Small-flowered Evening-primrose. Oenothera parviflora.
Onagraceae, the evening-primrose family.

Plant: Straight somewhat hairy stalk 1'-3' high with alternate leaves and an extended terminal spike of four-petaled yellow flowers that bloom progressively. Biennial.
Flowers: Yellow, 3/4"-1.5", with four petals, notched so to appear heart-shaped. Stigma split in four parts, stamens as long as the petals with long, center-mounted anthers.
Leaves: Narrowly ovate to lanceolate, 4"-8" long below, smaller above.
Fruit: An elongated, nearly cylindrical capsule, narrowing somewhat towards the end, hairy, and interspersed with leaf-like bracts.
Blooming: July-September.
Habitat: Fields, roadsides.

Comments: There are several species of yellow-flowered evening-primroses in this area, some quite similar. O. parviflora (small flowered) is my best guess for the one depicted here. It is quite similar to the common evening primrose O. biennis, and differs primarily in being slightly smaller and in some technical characteristics. The genus is unrelated to the true primroses primula. Most of the species are biennial, forming a low rosette the first year, and blooming the second. This first year growth stores nutrients in a large parsnip-like root, which is edible if gathered in late fall or very early spring and boiled in a couple of changes of water.

Where to find it: A few scattered locations in the park, sometimes where paved areas abut weedy overgrowth. These pictures were taken in an area near the east boundary, adjacent to professional building parking lots. Since the plants are not perennial, locations vary year to year.