Why not rope off the area you want to keep?
A number of canes pushed in around the perimeter and then cord tied from one
to the other. Then when he comes up to it he should remember it's special.

Its too late for my nettles but they will grow again soon I hope and
yes it is a good idea to fence/rope of the special areas,my daughter has
already suggested that so now I must act, tomorrow is another day :-)