Asclepias syriaca (common milkweed)
Image credit: James LeeIdentification tips
Tip #1: the name of the genus, Asclepias, celebrates Asklepios, the Greek god of medicine. Syriaca, means "from Syria" about Linnaeus' mistaken belief that this species originated from Syria.
Plant shape
2º Identification tip: the plant is perennial herbaceous, which when it is established it appears in communities because it can generate new plants through the root. A great explanation of how to identify this plant in video.
Leaves
3º Identification tip: the leaves have serosity, are dark greens from the top of the plant contrasting with a lighter green at the bottom. The leaves are oval and opposite, each leaf node grows opposite to the other. The central rib of the leaves tends to be reddened when they are new and become whitish when older.
Stem
4º Identification tip: the stem is hollow and releases milky sap when broken, the stem can be green or reddened according to the age of the plant, have many small hairs.
Flowers
5º Identification tip: the flowers are purple and are arranged in globular curls.
Seeds
6º Identification tip: the seeds are flattened, thin and wide, are brown/straw-colored and have white hair.
Fruits
7º Identification tip: the capsules are shaped like an American football with thorns, these open and release the seeds when ripe.
Roots
8º Identification tip: the root system has rhizomes, promoting the vegetative dissemination of this plant. These also make it difficult to control this weed.
Seedlings - the terror of all
9º Identification tip: Seedlings can originate from seeds or roots when it originates from roots they have reddish coloration and do not have cotyledons. When originated from seeds they have spatula-shaped cotyledons and the true leaves are longer but similar in shape.
Extra references:
https://www.fs.fed.us/wildflowers/plant-of-the-week/asclepias_syriaca.shtml
https://www.illinoiswildflowers.info/weeds/plants/cm_milkweed.htm