(Asclepias syriaca). Monarch butterflies (Danaus plexippus) only lay eggs on plants in the milkweed family. The Milkweed host-plants are the only food Monarch caterpillars eat. Milkweed is no longer listed as an Ontario noxious weed. Do not consume any part of a milkweed plant as the sap contains cardiac glycosides that are poisonous to humans and livestock, if ingested in significant quantities. Flowers are attractive, fragrant and extremely popular nectar and pollen sources for pollinators (bees in particular). Perennial hardy to Zone 3.