Lemon balm (Melissa officinalis) is a popular perennial herb known for its lemony fragrance and flavor. Since it belongs to the Lamiaceae family, the same botanical family as mint, gardeners often worry it shares mint’s notorious tendency to spread uncontrollably. Understanding the distinct growth habits of these two relatives is important for anyone considering adding the herb to their garden space. While both can become vigorous, their mechanisms for expansion are fundamentally different.
The Aggressive Growth of Mint
Mint species, belonging to the genus Mentha, are infamous for their aggressive and rapid horizontal expansion. This vigorous growth is driven by specialized underground stems called rhizomes, which function as subterranean runners. These rhizomes creep horizontally beneath the soil surface, acting as a cloning mechanism for the parent plant.
The rhizomes possess nodes that sprout new, genetically identical mint plants several feet away from the original planting location. This network creates a dense, interconnected root system, allowing mint to quickly colonize and dominate an area, easily outcompeting nearby plants for resources. Even a tiny fragment of a broken rhizome can regenerate into a new, fully established plant, making eradication difficult once established in the ground. Gardeners often plant mint in contained environments to prevent its relentless spreading.
How Lemon Balm Spreads
Lemon balm does not spread using the aggressive, far-reaching rhizomes characteristic of its mint relatives. Instead of sending out underground runners, lemon balm primarily grows as a clump, slowly expanding outward from a central root crown. The plant’s immediate root mass will grow wider over time, but it will not send shoots up across the garden bed.
The primary way lemon balm becomes difficult to manage is through prolific self-seeding. Throughout the summer, the plant produces inconspicuous, small white or pale yellow flowers. If these flowers are allowed to mature, they will set seed that drops directly onto the surrounding soil.
These dropped seeds germinate readily, often resulting in a dense cluster of new seedlings near the mother plant. This dispersal method is significantly different from mint’s root-based invasion, which allows it to appear in unexpected locations far from the original plant. The resulting small seedlings from lemon balm are much easier to remove than the established, woody rhizome network of mint. However, if left unchecked for multiple seasons, lemon balm can still create a dense, interwoven clump of roots and stems, making large established patches challenging to remove.
Managing Lemon Balm Growth
Since lemon balm’s main spreading strategy is via seed dispersal, controlling its growth requires interrupting the reproductive cycle. The most effective preventative measure is consistent deadheading, which involves removing the small flowers before they have the chance to mature and set viable seed. This action eliminates the source of the aggressive self-seeding that causes the plant to spread.
Regular harvesting and pruning throughout the growing season also help maintain the plant’s size and prevent it from flowering in the first place. Cutting the plant back several times a year encourages bushier growth and reduces the likelihood of seed development.
Gardeners often choose to grow lemon balm in containers to eliminate the risk of spread. While a container is essential for physically blocking the rhizomes of mint, for lemon balm, this method simply isolates the plant and prevents its seeds from dropping into the surrounding garden soil. If planting in the ground, adding a layer of mulch around the base of the plant can also help by preventing fallen seeds from germinating.