I'm replacing my garden edging (the separator between lawn and flower beds), and I have some treated pine I would like to use.
What's the best way to stop the pine from rotting?
You might want to consider non-toxic alternatives:
You're kinda fighting a loosing battle. A high grade marine varnish such as Epifanes would do the best in terms of water and UV protection, but I doubt it would give you the look you want. Here's a link to some other alternative marine finishes.