Blanket Weed Is a Symptom, Not the Problem

Few things frustrate wildlife pond owners more than the arrival of blanket weed. One week the water is clear, the next it is full of long green strands that tangle around your fingers, smother the hornwort and turn the whole pond into something resembling pea soup. It is worth saying at the outset that blanket weed is not really a disease of the pond. It is a symptom of a pond that has more nutrients than its plants can use.

In the UK, the worst period runs roughly from March to June. Light levels rise quickly in spring, but the pond plants are still waking up. Filamentous algae are opportunists; they grab that light and the available dissolved nutrients before the slower-growing higher plants get going. Frog and newt spawn, insect larvae and the casual observer all suffer, and the temptation to reach for a bottle of algaecide is strong. Please don't. Chemicals knock back everything in the pond, dead algae sink and rot, oxygen crashes, and you end up with a worse problem within weeks.

Where the Nutrients Come From

Phosphates and nitrates are the fuel. If you can reduce the supply, you can reduce the algae. Common sources around a UK garden pond include:

• Fertiliser runoff from lawns, beds and container plants, especially after heavy rain
• Fallen leaves, grass clippings and dead plant material rotting in the water
• Fish food, duck food and bread thrown in by well-meaning visitors
• Soil washed in from bare banks or from a compost heap sited too close
• Tap water used for top-ups, which carries nitrates and phosphates
• Overstocking with fish, which is the single quickest way to overload a small pond

A tiny, heavily fed pond with no plants and a dozen goldfish will produce blanket weed no matter what you do. A planted, unfed pond with a rough grassy margin will almost always settle down.

Cutting the Supply at Source

Start with the garden around the pond. Leave an unmown buffer strip of rough grass and wildflowers, ideally 30 to 60cm wide, between any treated lawn and the water. That strip filters out a surprising amount of runoff. If you must feed the lawn, do it well away from the pond and never before forecast rain.

In autumn, net out fallen leaves regularly rather than letting them sink and decay. A handful of leaves in a pond is fine and feeds the invertebrate life; a thick carpet on the bottom is a nutrient reservoir that will fuel next spring's algae. Use rainwater from a butt for top-ups wherever possible, and if you use tap water, let it stand and add it sparingly. Do not feed fish or ducks at all in a wildlife pond. Fish are not required, and the wildlife will look after itself.

Plant for Competition

Plants are the cheapest, most reliable algae control you will ever have. They take up dissolved nutrients directly and shade the water surface, denying algae the light they need. Aim for roughly 30 to 50 per cent surface cover by midsummer and at least a third of the pond planted with submerged oxygenators.

• Rigid hornwort (Ceratophyllum demersum) and spiked water-milfoil grow fast and soak up nutrients
• Common water-starwort (Callitriche) is excellent for early spring, when it gets ahead of the algae
• Water crowfoot provides both submerged growth and pretty white flowers
• Marginals such as water forget-me-not, marsh marigold and purple loosestrife compete for nutrients and give cover for amphibians
• Floating plants like frogbit and a native water lily cast the shade that keeps algae in check

Plant in baskets of loam or use a low-nutrient aquatic compost. Ordinary garden compost is far too rich and will simply feed the problem.

Removing Blanket Weed by Hand

Hand removal is tedious but effective, and it is by far the kindest option for the creatures living in the pond. Twirl the strands around a bamboo cane, a garden fork or a gloved hand, lifting slowly so you do not scoop up newt larvae and damselfly nymphs along with it.

Work little and often, maybe every week or two through spring, rather than waiting for a solid mat. Leave everything you pull out on the pond edge for a day, so that any invertebrates can crawl back into the water. Then move it well away from the pond, either to the compost heap or as a mulch on a border, where the nutrients it contains will feed your plants rather than your algae.

Do not strip the pond bare. Some algae is habitat, and tadpoles graze on it. Removing too much at once releases nutrients back into the water and can cause a sudden oxygen swing.

Patience, and a Little Barley Straw

Barley straw is a traditional UK remedy and many pond keepers swear by it. The evidence is mixed, and it works best as a preventative measure added in early spring, before the algae gets going. It only works in oxygen-rich water, so if it is rotting anaerobically it will make matters worse. Treat it as one small tool rather than a cure.

The real answer is time. A new pond often takes two or three seasons to find its balance, as plants establish and the invertebrate community builds up. Feed the pond less, shade it more, plant it generously, and pull the weed by hand while you wait. By year three, most wildlife ponds quietly sort themselves out, and the blanket weed becomes a minor spring nuisance rather than an annual emergency.