Lemon balm thrives in moist, well-drained soil. It is important to keep the soil from drying out but take care not to over-water. Root rot and powdery mildew have been known to affect plants that get too much water. A drip system or soaker hose will help keep the soil moist in summer. Check the plants daily to be sure they are getting an adequate water supply. The mulch you used should help keep it damp as well.
The mulch should also keep weeds down. However, if you do see weeds, it is important to eradicate them immediately. Remove them by hand, taking care not to disrupt the plant's shallow roots.
To ensure fresh, fragrant leaves throughout the growing season, prune the plant close to the ground regularly. It will grow back quickly and give you a higher yield. For a bushier plant (and more leaves for harvesting), pinch back some of the growing tips at least twice during the growing season.
To help maintain good growth, you should also apply an all-purpose fertilizer once in spring and once in fall. Be careful not to over-fertilize as this will produce less flavorful leaves. [doityourself]