When you think about gardening and preschoolers, a spindly bean sprout emerging from a half-pint cardboard milk carton might come to mind. Every child deserves to be initiated into a "green thumb society" with this classic gardening activity.

But there's another favorite my kids enjoyed when they were young, even before they could say "rutabaga." We called it a saucer garden. It uses kitchen-scrap throwaways from vegetables such as carrots and beets, offers life lessons about care and responsibility and, like the bean seed, this simple garden grows to maturity before the kids go off to coll...