GIANT SEQUOIA TREES AT BENMORE BOTANIC GARDEN SAVED

When the 49 giant redwoods (Sequioadendron giganteum) started showing signs of distress, Curator Peter Baxter and colleagues at the Royal Botanic Garden Edinburgh (RBGE) agreed radical action was required. As branches defoliated and the crowns of the trees started to thin, there was a real risk the trees of the 350m avenue, would succumb to diseases such as phytophthora, the group of plant pathogens capable of inflicting rapid devastation to trees. The race was on to save the stunning specimens and the experts believed the answer was to tackle the situation from the roots up.

Peter Baxter explained: “We were observing the trees going into decline from about 2016. However, the process accelerated worryingly and by 2020 they were existing, rather than growing. My concern was that we were reaching a point of no return.”

“The reasons for the decline were complex and multifaceted but soil compaction was thought to be central to the problem. Planted in 1863 by the wealthy US landowner James Piers Patrick, the trees were among the first redwoods introduced to Europe from California and the Avenue was planted along the main driveway to Benmore House. Only a very thin layer of topsoil was supporting the turf over the original hard-core road and this, combined with a typically wet west coast climate and limited drainage, caused serious waterlogging and puddling around the roots, creating a favourable habitat for pathogens, and increasing trees’ diseasesusceptibility through stress.

“We believed the best approach would be to employ a combination of breaking-up compacted soil and applying a drainage layer of specialist growing medium to prevent future puddling and waterlogging around the trees to significantly reduce the pathogen pressure they faced. But, that would be an expensive exercise and we didn’t know whether available equipment could work in our climatic conditions.”

Briefly halted by the covid lockdown, the £110,000 project finally took off at the start of the decade. The process involved killing off the grass around the redwoods, then utilising a geo injector to puncture the ground and blow air beneath the surface, breaking-up the compacted soil and making room for the addition of granular materials to largely reduce the likelihood of re-compaction. Breaking the soil improved its structure and aeration, allowing the roots to have the stimulation they need to strengthen and thrive.

RBGE’s Director of Horticulture

Raoul Curtis-Machin said the need to save the iconic feature was beyond debate. Noting the virulence of plant pathogens, he also urged those responsible for gardens and estates to work together: “The Avenue is a great responsibility for us, and one we don’t bear lightly. These are landmark trees in the UK landscape, their statuesque dark green heads rising above most others. It has been a joy to see the fresh green growth re-appearing on the branches.

“We are very open to sharing our learnings and experience with other gardens and landowners because we are all in this together now. Climate change and pest and disease threats are coming thick and fast and they don’t respect national borders, county lines or property fences.”

While this battle looks to have been won, RBGE plant health and biosecurity scientist Matt Elliott, concluded on a salutary note on the challenges ahead: “Plant pests and pathogens have become a significant issue in the last 30 years. Gardens contain a wide range of hosts and, therefore, can be particularly susceptible to pest and disease outbreaks. The condition of the host is also crucial, if plants are stressed for some reason, such as climate, pollution and/or soil compaction, they will be more likely to become infected and the resulting disease more likely to be fatal. It is for this reason that we provide plants with the best growing conditions we can, to enable them to fight off diseases.”

Benmore Botanic Garden

Benmore Botanic Garden is part of the Royal Botanic Garden Edinburgh (RBGE). It sits on the A815, seven miles north of Dunoon on the Cowal Peninsula. Western Ferries sail from Gourock. It is open daily to the public from March 1 to October 31.

In its magnificent mountainside setting, Benmore is steeped in history and surrounded by dramatic scenery. The Garden’s 120 acres are home to a world-famous collection of plants from regions ranging from the Orient and the Himalaya to North and South America. In particular, it is renowned for its collection of more than 300 species of rhododendron.

The Royal Botanic Garden

Edinburgh (RBGE) is a leading international research organisation delivering knowledge, education and plant conservation action around the world. In Scotland its four Gardens at Edinburgh, Benmore, Dawyck and Logan attract a million visitors each year. It operates as a Non Departmental Public Body established under the National Heritage (Scotland) Act 1985, principally funded by the Scottish Government. It is also a registered charity, managed by a Board of Trustees appointed by Ministers. Its mission is “To explore, conserve and explain the world of plants for a better future.”