To gardeners who've been digging in the dirt a long time, it's amusing to hear that our favorite pastime suddenly has been proclaimed the No. 1 hobby in the United States. There are now 68-million gardeners, Gallup pollsters tell us. And last year, each one of us spent an average of $400 on supplies, tools and plants. Which is great for a thriving garden industry _ but are we better gardeners for it? Roger Swain, an editor at Horticulture magazine and host of The Victory Garden, thinks not. Earlier this month, he told the New York Times: "You can almost measure the quality of your gardening by how little you purchase. The better it's growing, the less you need of anything."