How to Stop Mint from Spreading in the Garden

Mint plants are perennial aromatic herbs from the Mentha genus, that are widely grown in temperate areas around the world, and have spread worldwide, due to their culinary, medicinal and aromatic value.

These are amongst the easiest plants for beginner herb gardeners to grow, as mint plants can be grown outdoors in pots and in the garden, in sun or shade, and can even be grown indoors near a bright window.

How Mint Plants Grow and Spreads

Mints grow best in moist, rich soils with a loose, friable, porous soil structure. They’re rampant, fast-growing plants, which can reproduce from seed, though most hybrids are infertile, but nearly all mint varieties can propagate to form new plants through their wide-spreading underground rhizomes.

Rhizomes are horizontal root-type structure which are actually modified and specialized underground stems that grow horizontally along the soil surface or just below it, and also serve as a storage organ containing the plant’s food reserves. New stems with leaves with their own roots grow from the rhizome, allowing mint plants to ‘run’ and spread, with new stems (suckers) appearing from the ground some distance from the parent plant.

The Four Best Ways to Stop Mint from Spreading

Given ideal conditions, mint plants may spread aggressively and become invasive in the garden, filling a whole garden bed completely!

The prolific growth habit of mint plants can be contained using the following four methods:

1. Plant only one variety of mint in its own small, separate garden bed where it can’t interfere with other plants. If more than one variety are planted together, they become a tangled mess that makes it hard to tell them apart when harvesting.
2. Grow mint in containers, large pots around 30cm wide can be very productive. Once again, one variety of mint per pot.
3. Plant mint with other plants and manually keep it under control. It may spread, but mint is fairly shallow rooted, and is relatively easy to pull up at the end of each summer, restricting its spread.
4. Create an in-ground root barrier by planting mint into a large bottomless container that is sunken into a garden bed.

How to Make a Root Barrier for Mint Plants

To construct an in-ground root barrier to contain the spread of rampant mint plants, use a plastic 30cm pot (or larger if desired), with the bottom removed.

Root barriers prevent the underground spread of the horizontal rhizomes.

1. Cut off the bottom of the plastic pot using garden secateurs, which cut through the thick plastic easily.
2. Dig a hole in the ground to fit the bottomless pot, make it deep enough so the pot extends around 5cm above the soil. When digging out the soil, especially if space is limited, it may help to put it into a bucket, this makes much easier to refill the hole when planting, and is a lot tidier too!
3. Fill the gap around the outside of the pot with soil, ensuring that the bottomless pot is sitting straight in the hole.
4. If the mint plant to be planted is in a pot, sit this pot in the hole, and add to the bottom of the hole, soil underneath the pot, to elevate the plant to the correct height. Unpot it to plant it in the same way that it would be when planted into the garden bed.
5. Plant the mint into the bottomless container, then water it gently with a watering can or a hose with a shower spray nozzle to settle the soil and give the plant a drink.

If the mint plant grows over the edge of the pot, and touches the soil around it, it may eventually root and spread, so prune off any long stems hanging over the sides.

A standard plastic 30cm pot has a volume of 13.5L, which is ample root space for most mint plants, and is approximately 30cm tall, which is quite deep for most mint rhizomes to run.

Smaller pots, such as a 20cm (8″) or 25cm (10″) wide plastic posts can be used when space is limited, such as in a narrow, raised garden bed that contains many other plants.

Smaller growing mint varieties such as peppermint grow well in smaller pots, and in smaller root barriers too.

This method of containing mint work successfully if we harvest mint regularly, so it grows bushier, doesn’t self-seed or lay down on the ground and take root outside the root barrier. It only takes little extra effort to save a lot of work!