Black mustard is a common weed which came from Europe.  It frequently grows in patches of dry ground, such as vacant lots and small neglected spaces like this, between a sidewalk and a wall.

Black mustard first establishes a rosette of basal leaves flat on the ground.  (I generally pull it out of my garden before it gets this large.) Then the plant grows up from the center of the rosette .

The flower buds are at the very tip of the growing stem.  As the flowers open, the stem gets longer between the flowers.

Below the flowers, the maturing fruit are spread along the stem.  The fruit is small and cylindrical, and may not contain seeds.

Since arriving with early settlers, black mustard has become well established in Southern California.  In the spring many fields and hill sides are bright yellow with wild mustard flowers.

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