Why marginals earn their place

Marginal plants do the heavy lifting in any wildlife-friendly water garden. They soften the hard line between water and bank, hide pond liner edges, filter run-off before it reaches open water and give frogs, newts, damselflies and beetles somewhere to shelter, hunt and climb out. In a brook-side garden they hold soil together and slow the flow during winter spates. Get the planting right and the pond looks settled within a season; get it wrong and you will spend your summers pulling out the wrong thug from the right place.

The trick is understanding that "marginal" covers a wide range of conditions. Some plants want their crown sitting in 10–15cm of shallow water. Others want roots that never dry out but a crown that stays above the surface. A few thrive in genuinely boggy soil that would drown a lavender. Match the plant to the spot and most of these will look after themselves.

Match the plant to the depth, not the other way round

Before buying anything, measure your planting shelf. Most UK garden ponds are built with a marginal shelf 20–30cm below the water surface, which suits a wide range of species when the plant sits in a basket with 5–15cm of water over its crown. A rough guide:

• 0–5cm of water over the crown: most irises, water forget-me-not, brooklime, creeping jenny, marsh marigold.
• 5–15cm: flowering rush, pickerel weed, lesser spearwort, water mint, bog bean.
• 15–30cm: arrowhead, water plantain, greater reedmace (only in a very large pond, and only if you want it), white water lily in deeper water beyond the shelf.
• Boggy soil, no standing water: astilbe, ligularia, rodgersia, hostas, candelabra primulas, meadowsweet.

If your shelf is deeper than 30cm, simply raise the basket on a couple of stacked bricks. If it is shallower, choose the 0–5cm group and let the taller species sit in adjacent damp soil instead.

Reliable tall marginals for structure

Tall marginals give vertical interest and a sense of enclosure around a small pond. Yellow flag iris (Iris pseudacorus) is the classic, but be warned: it seeds freely and can colonise a whole shelf. The variegated form is tidier and still flowers well. Iris laevigata and Iris ensata are better behaved and come in blues, whites and deep purples.

Flowering rush (Butomus umbellatus) is one of the loveliest, with pink umbels on stiff stems from midsummer. Pickerel weed (Pontederia cordata) offers spikes of blue into September and its leaves give excellent cover for tadpoles. For a brook-side or large pond, purple loosestrife (Lythrum salicaria) is superb for bees and butterflies, though it can self-seed widely in damp ground. In sheltered, mild gardens, Zantedeschia aethiopica looks exotic and flowers reliably if given a sunny spot and winter mulch.

Lower-growing marginals and edge fillers

These are the plants that knit the pond into the garden. Marsh marigold (Caltha palustris) gives you a burst of gold in early spring when little else is moving. Water forget-me-not (Myosotis scorpioides) flowers for months and is a magnet for small bees; brooklime (Veronica beccabunga) does a similar job with neat blue spikes. Water mint spreads agreeably and smells wonderful when bruised.

Creeping jenny (Lysimachia nummularia) will trail over stone edging and into shallow water, while Ranunculus flammula provides modest buttercup flowers and excellent spawning cover for amphibians. For a daintier touch, Sagittaria sagittifolia has arrow-shaped leaves and white flowers in midsummer.

Plants for boggy soil and brook banks

If your soil stays wet but never floods, you have more scope than you might think. Astilbes bring plumes of pink, red or white through summer and tolerate a fair amount of shade. Ligularias have dramatic, umbrella-like leaves and orange or yellow daisy flowers, though slugs love them while young. Rodgersias give bold, architectural foliage for a large bog garden.

Candelabra primulas are superb for a stream bank, seeding gently into damp ground and flowering in May and June. Meadowsweet (Filipendula ulmaria) is a native with frothy cream flowers and a honey scent that pollinators adore. Hemp agrimony (Eupatorium cannabinum) is another native worth making room for: it grows taller, flowers late and is covered in butterflies and hoverflies. Deschampsia cespitosa and Molinia caerulea add soft, tufted movement to the edges.

Planting, dividing and winter care

Plant marginals in aquatic baskets lined with hessian, using a loam-based aquatic compost rather than a fluffy multipurpose mix that will float away. Top-dress with washed gravel to stop fish and blackbirds grubbing the roots out. Late spring and early summer are ideal planting times, once the water has warmed.

Divide congested baskets every two or three years, in spring, discarding the spent centre and replanting the vigorous outer growth. Resist the urge to clear every dead stem in autumn: hollow stems and seedheads shelter overwintering insects, and the seed feeds finches. Cut back to just above the waterline in March, before new growth starts. Feed sparingly, if at all — marginals in a mature pond rarely need it, and excess nutrients encourage blanket weed and duckweed.

Finally, be honest about vigour. A plant described as "spreading" or "colonising" will do exactly that in a small pond. Plant the thugs in baskets with no gaps, or leave them out altogether. A little restraint at planting time saves a great deal of wet, cold work later on.