Planting marginal plants around a garden pond

Why marginal plants do more than look pretty

Marginal plants are the ones that grow in the shallows and along the damp edges of a pond: irises, rushes, marsh marigolds, water forget-me-nots and dozens more. They are the finishing touch that turns a hole full of water into something that looks as though it belongs in the garden.

Their value goes well beyond looks. Roots pushing through the shallows take up dissolved nitrogen and phosphorus, the very nutrients that feed blanketweed and green water. Dense growth shades the surface, keeping the water cooler in summer. Stems and leaves give frogs somewhere to sit, damselfly nymphs somewhere to climb before they emerge, and bees and hoverflies a reliable source of nectar. Put simply, a well-planted margin means less algae and more wildlife.

Know your pond's planting zones

Before you buy anything, work out the depths around your pond. Most gardens have three usable zones, and plants are far happier if you match them to the right one.

• Damp bank or bog garden — soil that stays reliably moist but is rarely submerged. Often the strip just above the liner edge, or a lined, gravel-topped area beside the pond.
• Shallow shelf — 0 to 15cm of water over the basket rim. This is where the widest choice of marginals thrives.
• Deeper marginal shelf — 15 to 30cm. Fewer plants cope here, but the ones that do make strong, architectural clumps.

Measure from the water surface down to the shelf, not the other way round, and measure when the pond is at its normal level rather than straight after heavy rain.

Choosing plants for each zone

Variety matters less than suitability. These are reliable, widely available choices that behave themselves in a British garden.

For the damp bank:

• Marsh marigold (Caltha palustris) — golden flowers in early spring, long before most pond plants wake up.
• Siberian iris (Iris sibirica) — slim foliage and elegant flowers in late spring.
• Creeping Jenny (Lysimachia nummularia) — low and spreading, ideal for softening a hard edge.

For the shallow shelf:

• Water forget-me-not (Myosotis scorpioides) — dainty blue flowers over a long season.
• Lesser spearwort (Ranunculus flammula) — modest in size, with buttercup-like flowers.
• Dwarf bulrush (Typha minima) — the compact alternative to the thuggish greater reedmace.
• Pickerel weed (Pontederia cordata) — handsome blue spikes in late summer.

For the deeper shelf:

• Yellow flag iris (Iris pseudacorus) — spectacular but vigorous; best kept for larger ponds.
• Flowering rush (Butomus umbellatus) — umbels of pink flowers on stiff stems.
• Soft rush (Juncus effusus) — vertical green structure that holds up through winter.

Avoid anything known to be invasive in the UK, including water fern, parrot's feather and New Zealand pygmyweed. They are difficult or impossible to remove once established, and can spread far beyond your garden.

How to plant marginals properly

Plant in late spring or early summer, once the water has warmed and new growth is well underway.

• Use proper aquatic baskets and line them with hessian if the mesh is coarse.
• Fill with heavy loam-based soil, not multipurpose compost. Compost floats, clouds the water and releases nutrients straight into the algae's favour.
• Top the soil with a 2cm layer of washed gravel to stop it washing out and to deter fish from rooting.
• Set the crown at or just above soil level. Burying it encourages rot.
• Lower baskets onto the shelf and check the rim sits at the depth the plant wants — raise it on bricks if necessary.
• Space clumps roughly 25 to 30cm apart so they have room to spread without smothering one another.

Keeping marginals in check year after year

The main job is restraint. Vigorous growers such as yellow flag iris and flowering rush will fill a basket within two or three years and start to crowd their neighbours. Lift and divide them in spring, cutting the rootball into quarters with an old bread knife and replanting the healthiest piece.

Through the growing season, remove yellowing leaves and deadhead spent flowers to keep things tidy and to stop self-seeders appearing in every corner of the pond. In autumn, cut back most foliage but leave hollow stems standing over winter — they are valuable shelter for insects, and seedheads such as those of the rushes feed finches. Never use general garden fertiliser near a pond; the run-off will feed algae for months. If plants look pale in midsummer, push a slow-release aquatic tablet into the basket instead.

Get the planting right and the margin will do most of the work for you: cleaner water, fewer algae problems, and an edge that softens the whole pond into the garden rather than stopping it dead.