Water and Sunlight
Too little water causes the lemon balm to produce weak leaves that die before they reach maturity. Drought conditions force the lemon balm into a dormant state, which weakens the plant. Water the lemon balm plant regularly, to keep the soil moist. Lemon balm plants grow best in partially shaded locations. Too much sunlight can be damaging to the lemon balm, even though it will grow in full sun.
Planting lemon balm plants too close together forces them to compete for resources like water, nutrients and light, which weakens the plant and can cause premature leaf death. Lemon balm plants grow as tall as 2 feet high; to allow for proper root space and healthy growth, plant the lemon balm plants 18 inches apart. Root crowding in close plants leads to premature leaf loss or death.
Pest problems, like aphids, caterpillars and mites can lead to defoliation of a lemon balm plant; aphids attack young leaves as they emerge from the plant, leading to weak, brown leaves that die off rapidly. Aphids and other pests feed on the sap of the leaves, taking nutrients from the plant and leaving the leaves susceptible to disease problems. Lemon balm leaves attacked by pests are more likely to turn brown and die. Apply a pesticide spray to the plant to keep it from being killed by pests.
A lack of nutrients in the soil will lead to weak growth on a lemon balm plant. Ensure that the soil is fertilized each spring to provide plants with the nitrogen, phosphorus and potassium needed to create healthy growth. Nutrients like zinc and magnesium are also important for the health of lemon balm plants. Unhealthy or unbalanced soil leads to lemon balm plants that are unable to put out healthy, adult leaves.