Blanket weed — the long, silky strands of green that tangle around your oxygenators each summer — is not really a weed at all. It is a colony of filamentous algae, and in a wildlife pond it is a symptom rather than a cause. Where light, warmth and dissolved nutrients meet, algae grow faster than anything else can keep up with. Plenty of ponds tick along happily with a thin haze of green on the stones; it is only when blanket weed forms dense matted ropes and smothers everything beneath it that action is needed.
The two nutrients that matter most are phosphate and nitrate, the same ones found in garden fertiliser. They arrive by way of run-off from beds and borders, soil washed in by heavy rain, rotting leaves, fish food, and even the tap water many of us use for topping up. Cut those inputs and you have already won most of the battle.
Start at the edges of the pond, because that is where the trouble usually begins.
Plants are the quiet workhorses of a healthy pond. Submerged oxygenators compete with algae for the very same nutrients while sheltering tadpoles, newts and invertebrates. Aim for a generous mix: rigid hornwort, water starwort, spiked water milfoil and water crowfoot all do well in British ponds. Do avoid the thugs — New Zealand pygmyweed and parrot's feather spread alarmingly and are illegal to plant in the wild.
Floating leaves shade the water and keep it cool, which slows algal growth. Water lilies, frogbit and broad-leaved pondweed are all good choices. Aim for roughly 50 to 60 per cent surface shade by midsummer, but always leave some open water — dragonflies need it for patrols and birds need it for bathing.
Marginals such as yellow flag iris, purple loosestrife, water mint and marsh marigold pull nutrients from the water at the edges, and their dense roots hide froglets and damselfly nymphs. Plant in baskets of low-nutrient loam rather than rich compost, and top with gravel to stop the soil drifting.
The kindest tool is a bamboo cane or a rough stick about a metre long. Lower it into the water and twirl — the strands wrap around it like candyfloss on a stick. Lift, unwrap, and repeat. A spring-tine rake works well on larger ponds, provided you go gently and do not drag it through the silt.
Whatever you pull out, leave it on the pond edge for a day so that water boatmen, damselfly larvae, snails and the occasional newt can crawl back in. Then move it well away from the bank to compost, or the nutrients will simply leach back into the water at the first rainfall.
Work little and often from early spring rather than waiting for a crisis in July. A short session every fortnight keeps on top of things without disturbing the pond's inhabitants. Do not strip it bare, either — a little blanket weed is habitat, and frog tadpoles graze on it happily. The aim is to break up the dense mats, not to achieve sterile clear water.
Barley straw has been used in British ponds for generations. Sink a loose bundle in a mesh bag in spring, once the water reaches about 10°C, allowing roughly one bundle per 1,000 litres. It takes four to six weeks to become active, and it does not kill existing algae — it simply inhibits new growth. Remember that decomposing straw consumes oxygen, so float it near a pump or in a well-oxygenated spot, and lift it out after six months.
What you should not reach for is algicide. Chemical treatments and blue dyes kill algae suddenly, and the resulting decay strips oxygen from the water overnight, harming newts, frogs, dragonfly larvae and any fish. They also leave you with the same nutrient load and the same problem a fortnight later.
In spring, twirl out the first strands as soon as you see them, divide overgrown lilies and place your barley straw. Through summer, skim the surface, top up with rainwater during dry spells and enjoy the dragonflies. In autumn, net falling leaves religiously and cut back dead marginal growth above the waterline, leaving the stems below as overwintering cover. In winter, do almost nothing — a cold pond needs its silt, its dormant larvae and its stillness.
A new pond takes two or three years to find its balance, and it will always wobble a little in a warm, bright June. Stick with the routine, keep the nutrients out, and the blanket weed will gradually settle into the background where it belongs. The goal is never a sterile pond, but a working one — buzzing, croaking and full of life.
Sedge Notes shares practical, down-to-earth guidance on british wetland gardening and wildlife for readers across the UK.
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