The ornamental design for a planterbox, as shown and described.
FIG. 1 is a perspective view of the planter box showing a front side and a left side in which a corresponding perspective view of the planter box showing a rear side and a right side is identical to the view shown;
FIG. 2 is a front elevational view of the planter box in which the opposing rear elevational view is identical to the view shown;
FIG. 3 is a right side elevational view of the planter box in which the opposing left side elevational view is identical to the view shown;
FIG. 4 is a bottom view of the planter box; and,
FIG. 5 is a top plan view of the planter box.
The broken lines in the drawings represent unclaimed environmental subject matter and form no part of the claim.