For gardeners lucky enough to be able to offer them the right conditions in USDA Zones 4 to 6(7), the Scottish heathers like Calluna vulgaris can transform a garden into a sweeping carpet of color in later summer and fall. This low-growing evergreen plant demands moist but very well-drained soil that's acidic but not fertile. For USDA Zones 7 and warmer, Mexican heather is an easy-to-grow alternative.