If you aren't familiar with the National Gardens Scheme, it's a pretty awesome program which features gardens all over England and Wales, most privately owned, which are open on a very limited basis to benefit the charity of the owner's choice. All told, there are about 3700 gardens included, and the NGS has raised 25 million pounds for charity over the past ten years. The individual owners typically pick dates when their respective gardens are looking their best, and will open up the garden to visitors for a nominal price, which goes to charity. The book is divided by county and open dates, so it's a great guide to have in the car when traveling through England.
