Portland, Oregon is a gardener's mecca. Visitors throng the world-famous International Rose Garden, the Peninsula Park Rose Garden, the Japanese Garden, and the Crystal Springs Rhododendron garden year-round. But the excitement for many lies in the city's myriad local neighborhoods, planted by homeowners for more than a century, unheralded by outsiders but revered by locals. Mid-February begins a cornucopia of color bookmarked by daffodils that lasts...