Giant Ironweed , Veronia gigantea
 
There is nothing more beautiful than an orange monarch butterfly landing on the fiery magenta-purple flowers of ironweed! It is a match made in heaven. Blooming from August to October, Ironweed provides excellent late-season nectar to monarch butterflies fuelling up for their migration to Mexico. Ironweed, in contrast to many native plants, is slow to establish. Therefore, I also offer 1-gallon pots, so the plants will bloom in the fall.
 
Ecology:
Ironweed is not only a butterfly magnet but also a bee magnet!
Ironweed pollen also supports various pollen specialist bees, who require ironweed pollen to provision their brood cells with bee bread made from saliva, ironweed pollen and nectar that will feed the emerging bee larvae. 
Ironweed is a larval host plant for moths in the caterpillar stage.
The abundant flowers also attract syrphid flies and soldier beetles, which are excellent beneficial insects to keep pests in a vegetable garden in check. 
The seeds feed songbirds in winter. 
 
Growing conditions:
Giant, also called tall ironweed, is found in prairies, old fields, and savannas in part-dry to moist soil. In the garden, it thrives in full to partial sun, is very adaptable to various soil types and prefers medium to moist soil conditions. Since it can tolerate drier soil occasionally, it is suitable for rain gardens. To tame the height, you can Chelsea-chop the plant in June by cutting off one-third of its height. It will grow shorter and bushier with more blooms.