Blame it on the algae

Don't blame it on the sunshine, don't blame it on the moonlight, don't blame it on the good times...

Yes, now that the Olympic swimming pool has turned green, I think it's time to turn the spotlight on algae.

In fact, it is one of the questions I get asked about most frequently: How do you stop a pond going all scummy with the stuff?

The answer, I'm glad to say, is not what they're having to do at the Olympics, where I believe they have been pumping in a supercharge of chlorine. But then few of us are likely to need to do a triple somersault with double twist into our ponds.

So what can you do to combat algae?

Understanding your enemy is a good place to start. There are lots of different types of algae - some types are microscopic but suspended en masse in the water they cause it to discolour brown or green; some form long threads which tangle together and cause the dreaded blanketweed; some cause a kind of bubbly scum on the water surface. One even causes birdbaths to turn red.

What algae loves is lots of nutrients in warm, sunny water where there are few other plants. And what the algae can then do is rob the pond of oxygen, making it difficult for other plants to survive.

I always warn that it is worst in new ponds, where aquatic plants haven't had time to establish and where nature hasn't found a balance. Here is the blanketweed in my new pond this week.

However, algae can strike established ponds too.

So, to reduce the risk of algae:

• Get your pond planted up so a third or so of the water surface is covered by aquatics, providing shade to the pond
• Try to fill and refill the pond only with rainwater. Tapwater tends to be high in nitrates and phosphates.
• Fish out algae where you can. Blanketweed can be wound around a stick and hoicked out.
• Don't introduce fish. Their excrement and fish food just add nutrients to a pond.
• Add pond snails - they can be efficient munchers of algae.
• Move the water about - I use solar fountains.
• Don't use normal soil or compost ANYWHERE in ponds - put a layer of washed gravel if you want to cover the liner, and plant up aquatic plants in baskets using a special peat-free aquatic (low nutrient) compost
• Don't allow run-off into the pond from fertile areas of the garden.

And this talk about tapwater being nutrient rich? Just take a look at this. This is me testing the waters this morning - the tapwater I drink on the left, my pond water on the right.

And here are the results. Check out the pink square at the top of each strip. On the left strip, deep pink shows lots of nitrates in tap water; on the right strip, hardly any nitrates at all in my pond water.

I don't use tapwater nor soil in my pond, so my algae, I believe, is due to it being a new pond in a sunny position with as yet not enough aquatic plant growth to shade the water or snails and other algae-munchers to combat it.

And as for 'don't blame it on the sunlight'? Well, as we've seen that isn't strictly true!