The tree collection is the backbone of the woodland garden and the chief glory of the various collections at Tregrehan. The garden was originally planted as an arboretum from the middle of the 19th century onwards. Then grew into a woodland garden with the understory filling in as the shade and shelter canopy developed. This is unusual especially in the westcountry as most valley gardens in Cornwall used the existing native canopy and natural topography to create their woodland microclimate.

By the 1st World War word had spread and WJ Bean and Jackson both from RBG Kew visited and wrote articles about the richness of the tree collection. Also around this time JC Williams brought Sargent from the Arnold Arboretum in Boston who declared that Tregrehan was the best thing of its type that he had seen, high praise indeed for a rapidly developing arboretum. 

The size and range of trees is stunning and the garden is rewarding to visit at any time of the year as so many of the trees are evergreen or coniferous. The collection has recently been reviewed by Owen Johnson from the British Tree Register who declared Tregrehan to be "The finest private garden in Britain and Ireland for its range and size of recently introduced tree species." He lists over 170 National Champions which puts it in the top half dozen collections in the country. He also lists a Picea sitchensis, (sitka spruce) growing on the valley floor at 153 feet which is the tallest tree presently on record in Cornwall.

The emphasis of much of the recent planting is on maintaining a "Safe Site" for trees that are endangered in the wild. Formal links have been established to access known source wild plant material such as the Conifer Conservation Group set up by RBG Edinburgh. The idea being that viable populations of endangered trees are established in ex situ collections as a back up to preserving these trees in their native habitats.   

Tregrehan was also chosen  to be represented in "New Trees. Recent Introductions to Cultivation" published in 2009 by Kew.