Lemon balm, Melissa officinalis, is a perennial herb known for its bright, lemony scent. However, like many members of the mint family, lemon balm possesses an aggressive nature in the garden that can lead to rapid, sometimes overwhelming, growth. This vigorous expansion is the main concern for those planting it, stemming from its highly effective dual mechanisms of reproduction.
Understanding Lemon Balm’s Dual Spreading Mechanisms
Lemon balm is a successful colonizer because it employs two distinct biological strategies to spread. The first involves the production and dispersal of numerous viable seeds, allowing the plant to expand its presence over long distances. The second utilizes underground runners, enabling a dense, localized takeover of the immediate surrounding area.
The plant produces small flowers, typically between July and September. Once mature, they develop into tiny, hard seeds that are easily dropped to the soil surface. Lemon balm seeds are notably small, requiring light to germinate successfully, meaning they thrive in disturbed or open soil patches near the parent plant. This self-seeding is the primary method for new plants to appear unpredictably across a garden.
The herb spreads through specialized underground stems known as rhizomes. These rhizomes creep horizontally beneath the soil surface, periodically rooting and sending up new shoots that form independent plant crowns. This process leads to the formation of dense, expanding clumps that are genetically identical to the original plant.
Conditions That Determine How Quickly Lemon Balm Expands
The rate at which lemon balm spreads is heavily influenced by ideal environmental conditions. When provided with optimal resources, the plant channels more energy into both rhizome development and abundant seed production, accelerating its expansion.
Lemon balm thrives in rich, fertile, and well-drained soil, often preferring a neutral pH range between 6.5 and 7.0. This composition allows for easy rhizome travel and provides the necessary nutrients for the plant to establish a large, healthy root system. A robust root system supports a larger above-ground structure capable of producing more flowers and subsequent seeds.
The plant requires consistent moisture. While lemon balm is drought-tolerant once fully established, steady access to water encourages lush foliage and promotes the rapid elongation of underground runners. Furthermore, while the plant tolerates full sun, partial shade, especially during the hottest parts of the day, can be ideal. This protection from intense heat reduces plant stress, allowing it to maintain vigorous growth and maximize its reproductive output.
Strategies for Controlling Unwanted Spreading
Effective management of lemon balm requires a two-pronged approach that addresses both its seed dispersal and its underground growth. The most important step in controlling the herb is preventing it from producing a new generation of seeds.
Controlling Seed Dispersal
Regularly harvesting the top third of the plant’s foliage is an excellent way to maintain a bushy shape while simultaneously delaying or stopping the flowering process. If small white flowers do appear, they must be removed immediately to prevent the formation of viable seed. Applying a thick layer of organic mulch around the plant can also help by blocking light to the soil, which prevents any dropped seeds from germinating.
Controlling Underground Runners
To manage the local spread via underground runners, physical barriers offer the most reliable solution. Planting lemon balm in a container is the simplest method, as the pot walls entirely restrict the rhizomes’ ability to expand. If planting in the ground, a barrier made of plastic or metal can be sunk vertically into the soil around the plant to a depth of at least 10 to 12 inches.