Ambrosia trifida (Giant ragweed)
Image credit: Mike EnerioIdentificação tips
Tip #1: to remember the scientific name, the name of the genus comes from the Greek ưμβροσία "food of the gods", traditionally one considers the food of the gods a dessert called ambrosia, already trifida comes from the Latin crossing or crossroads, due to the characteristic of leaves and knots.
Plant shape
2º Identification Tip: is an annual can reach more than two meters in height.
Leaves
3º Identification tip: the leaves are wide and have three to five large deep lobes, they are opposite and their shape resembles a path dividing into several. The leaf branches have leaves without lobes, having an oval shape and thin tip.
Stem
4º Identification tip: the stem has hair and purplish green coloration. The nodes also resemble crossroads, due to the angulation of the branches.
Flowers
5º Identification tip: the flowers are practically unique, having very peculiar characteristics, resambling to a pot of ambrosia, can easily be confused with Ambrosia artemissiifolia.
Seeds
6º Identification tip: seeds have straw coloring, black and moss, have a peculiar characteristic looking like a head with a small crown and a pointed hat, have to be careful not to confuse with the seed of Ambrosia artemisiifolia Common ragweed, this has the crown and hat much smaller.
Seedlings - the terror of all
7º Identification tip: seedlings have oval cotyledons/spatulas or with thin tips, the first true leaf is that facilitates identification because it already presents the deep lobes dividing the blade limbus.
Extra references:
http://www.weedinfo.ca/en/weed-index/view/id/AMBTR
https://images.bugwood.org/series.cfm?coll=85031&color=transparent
http://agron-www.agron.iastate.edu/~weeds/Ag317-99/id/WeedID/rag.html