(Asclepias syriaca). Zones 4 - 7. This milkweed has fascinating two-tiered vivid flowers arrayed in a dense umbel. A five-pointed star or horn-like stamens comprises the upper tier, while the lower tier consists of the petals which bend backward or downward. The two to three foot tall milky sapped plants spread from an underground root system. The bloom from early to mid-summer along road edges and moist areas at the lower elevations of the Rocky Mountain range.