Buckthorn tree? There are three

A READER emailed me to ask the best way to propagate buckthorn trees.

He didn’t say which species of buckthorn — there are three.

The common, or purging, buckthorn is a native Irish shrub that can grow 10 metres tall. It is common on lime-rich soils in the Midlands and, as a prostrate shrub, is found in the Burren.