I offer this quote from a user named “sagedavis” from the Helpful Gardener Gardening Forum: “It did, somehow, make my potted mint grow way faster than I expected it to, and the roots basically strangled it out before I realized what was causing the death of the the mint, and obviously, before I decided it was time to transplant it.”
Do a CTRL + F and search for “mint”.

In pot-grown mints, there are two main kinds of roots: feeder roots, which draw nutrients upwards throughout the plant; and tap roots, to stablize the plant. In a pot, the roots are constrained, but they still continue to grow in the constrained area. Both types of roots grow, and over a period of more than a year, can fill up the whole pot so much that the root mass starts growing out from the drainage holes at the bottom of the pot. In the pot itself, there is no more space to expand. So the roots become so tight that they literally strangle themselves. Nutrients and water are likely unable to reach the feeder roots easily, and the plant might die from dehydration and also nutrient deficiency.

The way to prune the mints’ roots are as follows:

Step one
Grab hold of all the stalks of your plant into one bunch. Hold them as close to their base as possible. Pull the whole clump gently out of the pot. Technically, your root ball should look much tighter and messier than the one shown in my picture before you prune it. But I prune my mints every nine months or so to encourage more vigorous growth, and to do a soil change for them.

Step two
Take a pair of clean secateurs and cut between 30 to 40% of the lower root mass off (the part farthest from the stalks). I usually do a 50% cut, but for others who are not sure, I recommend a 30% prune.

Step three

Fill your pot with fresh soil mix

Step four

Place your pruned plant gently onto the fresh soil, and firm it down. Fill up any gaps between the root mass and the pot, water your plant, and place it in an area with bright shade (not full sun) for a few days until you see the leaves perk up again, which shows that it has gotten over the shock to its roots.

Personal experience: The mints (and also other plants) all seem to do better with semi-regular root prunings. However, you must make sure you know if a plant is exceptionally sensitive to root disturbances (like the lemon myrtle) then a minimal root pruning of 5% of the whole root ball is enough.

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