The ornamental design for a plantersupport, as shown and described.
FIG. 1 is a perspective view of my new design;
FIG. 2 is a side view of the design of FIG. 1;
FIG. 3 is a top view of the design of FIG. 1; and,
FIG. 4 is an exploded perspective view thereof.
The broken lines showing planters and another planter support are included for the purpose of showing features that form no part of the claimed design.