Mint (Mentha species) is a highly favored herb in the home garden, popular for its aromatic leaves used in cooking, beverages, and essential oils. The genus Mentha includes varieties such as peppermint (Mentha × piperita) and spearmint (Mentha spicata), valued for their vigorous growth and strong flavor profiles. Gardeners appreciate mint’s ease of cultivation, but its notorious habit of spreading quickly raises concerns about whether it is an invasive plant.
Aggressive Growth vs. True Invasive Species
The terminology used to describe a plant’s spreading habit is often confused, but there is a clear distinction between an “aggressive” plant and a legally defined “invasive species.” An aggressive plant spreads rapidly and dominates its immediate environment, such as a garden bed, often outcompeting nearby cultivated plants. Mint certainly fits this description, as it quickly takes over the space allocated to it and can be difficult to remove once established.
The term “invasive species,” however, is an ecological and legal designation. A plant is classified as invasive if it is non-native to the ecosystem and causes, or is likely to cause, environmental or economic harm. While some mint varieties are considered weeds, common culinary mints are generally not classified as true invasive species in most jurisdictions. This is because they rarely escape cultivation to disrupt natural, wild habitats in the same manner as plants like kudzu.
The Root System Driving Mint’s Spread
Mint’s remarkable ability to spread is rooted in its specialized underground structures. Mint plants propagate mainly through modified horizontal stems called rhizomes, which are distinct from roots. These rhizomes grow laterally beneath the soil surface, acting as an extensive underground network.
The rhizomes possess buds along their length that sprout new shoots and roots, allowing the plant to rapidly colonize the surrounding area. This process creates clonal offshoots that can emerge far from the original planting site. Because these structures are underground and can extend deep into the soil, simply pulling up the visible top growth is ineffective, as the rhizomes remain intact to send up new growth. This vegetative spreading mechanism allows mint to quickly cover an area, making it a garden “thug” that outcompetes slower-growing neighbors.
Practical Strategies for Containment
Gardeners who wish to enjoy fresh mint without managing its spread must employ physical containment strategies. The most frequently recommended method is to plant mint in containers, as this restricts the rhizomes from spreading into the surrounding soil. For the best results, use a container that is at least 12 to 18 inches deep and wide, ensuring it has adequate drainage holes.
In-Ground Container Method
If you prefer the look of mint in the ground, sink a bottomless pot or a plastic container with the base removed directly into the garden bed. This creates an underground wall that blocks the horizontal growth of the rhizomes while still allowing the plant to access the nutrients and moisture of the garden soil. Leave the rim of the container about one inch above the soil line to prevent the rhizomes from creeping over the top edge.
Physical Root Barriers
Alternatively, a physical root barrier can be installed around the planting area to contain the herb’s growth. This barrier should be a solid material, such as metal flashing or a thick plastic sheet, and must be buried at least 12 to 18 inches deep to effectively block the traveling rhizomes. The barrier must also extend a few inches above the soil level to prevent above-ground runners, known as stolons, from rooting outside the designated area. Regular pruning of the plant, including cutting back any stray runners, is necessary to maintain control.