The design consists of the features of shape and configuration of a cover for plant-growing media shown in solid lines in the drawings.FIGURE 1 is a top perspective view of a first variant of a cover for a plant-growing media;FIGURE 2 is a bottom perspective view of the cover of Figure 1;FIGURE 3 is a first side view of the cover of Figure 1;FIGURE 4 is a second side view of the cover of Figure 1;FIGURE 5 is a third side view of the cover of Figure 1;FIGURE 6 is a fourth side view of the cover of Figure 1;FIGURE 7 is a top plan view of the cover of Figure 1;FIGURE 8 is a bottom plan view of the cover of Figure 1;FIGURE 9 is a top perspective view of a second variant of a cover for a plant-growing media;FIGURE 10 is a bottom perspective view of the cover of Figure 9;FIGURE 11 is a first side view of the cover of Figure 9;FIGURE 12 is a second side view of the cover of Figure 9;FIGURE 13 is a third side view of the cover of Figure 9;FIGURE 14 is a fourth side view of the cover of Figure 9;FIGURE 15 is a top plan view of the cover of Figure 9;FIGURE 16 is a bottom plan view of the cover of Figure 9;FIGURE 17 is a top perspective view of a third variant of a cover for a plant-growing media;FIGURE 18 is a bottom perspective view of the cover of Figure 17;FIGURE 19 is a first side view of the cover of Figure 17;FIGURE 20 is a second side view of the cover of Figure 17;FIGURE 21 is a third side view of the cover of Figure 17;FIGURE 22 is a fourth side view of the cover of Figure 17;FIGURE 23 is a top plan view of the cover of Figure 17;FIGURE 24 is a bottom plan view of the cover of Figure 17;FIGURE 25 is a top perspective view of a fourth variant of a cover for a plant-growing media;FIGURE 26 is a bottom perspective view of the cover of Figure 19;FIGURE 27 is a first side view of the cover of Figure 25;FIGURE 28 is a second side view of the cover of Figure 25;FIGURE 29 is a third side view of the cover of Figure 25;FIGURE 30 is a fourth side view of the cover of Figure 25;FIGURE 31 is a top plan view of the cover of Figure 25;FIGURE 32 is a bottom plan view of the cover of Figure 25;FIGURE 33 is a top perspective view of a fifth variant of a cover for a plant-growing media;FIGURE 34 is a bottom perspective view of the cover of Figure 33;FIGURE 35 is a first side view of the cover of Figure 33;FIGURE 36 is a second side view of the cover of Figure 33;FIGURE 37 is a third side view of the cover of Figure 33;FIGURE 38 is a fourth side view of the cover of Figure 33;FIGURE 39 is a top plan view of the cover of Figure 33;FIGURE 40 is a bottom plan view of the cover of Figure 33;Drawings of the design are included.