Lemon Balm

I have lemon balm growing in my back yard in Dallas. I water mine every day, and don't know if you can overwater them. Lemon balm makes a great tea mixed with peppermint and spearmint, although I boil mine about 30 minutes--longer than usual. At that point the greens are tender and can be eaten like any potherb.

Check the soil every day. If it feels moist, don't water. If it is dry, give it some water.

Giving you a frequency is difficult. It depends on what materials the pot is made from. Some are more porous than others and dry out faster. It depends on the condition of the soil and plant. If it is root bound, it will need more frequent watering.

Best thing to do is monitor the moisture in the soil and water as needed. You'll soon get an idea of the schedule your specific plant needs to thrive.

Also on humidity.

I only grew it once, in a very temperate climate, in semi shade.
When I would water would depend on the weather and the look of the leaves.
I try to avoid nurseries that purvey root bound stuff, but sometimes that's unavoidable. I tend to unbind them into good-ish soil, if I can, and then, yes, water.

My later business partner had a wonderful small nursery before we set up a design firm, and I doubt anything there was root bound. She knew her business.

Maintaining Growth

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]