How do I keep squirrels out of my window-boxes?

I planted a windowbox last winter--petunias, a flowering cabbage, and ivy. It did wonderfully all winter, but apparently, the squirrels buried their stash in my box, because now, every day, a different plant has been rooted up. First, they chewed the roots out of my petunias, then the leaves off my cabbage, they ripped most of the ivy out, and when I came home yesterday, they'd gnawed through the stem of the cabbage--it looked just like a beaver had felled a tree, only much smaller, and on my porch. There's nothing left in my planter now, and I wanted to plant a new spring box, but I'm scared the little buggers will just root it all up again. How can I keep them away? Do you think they'll come back now that the planter has been empty for a few weeks?