Why algae appears in the first place

Almost every wildlife pond goes green at some point, and it is rarely a sign that you have done something wrong. Algae are the opportunists of the pond world. Give them light and dissolved nutrients and they will multiply faster than anything else in the water. Blanket weed forms those familiar strands of green cotton, whilst single-celled algae turn the water itself into pea soup.

The key to managing them naturally is to stop thinking of algae as an enemy to be poisoned and start thinking of them as a symptom. If you can work out what is feeding them, you can tip the balance back in favour of the plants, invertebrates and amphibians you actually want. Chemical algicides might clear the water for a fortnight, but they also strip oxygen, harm tadpoles and leave a nutrient soup behind that the next algal bloom will happily exploit.

Get the light right

Sunlight drives photosynthesis, and a pond in full sun with no cover will always be more vulnerable. Aim for around half the surface to be shaded at midday during summer. You do not need to dig up the pond — you simply need to plant around it.

• Allow marginal plants such as iris, purple loosestrife and marsh marigold to grow tall on the southern side.
• Let a shrub or small tree cast dappled shade without dropping too much leaf litter into the water.
• Float plants such as water soldier or frogbit across part of the surface to shade the water beneath.
• Avoid moving the pond or felling trees just for shade; working with what you have is far less disruptive.

Be careful not to go too far the other way. A pond in deep shade will struggle to support flowering plants and the insects that depend on them, and the water will stay cold and slow to warm in spring.

Starve the algae, not the pond

Nutrients are the real driver. Phosphates and nitrates entering the water from soil, fertiliser, bird droppings or decaying matter will feed blanket weed relentlessly. In a wildlife pond the most common culprits are surprisingly mundane.

• Runoff: a sloping lawn above the pond will deliver nutrients every time it rains. A planted buffer strip of long grass or bog plants slows and filters that water.
• Fallen leaves: skim them out in autumn before they sink and rot. A net stretched over the pond during leaf fall saves hours of work later.
• Fish food: overfeeding goldfish or koi is one of the fastest routes to green water. If you keep fish, feed sparingly and remove what is not eaten within a few minutes.
• Duck and goose droppings: if waterfowl visit regularly, their waste adds considerable nutrients. Encouraging them to move on with a barrier of tall marginal planting usually works better than chasing them.

If your pond has accumulated years of silt, a partial autumn clean-out — removing perhaps a third of the sludge from one section — can reduce the nutrient store without destroying the habitat. Never empty a wildlife pond completely in one go.

Let plants do the work

Healthy plant growth is your best long-term defence. Submerged oxygenators such as hornwort, water milfoil and spiked water milfoil compete directly with algae for nutrients and light. Aim to have around a third of the pond volume occupied by submerged plants by midsummer.

For green water specifically, floating plants are excellent because they shade the water column and their roots take up nutrients from the surface layers, where single-celled algae thrive. Native options include frogbit and water soldier, though both need thinning so they do not blanket the entire surface. Remember that oxygenators release oxygen during the day but consume it at night, so a pond stuffed solid with them can become oxygen-poor in warm weather.

Practical interventions that respect wildlife

Sometimes you need to act quickly, especially if the water has turned opaque and you cannot see the newts. Several gentler methods are worth trying before anything more drastic.

• Barley straw: a well-established British technique. Place a mesh bag of barley straw in the pond in spring, roughly 50g per square metre of surface area. As it decomposes it releases compounds that inhibit algae without harming other life. It works slowly and needs oxygen to function, so keep it near a pump or fountain if you have one.
• Manual removal: twirl blanket weed around a rough stick or a garden fork. Do this in the morning and leave the pile on the bank for a day so any trapped invertebrates can crawl back into the water.
• Partial water changes: replacing a quarter of the volume with rainwater from a butt can dilute nutrients, but only if the replacement water is itself low in nutrients. Tap water often contains phosphates, so avoid it where possible.
• Encourage daphnia: these tiny water fleas graze on single-celled algae and are a favourite food of newt larvae and sticklebacks. A pond with plenty of cover for them will usually clear green water on its own within a few weeks.

Work with the seasons

Algal blooms in a wildlife pond are usually a spring and early summer phenomenon. As the pond matures and your plants fill out, the problem tends to ease. Expect a flush of green water in April when light levels rise but your oxygenators have not yet started growing. By June, if the balance is right, the water should be clear enough to see the bottom in the shallows.

Keep an eye on things as the season progresses. If blanket weed returns every year in the same spot, that is a clue — perhaps a hidden pocket of silt, or a trickle of runoff you have not noticed. Address the cause, be patient, and your pond will reward you with frogs, dragonflies and water that clears itself.