I visited southern NJ again on May 12th and found the following plants blooming in the pine barrens.
Lance-leaved violet (Viola lanceolata) is normally found on moist sandy soils bordering ponds, lakes, and rivers. These plants seemed to be quite drought tolerant, growing on an open sandy site without any nearby water source.
A similar species is primrose-leaved violet (V. primulifolia), which some botanists believe is a hybrid of V. lanceolata and V. pallens though it can occur without one or both parents nearby.
A common biennial plant in the pine barrens is blue toadflax (Nuttallanthus canadensis). Since its life cycle is heavily dependent on successful annual seed production this plant is a prolific bloomer.
Another interesting plant blooming in pine barrens this time of year is American ipecac (Euphorbia ipecacuanha). The root of this plant has been used to make an emetic (syrup of ipecac, which induces vomiting), hence the common name and specific epithet.