Lemon balm (Melissa officinalis) is a popular herb, valued for its lemon scent and uses in teas, culinary dishes, and remedies. If your lemon balm leaves turn purple, it signals the plant is reacting to its environment or internal conditions.
Understanding Purple Leaf Coloration
Purple coloration in plants, including lemon balm, results from pigments called anthocyanins. These water-soluble flavonoids appear red, purple, or blue, depending on the plant’s cell sap pH. Plants synthesize anthocyanins as a protective mechanism against environmental stressors. They absorb light, protecting plant tissues from damage. When a plant experiences stress, it increases anthocyanin production as a defense response, causing visible color changes.
Common Causes for Purple Lemon Balm Leaves
Nutrient Deficiencies
A common reason for purple lemon balm leaves is a nutrient deficiency, particularly a lack of phosphorus. Phosphorus is a crucial macronutrient for energy transfer and plant growth. When insufficient, it disrupts metabolic processes, triggering anthocyanin production and a purplish hue, especially on older foliage. This deficiency can occur even if phosphorus is present but unavailable due to soil pH or cold temperatures.
Temperature Stress
Temperature fluctuations, especially cold temperatures, frequently cause lemon balm leaves to turn purple. When exposed to sudden drops or prolonged cold, the plant produces anthocyanins as a protective measure. This pigment shields cells from cold damage and improves chilling tolerance. Purple coloration can appear even in cool, non-freezing conditions.
Environmental Factors
Other environmental factors can contribute to purple lemon balm leaves. Incorrect soil pH limits nutrient availability, leading to purpling. Lemon balm prefers well-drained soil with a pH between 6.5 and 7.3. Excessive sun exposure, particularly in hot climates, can also stress the plant, prompting anthocyanin production. While lemon balm tolerates drought, prolonged dry spells cause stress, leading to color changes as the plant conserves moisture.
Diagnosing the Cause of Purple Leaves
To determine why your lemon balm leaves are turning purple, observe the entire plant and its environment. Check soil moisture; overly wet or dry conditions induce stress. Consider recent temperature changes, especially cold snaps or prolonged cool periods, as these trigger anthocyanin production. Examine the purple leaves’ location—older leaves, new growth, or evenly distributed—for clues about nutrient deficiencies or environmental stressors. Performing a soil test can reveal pH or nutrient imbalances.
Look for additional symptoms accompanying the purple coloration. Stunted growth, wilting, or yellowing can indicate nutrient issues or watering problems. For example, if older leaves are purpling and growth is slow, a phosphorus deficiency is likely. If the plant appears healthy and purpling occurs after a cold night, temperature stress is more likely. These details help narrow down potential causes.
Solutions and Prevention
Addressing purple leaves means rectifying the underlying stressor. If a phosphorus deficiency is suspected, amend the soil with a balanced fertilizer or one higher in phosphorus. Organic matter like compost also gradually releases nutrients, improving soil fertility. Ensuring proper soil pH (6.5-7.3) allows the plant to absorb nutrients efficiently.
To prevent temperature stress, protect lemon balm during cold snaps with row covers or by moving potted plants indoors. Water consistently; aim for moist soil, allowing the top inch to dry between waterings to prevent drought and root rot. If excessive sun is the issue, provide partial shade during the hottest part of the day. Regular harvesting encourages new, healthy growth and maintains plant vigor.
When Purple is Not a Concern
While purple leaves often indicate stress, slight purpling on lemon balm is sometimes not a concern. Certain cultivars may naturally exhibit purple or reddish hues, especially on new growth or leaf margins. This genetic predisposition means the plant produces anthocyanins as part of its normal development, not due to an adverse condition. In such cases, the plant appears robust with vigorous growth and no distress signs.
If your lemon balm is actively growing, producing new leaves, and looks healthy despite some purple coloration, it might be a natural characteristic or a benign reaction to minor environmental shifts. For instance, a temporary cool spell might induce a temporary color change without harm. Observe the overall health and growth pattern to distinguish between cosmetic variation and genuine stress.