Why Mint Loses Its Vigour

Mint is the most enthusiastic herb in the kitchen garden. Given half a chance it will romp across a bed, send runners under the path and appear cheekily among the raspberries. That same vigour is what makes it fade in a pot or a confined corner of the plot. Within two or three seasons the roots become a dense, woody tangle, packed tight against the sides of the container with barely any compost left between them.

The warning signs are easy to read. Leaves get smaller and paler, the middle of the clump dies out, and the whole plant starts to look thirsty even when the compost is damp. Flavour falls away too — the oils that give mint its bite are produced by young, actively growing roots, and a congested crown has precious few of those. Dividing the roots in spring is the simplest remedy in the allotment repertoire, and it costs nothing but half an hour and a bag of compost.

Choosing the Right Moment

In most of Britain the ideal window runs from late March to the end of April, just as the first new shoots are pushing through and the soil has begun to warm. Aim for shoots around two to five centimetres tall — big enough to handle, small enough to settle in without sulking. Gardeners in the north, or on cold heavy clay, are best waiting until early May.

You can divide mint in autumn, but the plants have less time to root before winter and wet crowns often rot. Spring divisions establish quickly and will be cropping properly by late May. Choose a mild, overcast day if you can, and water the pot or clump thoroughly the day before to make lifting easier.

Lifting and Inspecting the Clump

• If it is growing in a pot, run a knife around the inside rim, tip the pot on its side and ease the rootball out. A firm tap on the base usually does it.
• If it is in open ground, fork around the clump generously — at least a spade’s width from the edge — then lever it up in one piece.
• Shake off as much old compost as you reasonably can and rinse the roots under a tap or in a bucket. You need to see what you are dealing with.
• Discard the woody, hollow centre of the crown. This is the oldest material and never regrows well.

While the roots are bare, check the leaves and stems for mint rust, which shows up as bright orange pustules on the undersides of the foliage. Badly affected pieces should go in the bin, not the compost heap, and it pays to choose divisions only from clean, healthy growth.

Splitting Into Strong Sections

For a small clump, two garden forks pushed back to back into the middle and levered apart will separate it neatly. Bigger crowns may need a sharp knife or an old pruning saw — do not be timid, mint is tough and a clean cut heals faster than a torn one.

Break the clump into pieces roughly the size of your fist, each carrying three to five plump shoots and a good spread of pale, healthy roots. A single congested pot will often yield six or eight divisions, which is why dividing is the cheapest way to fill a herb bed. Any stray runners you pull out can be trimmed to a few centimetres and potted up separately, where most will root within a fortnight.

Replanting in Fresh Compost

Mint is greedy and appreciates a rich start. Use a soil-based compost such as John Innes No. 2, or a good multipurpose mixed with a handful of grit for drainage. A 25–30cm pot suits one division nicely.

Set the root piece so the shoots sit just at the surface, firm the compost gently around it and water well. Keep the pots in light shade for three or four days, then move them into their final spot. If you are planting into the ground, sink the pot rim just above soil level, or line the hole with a bottomless bucket — buried mint will travel, and you will spend the rest of the summer pulling it out of the onions.

Aftercare and Keeping It in Bounds

• Water regularly for the first month; mint resents drying out, but it will not forgive waterlogging either.
• Feed with a dilute liquid seaweed every fortnight through summer for tender, aromatic leaves.
• Cut plants back hard after flowering to encourage a fresh flush of young foliage.
• Divide again every second spring, before the crown has a chance to seize up.

Grow a couple of varieties rather than one — Moroccan mint for tea, spearmint for sauces, apple mint or a peppermint for puddings and cold drinks. The flavour difference between a revived plant and a tired one is genuinely startling, and by June you will be picking leaves so sweet and sharp that you will wonder why you ever let the pot go untouched for so long.