Marsh pennywort

How to recognise it

Habit

Low, creeping plant of 5–15 cm, with thin stems that root at every node and form mats.

Leaf

Round, 1–5 cm across, glossy green with a scalloped edge; the long stalk is attached to the centre of the leaf (peltate).

Flower

Tiny, pinkish-white to greenish, in small whorls on short stalks, usually hidden beneath the leaves, from June to August.

Fruit/Seed

Small, flat, round fruits of 1–2 mm that split in two.

Roots

Fine roots at every node of the creeping stems.

Confused with

Floating pennywort is on the EU list of invasive species: it has kidney-shaped, lobed leaves with the stalk at the edge, grows much larger and forms floating mats on the water. Navelwort also has peltate leaves, but they are thick and fleshy, and it grows on walls and rocks.

Flowering: June–August · Pruning: September

Why grow it?

• A fine, low ground cover for wet places where little else grows.
• Its mats give shelter to frogs, newts and small insects along the water’s edge.
• The small flowers are visited by tiny flies and beetles.
• A characteristic plant of valuable, nutrient-poor wetland, alongside cottongrass, cross-leaved heath and small sedges.
• The leaf with its stalk in the middle is one of the most recognisable leaf shapes in our flora: fun to discover with children.

Marsh pennywort: where does it grow best?

• In the wild: wet heathland, the edges of heathland pools, bogs, damp dune slacks, purple moor-grass meadows and poor, wet pastures.
• Sun to partial shade.
• Wet to waterlogged, nutrient-poor, acid to neutral soil; it tolerates very shallow water.
• In the garden: along the shallow edge of a pond, in a bog garden or in a damp, poor corner of a lawn. In rich soil it loses out to larger plants.

Goes well with

• Common cottongrass: White tufts in summer, in the same wet, nutrient-poor soil.
• Cross-leaved heath: Pink bells in summer on wet, acid soil.
• Marsh cinquefoil: Deep red flowers at a wet, peaty pond edge.
• Common marsh-bedstraw: Fine white flowers among the round leaves.

Marsh pennywort: when and how to plant it?

1. Buy plants from a nursery for native water and marginal plants, and check the leaf: the stalk must be in the middle.
2. Plant in April–June in wet, sandy or peaty soil at the pond edge, 20–30 cm apart.
3. You can also set it in very shallow water, on the gently sloping edge of the pond.
4. Do not use potting compost or garden compost: poor soil suits it and keeps it compact.
5. Keep the soil wet at all times; if it dries out, it dwindles.
6. Never dig up plants in nature reserves: heathland pools and bogs are fragile, protected habitats.

Care

Water

The soil must stay wet. Top up the pond with rainwater, not tap water.

Feeding

Never: fertiliser clouds the water and lets large, vigorous plants take over.

Spreading

In a damp spot it can spread widely. Simply pull out runners at the edge.

Mowing

In a damp corner of the lawn you can mow it along with the grass; it copes well.

Checking

Inspect new water plants carefully: floating pennywort can arrive as a stowaway in the pots.

Tip: Finish the edge of a new pond with poor sand and plant marsh pennywort there: it quickly forms a green fringe that softens the transition from water to land.

Pruning and mowing

Pruning months: September.

When: In September, when the mats get too wide.

How:

• Pull or cut away over-long runners at the edge.
• Leave the rest: the mats are a hiding place for frogs and insects.
• In autumn remove fallen tree leaves from the plants so they do not smother them.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Division

In April–June cut a piece of the mat with roots and replant it elsewhere in wet soil; it soon takes.

Cuttings

Lay a piece of stem with a few nodes on wet soil and pin it down; roots form at every node.

Marsh pennywort: how does it get through the winter?

Hardy to −25 °C.

• In mild winters some leaves stay green; in hard frost the parts above ground die back.
• The creeping stems overwinter in the wet soil and sprout again in spring.
• Hardy to about −25 °C.
• Small creatures overwinter under the mats along the water’s edge.

Diseases and pests

• Mix-up with floating pennywort: This invasive non-native plant chokes ponds and ditches. Buy only from a reliable nursery and check the leaf: stalk in the middle means marsh pennywort.
• Drying out: In dry summers it shrivels. Keep the pond edge wet with rainwater.
• Being overgrown: In rich soil, reed, soft rush or grass grows over it. Keep the soil poor and cut the bank now and then.
• Slugs: They sometimes eat the leaves, but usually without serious harm.

Through the year

• March: The first new leaves appear.
• April: Plant or divide at the pond edge.
• May: The mats grow; keep the soil wet.
• June: The tiny flowers appear beneath the leaves.
• July: In flower; in dry spells top up the pond with rainwater.
• August: Last flowers and small fruits.
• September: Trim back mats that have spread too far.
• November: Remove fallen leaves from the mats.
• December: Rest; partly green in mild winters.

Did you know?

• Hydrocotyle comes from the Greek hydor, water, and kotyle, a small cup: after the round, saucer-shaped leaves in wet places.
• It used to be placed in the carrot family; DNA research showed that it is more closely related to ivy, in the ivy family.
• In England it was once blamed for disease in sheep grazing wet pastures, hence old names such as ‘white rot’; the real cause was liver fluke, a parasite of the same wet places.