The ornamental design for a plantmarker, as shown and described.
FIG. 1 is a front side view of a first embodiment of the plant marker.
FIG. 2 is a back side view of the first embodiment of the plant marker.
FIG. 3 is a right side view of the first embodiment of the plant marker.
FIG. 4 is a top side view of the first embodiment of the plant marker; and,
FIG. 5 is a bottom side view of the first embodiment of the plant marker.
The broken lines in the drawings showing the writing and the top end of the plant marker depict portions that are not part of the claimed design, but rather show the environment in which the design is associated. The diagonal shading and stippling on the plant marker indicates a contrast in materials or color.