The method used by Popham to establish the arboretum was so special that it has come to be known as the Popham method. The method was influenced by his dislike of tampering with nature's ways. He “cleansed” parts of the property – that is to say he selectively cleared away the scrub jungle and thereby released seedlings of the earlier evergreen forest.
The Popham method is best described in the words of the man himself, who wrote:
"I have egged Nature on to call the tune; I have left the trees to get on with the task. They cope very successfully on their own, and my help is needed for the most part only in their formative years. They grow where their seeds fell…During early growth they are encouraged, by stempruning and crown lifting…Last comes thinning-out"
– taken from 'Dambulla – A Sanctuary of Tropical Trees.'
The property was divided into 12 blocks and these blocks were cleared at different times, which means that one can now see woodlands in different stages of growth. Some blocks have not been cleared at all and continue to be covered by the scrub jungle.
The rationale for the staggered clearing was that only an area which could be nurtured and protected from the exigencies of wind, drought, floods etc, by Popham and his team would be cleared at any one time.