FIG. 1 is a perspective view of the new design of the plant base; FIG. 2 is a right-side view thereof; FIG. 3 is a left-side view thereof; FIG. 4 is a front view thereof; FIG. 5 is a rear view thereof; FIG. 6 is a top view thereof; and, FIG. 7 is a bottom view thereof. The portions shown in broken lines do not form part of the claimed design.