Pilewort. Erechtites hieracifolia.
Asteraceae, the daisy, sunflower, or composite family.

Plant: Short to tall, coarse plant, to 6', with large, coarsly-toothed leaves, topped by loose clusters of peculiar green white-tipped, closed, flower heads. Annual.
Flowers: Flower heads are a peculiar cylindrical shape, with a swollen base, mostly green, but white on the tip where the disk flowers are exposed. The flowers are grouped in loose, upright clusters (determinate, hence technically cymes).
Leaves: Alternate, elongated, sessile, 2"-8", and coarsely toothed or lobed.
Fruit: A dry seed-like body attached to silky hairs. At maturity the flower head opens to produce a globose puff, like a dandelion.
Blooming: August-October
Habitat: Clearings, wood margins, burned areas.

Comments: This plant is also called fireweed, a reference to its tendency to grow in burned areas. It is probably better to reserve this name for the more familiar bright pink and purple members of the evening primrose family (e.g. Epilobium angustifolium). The name pilewort refers to the plants supposed use in treating hemorrhoids.

Where to find it: I have found this plant growing on the east boundary of the park near the medical building parking lots.