Monarch butterflies need milkweeds as host plants for their caterpillars. The tropical milkweed is my favorite---it's lovely, easy and butterflies and pollinators love it. Although not a true perennial, it self-sows readily. We haven't planted new ones in our demonstration beds for years, relying on seedling that come up in the Spring. Available here at K+K in 1 gallon pots for $9.95.