Cyclachaena xanthiifolia (Marshelder)
Image credit: Jason LeungIdentification tips
Tip #1: The name of the genus comes from the Greek cyclo meaning circular, xanthi comes from yellow latin and folia means leaf. So it would be something like yellow circular leaf, but what is yellow in the plant is the flower.
Plant shape
2nd identification tip: the flowers look a lot like the flowers of common ragweed and giant ragweed and the leaves resemble sunflower leaves.
Leaves
3º Identification tip: the leaves have oval to oblong shape, with toothed margins and rough surface, the leaves near the flowers are thinner but kill the rough texture and toothed margins, with spear shape. The leaves have a silvery appearance at the basal part of it.
Stem
4º Identification tip: the stems are not branched, except in the group of flowers, without hair on the lower plant, but with thin and sticky hair on the flowers.
Flowers
5º Identification tip: the flowers are dense and branched clusters at the tip of the main stem and resulting from leaf armpits in the upper plant. The flowers are disco-shaped, most often face down, without petals, greenish to yellow.
Seeds
6º Identification tip: seeds resemble sunflower seed.
Seedlings - the terror of all
8º Identification tip: the cotyledons have a spatula shape and the first pair of true leaf has long petiole and covered hair, have rough texture, the second pair of sheet have toothed edges and oval shape.
References
https://www.minnesotawildflowers.info/flower/giant-sumpweed
http://swbiodiversity.org/seinet/taxa/index.php?taxon=32954&clid=3510