Harcourt Arboretum has been part of the University of Oxford since 1963. The site comprises 130 acres containing the best collection of trees in Oxfordshire with some of the oldest redwoods in the UK. Seasonal highlights include wildflower meadows, rhododendrons and bluebell woods. It is open through the year.
For details see: http://www.botanic-garden.ox.ac.uk/harcourt-arboretum