Home of Proven Winners® Perennials 'String Theory' Amsonia PPAF CPBRAF This durable native perennial has threadleaf foliage that adds fine texture to the landscape. Periwinkle blue flowers cover the plant in late spring with the leaves turning a golden orange color in fall for season long interest. Zones 4-9. Proven Performance. All Season Long. TM Premium Perennials. Rewarding Results.