Why small ponds turn green so quickly

A small pond has very little water to buffer it. It warms up fast in spring, catches whatever nutrients blow in, and holds them there. Green water and stringy blanket weed are the result: algae making the most of two things you cannot easily remove — light and nutrients. The trick is not to fight the algae with chemicals, but to make the pond a less generous home for it.

It is worth saying straight away that a little algae is normal and useful. A film of green on the stones, tadpoles grazing the surface, water fleas drifting about — that is a working wildlife pond. What you are managing is the excess, not the presence. Clear-ish water and plenty of life is the goal; an empty, sterile aquarium is not.

Barley straw: the slow, gentle inhibitor

Barley straw works by releasing compounds as it decays in oxygenated water, and these gently inhibit new algae growth. It does not kill existing algae, so it is a preventive, not a rescue. Used in the right place at the right time, it takes the edge off both green water and blanket weed.

• The rate: roughly 50g of loose barley straw per square metre of pond surface area. Double it for a badly affected pond.
• The method: pack it loosely into a mesh bag with a stone or two, and suspend it just below the surface in a sunny, well-oxygenated spot, not on the bottom where it will rot.
• The timing: put it in when the water reaches about 10°C in spring, before the algae gets going. It takes two to four weeks to start working and lasts around six months. Remove the spent bag in late summer and replace it the following spring.

Barley straw will not do much in cold, stagnant, airless water. Pair it with the plants and aeration below and it behaves far better.

Oxygenating and floating plants do the heavy lifting

Plants compete with algae for the same dissolved nutrients and release oxygen as they grow. Oxygen is what keeps the whole system sweet and helps your barley straw break down properly. Aim for a good mix of layers.

• Submerged oxygenators: hornwort, water milfoil, water starwort and common water crowfoot. Plant a few bunches per square metre and let them romp. Thin them in summer if they fill the pond.
• Floating leaves: a dwarf water lily, frogbit or water soldier. Let floating cover shade a third to a half of the surface — shade is your best algae control.
• Marginals: marsh marigold, purple loosestrife, water forget-me-not. Plant them in baskets of low-nutrient aquatic compost, topped with gravel. Never ordinary potting compost or rich garden soil; that is simply algae food.

Feed the pond, not the algae

Most stubborn green water in small ponds is caused by too many nutrients arriving from something the gardener controls.

• Fish food: feed only what the fish clear in two minutes, once a day in summer, less in spring and autumn, and nothing at all once the water drops below 10°C. Net out anything uneaten.
• Stocking: a small pond suits a few small native fish, or none at all. It cannot carry koi or a crowd of goldfish, however tempting.
• Leaves: net fallen leaves out promptly in autumn. Left to sink, they rot down into a rich soup that feeds algae all winter.
• Runoff: keep lawn fertiliser, weedkiller and compost heaps well away from the pond edge, and leave a buffer of rough grass to filter what runs in.
• Tap water: top-ups bring in nitrates and phosphates. Use rainwater from a butt wherever you can.

A simple seasonal routine

An hour or two of pond care spread across the year keeps things clear without a single chemical.

• Spring: divide overgrown plants, start the barley straw, top up with rainwater, and twirl blanket weed out with a cane before it takes hold.
• Summer: keep the water level up, thin floating plants so half the surface stays open, and feed fish sparingly.
• Autumn: net leaves, trim excess growth but leave some stems standing for overwintering insects, and take out the spent straw bag.
• Winter: stop feeding, keep ice clear with a floating ball or a gentle air pump, and never smash the ice — the shock harms everything below.

When to leave well alone

Please resist the urge to drain the pond in spring and scrub it clean. That wipes out the whole community of invertebrates, frogspawn, newt eggs and dragonfly larvae that makes a pond worth having. It also creates a fresh burst of nutrients that guaranteed green water within weeks.

Skip the algaecides, dyes and miracle clarifiers. If the water still clouds, the answer is almost always the same: more plants, less nutrient, a little more shade, more oxygen. Give any change a fortnight before judging it, and let the pond find its own balance. A slightly green, gloriously busy pond is a far better thing than a bare one.