The ornamental design for a flowerpotholder, as shown and described.
FIG. 1 is a front elevational view of a flower pot holder showing my new design;
FIG. 2 is a rear elevational view thereof;
FIG. 3 is a left side elevational view thereof, the opposite side being a mirror image of the shown;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a top, front, and right side perspective view thereof, with the broken-line disclosure of a fragment of a wall and a flower pot being shown for illustrative purposes only and forming no part of the claimed design.