I claim the ornamental design for an edging for modular planting system, as shown and described.
FIG. 1 is a perspective view of an edging for modular planting system showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof; and,
FIG. 5 is a top plan view thereof.
The broken lines are for illustrative purpose only. The claimed design is broken on one side to indicate indefinite length. None of the broken lines form part of the claimed design.