TURNING PROBLEM RHODODENDRONS INTO 'BLACK GOLD'

How the National Trust garden at Inverewe is making biochar - and better soil - from invasive shrubs in the wet, acidic West Highlands. Charlotte Sterling finds out more

- Charlotte Sterling

Inverewe's spectacular coastal setting in northwest Scotland is both a blessing and a challenge.

Shallow, peaty soils sit over rock, rain is frequent and heavy, and nutrients wash through before young roots can grab them. Regeneration plantings also have to contend with a long legacy of Rhododendron ponticum – dense thickets that cast deep shade and litter the ground with leaves and woody debris containing allelopathic compounds that can suppress germination and slow establishment.

Clearing those thickets creates new light and space – but it also leaves a mountain of woody material. Burn it, and you lose most of the carbon to the atmosphere; compost it, and you need space, time and constant turning, with a large share of carbon and nitrogen lost on the way.

THE SIMPLE, SMART IDEA

Instead of treating rhododendron brash as waste, the team converts it to biochar – heating the wood at high temperature in a low-oxygen environment (pyrolysis) inside a kiln or retort. The process drives off gases and tars - the hot gases are re-burned to keep the system running cleanly, while the woody skeleton is transformed into a carbon-rich, highly porous solid: biochar.

Two things make biochar a notably good fit for Inverewe:

Porosity and surface area – The microscopic pores act like tiny store cupboards, holding onto water-soluble nutrients that would otherwise be leached by rain. That helps keep food in the root zone where young plants can use it.

Chemistry – Fresh rhododendron debris tends to acidify. Biochar from woody feedstocks is usually mildly alkaline. In small, well-blended amounts, it can buffer acidity and help stabilise pH in peaty, rain-leached soils.

In short, it turns a persistent problem material (rhododendron) into a stable, nutrient-retaining soil amendment that lasts for decades – even centuries – in the ground.