I must be getting old. Not only did I buy this book recently believing it to be a Penguin Handbook - which it isn't, even a superficial examination would have told me that, but I had been reading it for a little while before I realised that the plant on the cover was cannabis.
Since the book dates from the 1970s presumably the idea was the make it attractive to hippy pot heads. It does seem rather incongruous though on a book by someone who'd been writing gardening books since the early 1930s though.