Capsella bursa-pastoris (Shepherd's purse)
Image credit: Pretty DrugthingsIdentification tips
Tip #1: The name of the genus Capsella comes from the Latin "little box", and the name of the species, bursa-pastoris means shepherd's purse, these names are associated with the plant's seed capsule that looks like a sheep shepherd's pouch. How do I know what a pasture bag looks like!? I have no idea, but take a look at the image bellow, in fact the common name of the plant is also shepherd's purse."
Plant shape
2º Identification tip: it has its initial development in rosette, presenting stems that protrude from the rosette, this characteristic is seen in almost all plants of the mustard family.
Leaves
3º Identification tip: the leaves are not the best parameter for identification because they can vary greatly, and can be more or less lobed (openings in the leaves, appearance of saw), usually the leaves of the rosette are the largest leaves and more concentrated, the leaves of the stems are thin and much smaller than the leaves of the rosette. Usually the older leaves are rounder, but it is not the rule.
Stem
4º Identification tip: the stems are upright, green to purple, have hair. Plants are usually have few stems, the leaves of the stems are well spaced and are sessile. The stems always end with a raceme of flowers.
Flowers
5º Identification tip: the flowers have 4 white petals and 4 linear and greenish oblong sepals. The flowers are grouped into round bunches.
Seeds
6º Identification tip: the seeds coloração are small orange and greenish, are inside the capsules.
Pod
7º Identification tip: the type of pod is the only unmistakable characteristic of the species, has a pouch shape, initially is green and passes to brown/straw with the ripening.
Seedlings - the terror of all
8º Identification tip: cotyledon is spatula shaped and the true leaves are oval opposed.
Extra references:
http://ipm.ucanr.edu/PMG/WEEDS/flixweed.htmlhttp://ipm.ucanr.edu/PMG/WEEDS/flixweed.html
https://www.illinoiswildflowers.info/weeds/plants/shepherd_purse.htm
https://www.minnesotawildflowers.info/flower/shepherds-purse