Characterized by lemon-scented foliage, lemon balm (Melissa officinalis) makes an attractive addition to the herb garden. Traditionally, it has been grown for culinary, medicinal and cosmetic use. This easy-care perennial grows about 2 feet tall and is hardy in U.S. Department of Agriculture plant hardiness zones 4 through 9.
Lemon balm grows best in locations with partial shade and fertile, loamy soil that stays moist but drains well. Water plants often enough to keep the soil from drying out. Make sure the soil doesn't stay soggy, though, because this can lead to root rot.
An all-purpose fertilizer applied twice a year provides all the nutrients lemon balm needs. Feed in the spring once the plants have started growing and again after flowering. Use a slow-release plant food, and follow package directions for application rates to perennial plants. A 12-4-8 plant food, for example, is applied to the soil at a rate of 1 tablespoon for each square foot.
In locations where the soil freezes over winter, mulch lemon balm in the late fall. You can also keep these plants mulched throughout the year to help conserve moisture around the roots. Use an organic mulch like wood chips or shredded leaves.
Pruning and Harvesting
You can harvest lemon balm throughout the growing season. Removing up to one-third of the foliage each month won't stress the plant and encourages healthy growth. If you don't plant to harvest lemon balm, use your fingernails to pinch the growing tips off each stem when plants reach 6 to 8 inches tall. This produces side growth and makes the plants bushy.
As the growing season progresses, older leaves become less desirable for harvest. After flowering, cut the plants back to the new growth near the ground. If there is no new growth yet, cut back just far enough to leave two or three sets of leaves on each stem. Then apply the second fertilizer application. The plants will put out new growth, which you can then harvest once a month until the plant dies down in the winter.
Always sterilize cutting tools before using them to prune or harvest lemon balm. This prevents disease transfer between plants and keeps the leaves clean for use in the kitchen. Soak cutting tools in a solution of one part bleach to three parts water for at least five minutes, and then rinse with clean water before use.
As a member of the mint family, lemon balm readily spreads in the garden. It can become invasive. To control the spread, harvest leaves regularly, dig around the roots to remove spreading stems and cut spent flowers before they set seeds.
Lemon balm is rarely bothered by diseases, but if plants are overwatered, powdery mildew may become a problem. You also avoid mildew problems by growing lemon balm in well-draining soil and not overfertilizing. Decrease watering, especially during times of high humidity, if you notice powdery growth on the leaves and stems. Remove and destroy all infected plant parts, making sure to disinfect cutting tools. Cultural controls should work for lemon balm, but if not, you can supplement with a fungicide like neem oil. Mix concentrated oil with water according to label directions, and spray on affected plants every 7 to 14 days.
Be sure to check warning labels for fungicides or insecticides to make sure they are safe to use on edible plants. For many products, you have to wait a certain number of days after application before harvesting plants for use in the kitchen. This varies depending on the brand and active ingredients.
Mites, aphids and cutworms can also feed on lemon balm, though the plants' high concentration of natural oils repels most pests. You can help prevent infestations by keeping plants weeded. If mites or aphids are a problem, spraying lemon balm with a strong stream of water can kill and dislodge insects. To repel cutworms, place a ring around the plants made of aluminum foil or cardboard that is partially buried in the soil and sticks up several inches above the ground.