Amaranthus tuberculatus (Waterhemp)
Image credit: bernard buyseIdentification tips
Tip #1: the name of the amaranth genus comes from the Greek amaranths, "unbeatable", referring to the long-lasting flowers. Just remember who this species is "sister", from Amaranthus palmeri so just remember Chuck Norris (unbeatable), because these days the A. palmeri is the Chuck Norris of the weeds. While the name of the species tuberculatus comes from tūberculum meaning "tubercle".
Plant shape
2º Identification tip: plants are variable from upright to wide, from few branches to many branches, from massive to robust. This species is the one that generates the most confusion when compared to A. palmeri.
Leaves
3º Identification tip: leaves are alternating, lanceolate, ovate-lanceolate or oblong, with smooth margins and a shiny top surface. The leaves tend to be larger in the lower thirds of the plant, and become thinner as they approach the inflorescences,the leaves do not have hair. The petiole is not bigger than the limbo of the leaf.
Stem
4º Identification tip: the stems do not have hair,they are green to reddish.
Flowers
5º Identification tip: the plant is dioic possessing male and female flowers in different plants, but what differentiates from A. palmeriis that the flowers are not prickly.
Seeds
6º Identification tip: it is not possible to identify the seeds among the genus amaranthus,so it is worth checking, small, black and shiny seeds.
Seedlings - the terror of all
He's not afraid of anything
7º Identification Tip: cotiledones are thin and spoon-shaped, as the first real leaves do not overlap cotiledones, having an oval shape.
Extra references:
https://www.illinoiswildflowers.info/weeds/plants/water_hemp.htm
https://www.minnesotawildflowers.info/flower/rough-fruited-waterhemp
http://www.kansasnativeplants.com/guide/plant_detail.php?plnt_id=743