Why mint refuses to stay put

Mint is one of the easiest herbs you can grow in the UK, and that is precisely the problem. It does not form a neat clump like thyme or rosemary. Instead it sends out stolons — thick, pale, fleshy runners that travel just below the soil surface, sometimes 30cm or more in a single season. Wherever a runner surfaces, it throws up a new plant, and the process repeats.

In open ground, a single supermarket pot of mint can colonise a border, creep into the lawn and thread itself through neighbouring herbs within two or three years. Digging it out rarely works completely, because any fragment of root left behind will regrow. Growing mint in its own container solves this at the source: the roots hit the side of the pot and stop. The plant still sprawls above ground, but it cannot invade anything else.

Choosing the right container

Mint has a big, thirsty root system, so give it room. A 20–25cm diameter pot holding around 5 litres of compost is a sensible minimum for one plant. A half-barrel or a 10-litre pot will support a much more generous crop and needs watering less often.

Whatever you choose, drainage matters. Mint likes damp soil but hates sitting in stagnant water, especially over winter.

• Choose a pot with plenty of drainage holes, and add a layer of broken crocks or gravel at the base.
• Terracotta looks lovely and dries out faster; glazed ceramic and plastic hold moisture for longer.
• A saucer is useful in summer, but empty it after heavy rain so the roots are not submerged.
• Never tuck mint into a mixed herb planter, however tempting the arrangement looks. It will swamp the parsley and overwhelm the chives.

Compost and planting

Use a good-quality peat-free multipurpose compost, or a soil-based John Innes No. 2 mixed with a little grit for weight and drainage. Soil-based compost suits mint particularly well because it holds moisture without becoming airless.

Plant so the crowns sit just below the surface, firm the compost gently and water in well. If you are growing from a supermarket pot, split the tangled rootball into two or three pieces first — each will grow away strongly and you will get a fuller container far quicker. If you want to sink the pot into a border for a tidier look, leave the rim a couple of centimetres above soil level so runners cannot simply hop over the edge.

Position, watering and feeding

Mint is happiest in partial shade, though it will tolerate sun if the compost stays reliably moist. A spot that gets morning sun and afternoon shade is ideal, and it suits a north- or east-facing kitchen windowsill or a shaded patio corner.

Containers dry out far faster than garden soil, so check daily in summer. In a heatwave you may need to water twice a day. Aim to keep the compost evenly damp rather than soaking it in bursts — dry spells followed by drenching encourage weak, leggy growth and can trigger rust.

Feeding should be light. A diluted liquid feed every three or four weeks from late spring to midsummer is plenty. Too much nitrogen gives you lush leaves with very little flavour, which defeats the point of growing your own.

Harvesting to keep plants bushy

Regular picking is the simplest way to keep a container of mint productive and tidy. Cut stems just above a pair of leaves rather than stripping individual leaves, and the plant will branch from that point. Pinch out the growing tips of young plants to encourage a bushier shape.

Remove flower spikes as soon as you see them if you want leaves for the kitchen — once mint flowers, the foliage turns coarse and the flavour dulls. If you would like the bees to enjoy it, let a few spikes open late in the season and cut the plant back afterwards.

Keep an eye on the rim of the pot. Runners will try to escape over the side and root into whatever is nearby, so snip them off and either compost them or pot them up as new plants.

Repotting, dividing and overwintering

By the end of a season, a pot of mint is usually a solid mass of roots. Every spring, tip the plant out, break it into sections and replant the healthiest pieces in fresh compost. Trim away the oldest, woody roots and any that are circling tightly. This one job keeps flavour strong and prevents the plant from exhausting its compost.

Mint is fully hardy in the UK, but roots in a small pot can freeze solid during a cold snap. Move containers to a sheltered spot against a wall, or wrap them in bubble wrap and raise them onto pot feet. Cut the stems back to about 5cm in autumn. Fresh growth will appear from the roots in spring, usually just as you are starting to think about the first salads of the year.

Common problems and quick fixes

• Rust (orange-brown spots on the underside of leaves): improve airflow, remove affected leaves, and avoid overhead watering.
• Leggy, pale growth: usually too little light or too much feeding. Move the pot and ease off the fertiliser.
• Wilting despite damp compost: the pot is probably root-bound. Repot or divide it.
• Mint appearing in a neighbouring pot: a runner has escaped. Pull it out promptly before it establishes.

Treat mint as a plant that needs its own space and a little discipline, and it will reward you for years — handfuls of leaves for new potatoes, mint tea, tabbouleh and summer drinks, without a takeover of the rest of the garden.