Flower:

Elongating clusters of tightly packed, short-stalked flowers at the ends of branching stems. Flowers are yellow, just over ½ inch across, with 4 petals and 6 yellow stamens in the center. Fruits quickly form along the stalk below the flowerhead.

Leaves and stems:

The leaves are 1½ to 7 inches long, ½ to 2 inches wide, irregularly toothed, mostly hairless, broadly oval to egg-shaped. Lower leaves are stalked, often with 1 to 3 pairs of irregular lobes near the base, becoming smaller, stalkless, and unlobed in the upper branches.

The stems are green or with reddish pigmentation, especially near the leaf axils, erect and amply branched, mostly smooth except for scattered, coarse hairs, more dense on the mid to lower stems.

Fruit:

Fruit is a slender, round, ½ to 1½ inch pod, including the beak at the tip which is about half as long as the entire pod. The pod angles out and up from the stem, and bulges where where the ripening seed is forming. Ripe seeds are round and small, less than 1/10 inch in diameter, dark brown to black, brown or reddish.

Notes:

Charlock is an old world species that was both cultivated for its seed as well as become a pesky weed in other crops, especially grain fields. It may well have been the mustard referenced in teachings of the New Testament bible. A serious weed in early American agriculture it appears to have been largely wiped out by modern herbicide methods and is relatively uncommon in the broader agricultural landscape, except for pockets along field margins - at least in my observations in Minnesota.