chainsaws roaring and axes hacking and the infected and dead spruce were removed swiftly and skilfully. This year 99 trees were felled, cleaned and cut into lengths ready to be sold (the beetle doesn’t actually damage the wood so the wood is still marketable).

All of the brash from the felled trees needed removing and the easiest, most efficient and practical way of doing so was to burn it on site. The team built fires at various points throughout the woodland where there was an area clear of canopy and not too close to any living trees. The team banded together, our backs bent, muscles taught and brows beading.

 Funeral pyres for D. micans. A fitting end for a worthy foe

The December cold was banished from our bones as the fires’ heat and the physical exertion warmed us through and the thought of our victory against D. micans fuelled our zealous commitment to the back breaking task at hand.

Garden manager David Perry doing the graft

The motley crew of pyromaniacs at RHS Garden Rosemoor

The woodlands are now cleared of brash but the logged trees still lay where they fell and are awaiting the next stage of their adventure.

In January, the logs will be removed using traditional heavy horses so as to minimise the amount of damage to the structure and ecosystem of the woodland floor that machinery would.

Watch this space for tales of our encounter with the shire horses next year!

Andrew Lane – Horticultural Trainee


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