Well, the results are in. My eco-friendly squirrel deterrent only worked for about a week. The little critters are stronger than I thought, because they recently pushed the rocks aside and did their digging thing:
The good news is that they didn't get to the lily of the valley itself. But one of the large leaves was already torn and a smaller leaf is yellowing. I know lily of the valley is hardy and I'm counting on these two rootings to pull through. I could always bring the planter closer to the house in hopes that the squirrels wouldn't be quite so daring with more active human presence. But I really like that container where it is.
A quick online survey indicates that most squirrel deterrents -- like moth balls, high-tech devices, pepper, fox urine -- just aren't effective. Yes, squirrel-proof birdfeeders are available and do work. But for other illegal outdoor (and indoor) activities, there's not a whole lot you can do besides trapping & removal.
The one idea that seemed viable for containers, which I found at www.squirrels.org, is to take a piece of metal screening, cut it to the shape of the container, cut out holes for the plants to come through, bury it just under the surface of the soil, and place some small rocks around the edges. If I go that route, I'll use a fairly wide mesh so new shoots of the plant can come through.
The photo above was taken on July 3, when I discovered the damage and moved the soil and rocks back into place. As of this morning the squirrels had not disturbed the planter again. So it's not an out-and-out war yet. However, it's becoming a matter of principle. I wouldn't go as far as having the squirrels removed with traps -- which I highly doubt would be a successful long-term solution anyway. They're part of the environment. But as far as my lily of the valley is concerned? It's on, little guys. It's on!