Alfalfa. Medicago sativa.
Fabaceae, the pea, bean, or legume family.

Plant: Prostrate plant, 1'-3' with clover-like leaves and clusters of small, purple, pea-like flowers. Perennial
Flowers: Purple to violet-blue, sometimes white, 1/4"-1/2" long, pea-like, in compact clusters (racemes).
Leaves: Alternate, three-parted, clover-like, with obovate leaflets 1/2"-1" long, toothed mainly at the ends. Stem of center leaflet typically bent upward.
Fruit: A small, twisted pod.
Blooming: May-October
Habitat: Old fields, meadows, roadsides.

Comments: Originally from Eurasia, alfalfa is the most commonly-grown forage plant in North America and is widely naturalized. Like many legumes, it harbors bacteria that enrich the soil with fixed nitrogen. It is highly nutritious fodder for grazing animals. The young sprouts are eaten by people as well.

Where to find it: Not common in the park. I have found a clump on the south side of the pond down from the highway, and another across the commercial park access road on the west side (which probably does not strictly count as in the park).