My Pond's Gone Green! Four Things To Know About Algae

1. Green water is normal in new ponds

The first thing to know about algae is that it is completely normal for new ponds to go green. In fact, this is more likely to happen than not.

Every year we hear from people who have recently built a pond, added a few plants, and then - about two weeks later - find that the water has turned green, or that slimy green strands have appeared. This is a normal stage in the life of a pond, and one that almost all established ponds have been through.

Green water (often described as a "pea soup" effect) is caused by microscopic, single-celled algae suspended in the water. Blanketweed, string algae or slimy algae are caused by filamentous algae, which usually attaches itself to surfaces such as pond sides or plant leaves, though it can sometimes float freely.

2. Algae doesn’t come from anywhere - it appears when conditions are right

The second thing to know is that you can't keep algae out of your pond, and you don't really "catch" it from anywhere.

People sometimes worry that algae has come in on plants they've bought. While algal cells can arrive on plants, they can also arrive on birds’ feet, on aquatic insects, or even on the wind. This isn’t the real reason ponds go green. Any new or recently disturbed body of water will develop algae once nutrients are released into the water.

These nutrients usually come from:

tap water used to fill the pond,

soil added with potted plants,

or, more rarely, disturbing sludge during cleaning.

Because algae appears soon after plants are added, it's easy to assume the plants caused it - but the timing is coincidence. Algae will appear whether plants arrive visibly clean or not.

The only way to truly exclude algae would be to disinfect the water constantly, as in a swimming pool, which is neither practical nor desirable in a garden pond.

3. Algae is mostly harmless

The third thing to know is that algae is usually harmless, and is in fact the base of the aquatic food chain.

Algae is often disliked because of how it looks, and there can be a strong urge to remove it quickly. Very heavy algal growth can occasionally cause water quality problems for fish (such as low oxygen levels or pH swings), but this is rare and can usually be avoided by circulating the water with a pump.

If your pond doesn't contain fish, algae is mainly a cosmetic issue. The most likely problem is that it may cut out light to your deep-water plants, and delay their establishing. It is very unlikely to kill them, but if the algae growth is heavy and prolonged and the plant is small and vulnerable, there is a small chance of this. If your plants seem to be really struggling for light, raising them higher in the water can help the light penetrate. Stringy or filamentous algae can usually be managed by physically removing as much as you reasonably can.

There are also a range of shop-bought algae treatments available, from barley straw to phosphate blockers. These are generally both effective and safe for plants and wildlife, provided they are applied as directed.

4. Algae usually settles down on its own

The fourth thing to know is that algae almost always reduces naturally over time.

As a pond matures, nutrients tend to leave the water column and become incorporated into the substrate and the plant material, and the algae clears. Invertebrates that feed on algae move in and it falls to the bottom in their droppings, and plant competition increases. In ponds with a good range of plants, established growth limits algae by competing for both light and nutrients.  In a pond without any plants at all, algae will persist but at a lower level. 

Provided there isn't a constant influx of new nutrients - such as from feeding fish, waterfowl droppings, ground run-off, or frequent sludge disturbance - the pond will usually settle into a stable state.

It's common for algae to bloom briefly each spring, since it grows faster than pond plants early in the year. This usually clears as the season progresses and plant growth catches up. Algae may bloom again briefly after a disturbance, such as adding plants or cleaning the pond, but this is typically much shorter lived than the initial bloom.

Managing ongoing algae problems

If your pond does receive a continual supply of nutrients (for example from feeding fish every day, waterfowl, or ground run-off), you can fight the algae by choosing faster-growing plants that will compete better with it. Thinning and removing excess plant growth is particularly effective, as composting removed foliage also removes the nutrients it contains (rather than leaving it to die off and re-release these nutrients back into the water).

If this still isn't enough, speak to your aquatic shop about additional measures such as UV units, phosphate blockers, or biological treatments.