As I suspected, the iris-l has as many experts on animals as on plants!
Many thanks to everyone who has sent me solutions, on and off line.
They are helpful although, to date, we still have a problem with the one
dog (who is about 10 years old). I am sure that the driving force is
the lack of access to grass in the winter and I will seek a solution.
At the moment, I will try to live with some plants that have been "cut
back" to a little more than the recommended level!
Ian, in Ottawa