Water mint plant: where to grow it, care and insects

Water mint (Mentha aquatica) is a native mint plant for wet soil and the shallow edge of a pond. From July to November, round lilac flower heads stand above the leaves. A crushed leaf smells of peppermint. Left to itself, the plant spreads through its runners into a wide mat.

Water mint at a glance

Leaf

April to November

Flower

July to November

Fruit

no noticeable fruit

JanFebMarAprMayJunJulAugSepOctNovDec

How do you recognise water mint?

The stem is square, hairy and often reddish purple. The leaves grow in pairs, each pair set crosswise on the pair below. Each leaf is oval to oblong, with a toothed or scalloped edge and a short stalk. The leaves are often tinged purple, and a crushed leaf smells strongly of peppermint.

The tiny flowers are reddish lilac. They sit tightly packed in whorls around the stem. The top whorls together form one small ball at the tip. Lower down there is often one more separate whorl. Four stamens stick out of each flower, which makes the ball look fluffy. After flowering the plant sets tiny seeds, which can still germinate after more than five years.

Corn mint (Mentha arvensis) looks very similar. Check the top of the stem: corn mint ends in a few leaves without flowers. Water mint ends in a round flower head.

Where does water mint grow best?

Water mint flowers most freely in full sun: more than six hours of direct sun on a midsummer day. With three to six hours of direct sun, the plant still grows. In the wild, water mint grows along watersides and in marshes, reed beds and wet grassland. Heavy soil, clay or loam, suits it well.

Places where water mint will not do well:

• A sandy border that dries out in summer. The stems go limp, and in dry soil water mint is more likely to get mint rust or powdery mildew.
• A small pond without a basket. Long runners grow above and below ground, and they soon take over the whole bank.

In autumn the plant dies back above ground. Left uncut, the clump is only dry, brown stems with dead flower heads in January.

AugustJanuaryThe same pond edge in August and in January. This is a generated image, not a photograph.

On a plan of your plot drawn to scale, with the buildings around it, you draw in your pond yourself. On that plan you can measure whether there is a metre of open space along the bank.

Planting and caring for water mint

Nurseries sell water mint all year as a young plant in a small nursery pot. Move it into a pond basket, which keeps the runners together. The basket goes in shallow water, usually 0 to 20 centimetres deep. Plant a piece of an existing clump in spring, right after dividing.

Nurseries allow 5 to 9 plants per square metre. On planting day that looks like a lot of open ground, because a young plant is only a few stems. The runners fill the gaps on their own. In or beside a pond there is no need to water. In a wet border, keep the soil moist all summer.

Birds breed mainly from March through July. So before you dig up a clump in spring, check for a nest in or beside it.

What goes wrong with water mint?

There are orange, dusty spots on the leaves. That is mint rust, a fungus. Later the spots turn yellow or black and the leaves drop. Cut away the diseased stems before the black spores appear. A spot that is too dry raises the risk of mint rust.

There is a white coating on the leaves. That is powdery mildew, another fungus that strikes mainly in dry spells. Keep the base of the plant wet.

The stems hang limp. The soil is too dry. Move the plant closer to the water, or into the pond itself.

Few flowers appear. Most likely the plant gets too little sun. In spring, move the clump to a sunnier spot.

The clump pushes other marginal plants aside. The runners do that. In spring, dig out part of the clump, or put the plant in a basket after all.

Which insects and birds use water mint?

The caterpillar of the mint moth (Pyrausta aurata) feeds on water mint and on other mints. This small, very common moth turns up mostly in gardens. In August the second generation is on the wing.

The caterpillar of the water ermine (Spilosoma urticae) also eats water mint. This moth lives in marshes, in wet grassland and along banks. The gold spot (Plusia festucae), a moth that flies from dusk, visits the flowers.

With fish in the pond, a dense edge of water mint helps against the heron. The growth makes it hard for the bird to reach the water.

Is water mint poisonous or edible?

No poisonings from water mint are known. Mints in general do carry a warning: large amounts of some mints, especially as concentrated oil, can cause a miscarriage.

A herbal tea is made from the leaves. As a flavouring in food, many people find them too sharp.