Ashes to ashes

Ash dieback in the UK can be traced back to 2012, when plants carrying the fungus responsible for the disease were imported into the country.

Wakehurst 22nd July 2021, Visual Air

The next year, major estates, such as Wakehurst, Kew's wild botanic garden in West Sussex, met the Forestry Commission to discuss how to identify ash dieback.

"By 2016, we were witnessing signs of ash dieback," recalls Wakehurst Arboretum Manager Russell Croft, "with the diamond-shaped lesions at the junctions of twigs and main stems, along with a yellowing crown."

In 2019, the site conducted its first large-scale roadside felling of diseased ashes, with some 90 specimens along the B2028 perimeter road going under the saw in stage one of an extensive dieback management programme.

"As a public attraction, our key priority is to ensure visitors can walk the site in safety," Russell states, "so we've focused on affected trees lining footpaths, as well as those along roads bordering us and boundaries with neighbours such as the South of England Showground, which stages public events throughout the year."

With a fair percentage of an estimated 1,000 diseased specimens already felled, some 250 along public access roads, work has switched to inside the gardens, but as Russell explains:

Left: Wakehurst Autumn, Claire Takacs Right: Wakehurst, Ash dieback management

"We don't always fell an entire tree, some are cut back. It's really a balance between necessity and cost, but highways work involves removing everything, whereas we adopt a different approach inside the garden."

Wakehurst has taken a considered view of how it tackles ash dieback. "I've had time to inspect the landscape over several years and can target trees growing round high usage areas and dwell points," he says.

Tree inspection is a nuanced strategy of managing individual specimens, he continues. "Those showing signs of tolerance we keep. We conduct annual canopy surveys to check tree health - 75-100% of the expected canopy density is fine but when only 50-75% is present, that indictates dieback. Once the percentage of leaf canopy is reduced to 25-50%, we have to fell."

He has plenty of ground to cover - of Wakehurst's 535 acres, 300 are open to the public, while 200 acres of native woodland claim SSSI status. The 400 ashes in the formal gardens have been surveyed and 280 felled. The 70-100 left have a reasonable amount of crown and are showing signs of tolerance."

Left: Wakehurst's Loder Valley, Visual Air Right: Wakehurst 27th October 2022, Visual Air

Measuring growth rate is not a reliable indicator of disease, he adds. "Some young trees may have grown tall in search of light, especially if they seeded in shady areas. But height is relevant in considering fall areas, which in turn can impact public safety."

Wakehurst is one of RBG Kew's two UK sites, the other being Kew Gardens in Richmond. Wakehurst's diverse range of habitats has led to its transformation into Kew's living laboratory, where Kew scientists can research across grasslands, wetlands and woodlands all in one place. This access to Kew scientists adds further insight for Russell to plan ash dieback strategy, as does on-site research capability. "Gene mapping can help discover what makes trees tolerant to disease," he says "but as ash dieback is purely windborne, biosecurity of our landscape is very difficult."

All images © RBG Kew 2022