Seeds from this Red Butterflyweed Asclepias curassavica. They blossom in zones 6-11 throughout the season and will bring more butterflies than you can imagine. These perennials grow 4 to 5 feet tall in full sun and sandy soil is best for them. Quite a drought tolerant plant. We recommend that tropical milkweed, which can be generally called Asclepias curassavica, should be cut back in the winter and fall months in the southern U.S. and California.