Giant Blue Porterweed (Stachytarpheta cayennensis) This offer is for a packet of 10 seeds. Plant type: Tropical/Tender Perennial Hardiness: Zones 9-11, Great pot plant in all zones Size: Up to 7 feet tall and wide. Can be maintained with pruning at nearly any size desired. Culture: Full sun to part shade in any soil. Best growth and blooming in full sun and moist fertile soil. Drought tolerant once established. Prune back hard after blooming to remove seeds and encourage another wave of blooms. Blooms throughout warmer months. One of the very best butterfly attracting plants. A must for any Butterfly garden. This Caribbean native is a blooming machine and a must have for any butterfly garden. It's a big and bold perennial that grows and blooms fast enough to be used as an annual where it isn't hardy. Be sure to collect some seeds for next year or take some cuttings in the fall to overwinter inside. In frost free gardens it can get up to 7 feet tall and wide and bloom continually. It also makes a great pot plant.