Why green water happens (and why it isn't the enemy)

Come late May, a pond that looked clear in April can turn the colour of pea soup almost overnight. That flush of single-celled algae is less a sign that something has gone wrong and more a sign that the pond is young, warm and well fed. Algae are the quickest things in the water to respond to light and dissolved nutrients, particularly phosphate and nitrate, and they get going long before your lilies have grown enough leaf to shade them out.

It helps to remember that a little suspended algae feeds tadpoles, newts and water fleas, so total clarity is not really the goal. What you want is a pond where you can see a frog's hind legs at ten inches, not one where you can see nothing at all. Green water is a symptom of an imbalance, and the fix is to correct the imbalance rather than to attack the algae.

Barley straw: the old remedy, used properly

Barley straw has been used in British ponds for decades and it does work, though rarely as dramatically as the folklore suggests. As the straw breaks down in oxygen-rich water it releases compounds that inhibit new algal growth. It is most reliable against filamentous algae — blanket weed — and rather more variable against suspended green water, so treat it as one part of a plan rather than a cure.

• Dose at roughly 25–50g of straw per square metre of pond surface. More is not better; too much can strip oxygen as it rots.
• Bag it loosely in a mesh net so it floats just below the surface, where oxygen is available for the decomposition that makes it work.
• Add it in spring, once the water is above about 10°C but before the bloom starts. It takes four to six weeks to become active.
• Replace it roughly every six months. Straw that has gone black and slimy is spent.
• It works poorly in very cold, very acidic or stagnant water. If your pond has no oxygen, fix that first.

Shade the surface with native plants

The most elegant long-term answer is simply to deny the algae light. Aim to have floating leaves covering somewhere between a third and a half of the surface by midsummer. In a larger pond, native white water lily (Nymphaea alba) or yellow water lily (Nuphar lutea) will do the job beautifully if grown in baskets to keep them in bounds.

Around the margins, plant generously with natives that also take up nutrients: yellow flag iris, purple loosestrife, marsh marigold, water mint, water forget-me-not and flowering rush. Below the surface, hornwort, spiked water milfoil and water starwort compete directly with algae for dissolved nutrients.

• Never plant floating pennywort, Australian swamp stonecrop, parrot's feather or water fern. These are invasive and banned from sale in the UK.
• Don't let submerged plants fill the whole pond. They can draw oxygen down overnight, and a mix of open water and cover suits wildlife better.
• Plant in aquatic baskets of loam topped with grit, never in ordinary compost, which leaches nutrients straight into the water.

Starve the algae: nutrient control that actually works

Plants and straw will only get you so far if the pond is being fed. Algae cannot grow without nutrients, and in most gardens the nutrients are arriving from somewhere obvious.

• Top up from a water butt rather than the tap. Tap water carries phosphate and nitrate, and a summer of topping up can fuel a summer of algae.
• Keep lawn fertiliser, weedkiller and mulch well back from the edge. A planted margin a metre wide is both a wildlife corridor and a buffer strip.
• Net falling leaves in October and November. A single autumn's leaf fall, left to rot in the bottom, can feed the following summer's bloom.
• Rake out dying foliage and excess blanket weed rather than letting it decay. Leave what you lift on the pond edge overnight so snails, beetles and newt larvae can crawl back in.
• Go easy on fish, or skip them altogether. Fish food and fish waste are among the most potent fertilisers you can put in a pond.
• Avoid algaecides and chemical "pond clear" products. They knock back the very invertebrates that graze algae, and dead algae decay into yet more nutrients.

Grazers and the seasonal tidy-up

A healthy pond has a workforce. Native pond snails, freshwater shrimp and water fleas graze algae continuously, and in turn feed newts, frogs and dragonfly larvae. Encourage them by leaving plenty of stone, log and leaf litter habitat, and by never introducing fish, which will eat the lot.

Every few years, in late September or early October before frogs and newts settle down for winter, lift a bucketful or two of silt from the deepest part of the pond. This is where nutrients quietly accumulate, and removing a little slows the whole process. Leave the silt on the bank for a day so anything caught up in it can return to the water.

Patience over potions

New ponds take two or three seasons to find their balance. Expect a vigorous algal flush in the first spring and often the second, followed by clearer water as plants establish and grazing invertebrates multiply. Each year, the clear period tends to arrive earlier and last longer.

A pond that goes green in April and clears by July is not failing. It is working, in its own unruly way. Keep the nutrients out, put the right plants in, add barley straw at the right moment, and let the creatures beneath the surface do the rest.