The ornamental design for a plant grow light, as shown and described.
FIG. 1 is a front and top perspective view of a plant grow light, showing my new design;
FIG. 2 is a rear and bottom perspective view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a right side elevation view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines in the figures illustrate portions of the plant grow light that form no part of the claimed design. The plant grow light is shown with a symbolic break in its length. The appearance of any portion of the article between the break lines forms no part of the claimed design.