Described as one of the finest entrances to a botanical garden anywhere in the world, the magnificent 159-year-old Redwood Avenue at Benmore Botanic Garden, on Scotland’s west coast, has been saved from an early demise through a daring conservation exercise to ward off its susceptibility to plant pathogens.

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, near Dunoon in Argyll, 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’ disease-susceptibility 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.”