Mary, like you and everyone else I too have found this same problem with Flynn-and he's only 10 weeks! I was quite shocked that he didn't seem to know where the grass ended and flowerbeds began
I've had enough of that, and am going to put a small fence around my flower beds, small enough so that I can still reach to put plants in but hopefully big enough so that Flynn will be unable to just run straight onto the bed. Not sure whether that will work, but as I only have a small garden I can't really divide it up for him. I've also decided, enough with the grass (or rather very small football pitch) and am going to pave it.
I've also found it impossible to do any gardening when Flynn's around, we had the most hilarious game with the sweeping the other day
and now if I have any plants to put in, Flynn stays inside, that way I can relax
Hope you find a good solution, if you do, let me know, save me all that paving
Carol