National Trust   Plenty of options here as with  English Heritage 

RHS Rosemoor. Depressingly superb   —   web site 

Castle Hill Gardens  —  web site 

Tapely Park House and Gardens   —  web site 

Marwood Hill Gardens   —  web site 

Stone Lane Gardens .  A very special garden (one of our favourites) featuring birch and alder trees and in the summer months a wonderful collection of garden sculptures for sale.  It is only only open in the afternoons but you can visit Castle Drogo  (NT) in the morning and have a pleasant lunch at The Sandy Park Inn which is between the two near Chagford. 

National Garden Scheme (Devon) - web site 

Powderham Castle near Exeter - web site  

The Garden House at Buckland Monachorum - Just south of Tavistock this is another favourite along with lunch at the very pleasant 16thC Drake Manor Inn in the village. 

Finally, whilst not really a garden, Lydford Gorge is well worth visiting for an extraordinary walk. Good walking shoes or boots and required and you may expect to see 4 foot wide butterflies and unicorns around every corner. It is really quite magical. And if you like good food then The Dartmoor Inn on the main road to Tavistock is a another favourite. 

RHS Encyclopaedia Series including: 

Pruning by Christopher Brickell 

Veg Growing by Tony Briggs 

Organic Growing by Pauline Pears and Sue Stickland 

The New Kitchen Garden – Penelope Hobhouse 

Any books by Graham Stuart Thomas 

Chris Cork – All Seasons South West  

Trevor Andrews – North Devon Landscaping  

Millwood Plants 

Society of Garden Designers - I am currently a pre-registered member. 

Health Benefits of Gardening for Older Adults