An oak (Quercus spp.)-dominated forest characterized the precolonial vegetation of most of the North American mid-Atlantic region, according to the palynological and written records. There is some evidence, however, of herbaceous communities, especially along estuaries, in very poor soils such as serpentinites, and in areas affected by disturbances caused by windthrows or native Americans. Numerous herbaceous plant taxa, such as Ambrosia spp., grasses (Poaceae) and sedges (Cyperaceae), are native, and predated the arrival of people on the continent. In this palynological study, I found evidence of extensive herbaceous plant communities along the Holocene floodplain of the Delaware River, near Trenton, NJ, between 3,000 and 8,500 years ago, providing evidence for the habitat of these taxa over long periods of time. The plant communities were dynamic, on the scale of millennia, and compare well with current riverine plant communities. This is some of the first evidence for these plant communities existing for millennia in eastern North America in a non-estuarine habitat.