Blackberry. Subgenus Rubus. Rubus fruticosus, etc.
Rosaceae, the rose family.

Plant: Woody bramble consisting of arching, prickly, biennial canes, growing 10'-12' or more, rooting when the tips reach the ground. Sometimes forming rounded, impenetrable clumps. White flowers are borne in the spring, and juicy black composite fruit in late summer and early fall. Perennial.
Flowers: White, about 1", with five petals, five sepals, and many stamens. The flowers are organized in loose cymose clusters with 5-15 flowers, mostly on the upper parts of the canes.
Leaves: Alternate, palmately compound with mostly 5, but sometimes 3 or 7 ovate, toothed leaflets, 3"-8" across, frequently more or less hairy.
Fruit: An aggregate consisting of many small druplets, shiny black, juicy and sweet when ripe.
Blooming: April-May
Habitat: Overgrown fields, waste areas, woodland boundaries

Comments: Well known and cultivated in temperate regions worldwide for its sweet, edible fruit. The black berries are technically a composite fruit formed from numerous druplets, each bearing a single seed. The classification of blackberries to species is complex, bordering on impossible. There is wide variation in the subgenus Rubus, and there are locally distinctive and fairly stable populations in many areas; however these hybridize freely, and the seeds have been widely distributed by birds for millions of years, and the plants by people for millennia. If there were originally a small number of more distinct species, this history is lost in the mists of time. Adding to the difficulty, typical morphological characteristics used for classification such as thorniness of stems, hairiness of leaves, size, typical number of leaflets, etc. do not correlate well with groupings derived from genetic studies. Perhaps best to just call them all blackberries.

Where to find it: Various forest boundary areas of the park, particularly on the east side.