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Fish mint, also sold as chameleon plant, spreads by brittle white rhizomes that travel horizontally at four to eight inches (10 to 20 cm) below the surface and keep going under paving, under edging boards, under a fence line and into the bed on the other side. Every fragment left behind carries buds and grows into a new plant, so the usual response of digging it out tends to multiply it instead.
This is not a plant that becomes a nuisance if you neglect it. It becomes a permanent feature of the garden the moment it is planted in open soil, and gardeners routinely spend three to five years trying to undo a single planting. Treat it the way you would treat bamboo or Japanese knotweed rather than the way you treat a herb.
The practical rule is short: a sealed container, or nowhere. If you cannot commit to a container that has no route into the soil, do not plant it, because the decision cannot be reversed cheaply later.
In some regions fish mint is listed as an invasive species and planting it in open ground is discouraged or restricted. Check your local list before planting, and never dispose of it in a green waste bin or a hedgerow.
What a sealed container actually means
Ordinary potting practice is not enough. Rhizomes leave through drainage holes, root where a pot stands on soil, and cross from one pot to the next where the rims touch. A container that holds fish mint has to be sealed at the bottom and lifted off the ground.
- Use a deep, rigid container At least twelve inches (30 cm) across and twelve inches deep. Rhizomes fill a shallow tray in a single season and then push out over the rim, so depth buys you time between divisions.
- Close or screen the drainage holes Line the base with fine metal mesh, or drill drainage holes only in the sides an inch (3 cm) above the base. A rhizome finds an open drainage hole within one growing season and roots straight into the ground below.
- Stand it on a hard surface, raised Put the pot on bricks, a slab or a stand so that it never sits directly on soil, gravel or a lawn. Use a saucer you can lift and inspect, and slide the pot sideways twice a year to check underneath.
- Keep two inches of free rim Fill the compost to about two inches (5 cm) below the rim. Stems that flop over a full pot touch the ground and root at the node, and that is the commonest escape route of all.
- Do not group pots edge to edge Leave a hand's width between the fish mint pot and anything else. Rhizomes and rooting stems cross short gaps readily, and a neighboring pot of something else is an easy target.
If you plant it in the ground anyway
Some gardeners want it as ground cover in a difficult wet corner. It will do that job better than almost anything else, but only inside a physical barrier, and only where you accept the whole enclosed area is now fish mint permanently.
| Requirement | Specification | Why it matters |
|---|---|---|
| Barrier depth | At least 18 inches (45 cm) below soil level | Rhizomes run deeper than mint and dive under a shallow board |
| Barrier height above soil | 2 inches (5 cm) proud of the surface | Level with the soil and stems simply creep over the top |
| Joints | Overlapped and sealed, no butt joints | A rhizome finds a gap of a quarter inch and passes through |
| Enclosed area | Fully surrounded, no open side | One open side means the barrier does nothing at all |
| Annual check | Walk the outside every spring | Escapes are easiest to trace in spring when shoots are small |
A bottomless pot sunk into the ground is not a barrier for fish mint. The rhizomes go deeper than most sunk pots and come out underneath within two or three seasons.
Position, soil and getting it established
The one genuine virtue of this plant is that it thrives in wet shade where very little else will grow. Damp ground under a downpipe, the north side of a shed, the shaded edge of a pond: those are the places where it earns its keep, and they are also the places where it grows most strongly.
It wants soil that stays moist, mildly acid to neutral, and shade or dappled light for most of the day. In full sun the leaves scorch at the margins by midsummer and the coloring in variegated forms bleaches. It is not fussy about fertility and grows perfectly well in ordinary all-purpose potting mix.
- Plant when the soil is workable Early spring, as shoots are just breaking, or early autumn. Dormant rhizome pieces establish easily at soil temperatures above about 50F (10C), and there is no need to wait for warm weather.
- Set the rhizome shallow Two inches (5 cm) deep, laid horizontally with the buds facing up. It will find its own depth within weeks whatever you do, so shallow planting simply gets it growing sooner.
- Water it in and then leave it Soak thoroughly at planting and keep it damp for the first month. After that it needs no attention to establish, and a plant that looks slow above ground is usually running hard below it.
- Mark the position and write down the date Note the planting date and the exact position of the container or barrier. When shoots appear somewhere unexpected in two years, that record tells you at once whether you have an escape or a seedling.
The smell, and the neighbors
Crushed leaves and cut stems release a strong, oily, distinctly fishy smell that carries several feet on a still day. Opinion on it divides sharply: some people find it appetising and some find it genuinely unpleasant, and neither reaction changes with familiarity.
This matters for placement. Do not put it beside a path where people brush past, next to a seating area, or under a window that gets opened. Mowing near it, strimming it or weeding around it releases the scent far more strongly than the intact plant does, and it stays on gloves and tools.
Before planting, crush a leaf and let everyone who uses the garden smell it. It is a five second test that prevents a decision that is difficult to undo.