Mint is a highly favored herb, prized for its refreshing aroma and diverse culinary applications. Its vigorous growth habit is a common challenge for gardeners, as it quickly overtakes other plants. This aggressive spread is rooted in the plant’s natural growth mechanism. Understanding this biological engine is the first step toward successful containment.
Understanding the Root System
Mint is an aggressive spreader because it propagates through specialized horizontal stems called rhizomes. These are modified underground shoots, not true roots, that grow outward from the main plant. Rhizomes continually produce new vertical stems and roots, allowing the mint to rapidly colonize new territory.
These underground runners are typically found in the upper soil layers, generally running between two and eight inches below the surface. Because the rhizomes are shallow and grow horizontally, they can easily bypass the root systems of neighboring plants. This unique growth pattern requires physical barriers to be effective, as the mint will continue to spread until it hits an obstruction or the edge of the garden bed.
In-Ground Containment Techniques
For gardeners who want to enjoy mint directly in the soil, installing a physical root barrier is necessary to restrict rhizome movement. Use a durable, non-biodegradable material such as thick plastic edging, metal sheeting, or a bottomless plastic bucket. This barrier must be installed deep enough to physically block the runners as they attempt to spread.
The barrier should extend at least 15 to 18 inches into the ground to ensure rhizomes cannot grow underneath it. Leaving a small portion of the barrier, typically one to two inches, protruding above the soil line is equally important. This elevated lip prevents above-ground runners, which can root where they touch the soil, from escaping over the top. An effective method is to plant the mint inside a large pot with the bottom cut out, which is then sunk into the ground up to the rim.
Growing Mint in Containers
Growing mint in containers offers the most straightforward way to ensure total isolation from your garden beds. Choose a pot that is wide and at least 12 inches deep to provide ample room for the plant to thrive. The container must have adequate drainage holes to prevent the soil from becoming waterlogged.
Prevent the rhizomes from escaping through the drainage holes and rooting into the ground below. Always place the container on a hard, impermeable surface, such as a concrete patio, a paver stone, or a wooden deck. If you choose to sink the pot into the ground for a more integrated look, place a solid barrier, like a saucer or piece of slate, beneath it to block any escaping roots. Alternatively, rotate the sunken container a quarter-turn every few weeks to prevent any developing roots from establishing a permanent connection with the surrounding soil.
Management of Established Patches
If a mint patch has already spread beyond its designated area, aggressive intervention is required to reclaim the space. Begin by physically digging out the entire patch, ensuring you remove every visible piece of the underground rhizome network. Since mint can regrow from tiny root fragments, sifting the soil or performing a “double-dig” to remove all remnants is highly effective.
Regularly removing the top growth, either through aggressive harvesting or mowing, is an effective long-term management strategy. Continually cutting the stems prevents the plant from photosynthesizing, which eventually depletes the energy reserves stored in the rhizomes. For severely overgrown areas, non-chemical methods like solarization or smothering can be used to kill the patch entirely. This involves covering the area with thick black plastic or heavy cardboard for six to eight weeks during the warmest part of the year.