A new variation on the ‘leaves are bad’ myth

My little animals (and bacteria and fungi) gradually skeletonise fallen leaves: this is one from the pond tonight

My little animals (and bacteria and fungi) gradually skeletonise fallen leaves: this is one from the pond tonight

Most guides to ponds say that leaves are ‘a bad thing’ and you should do your best to keep them out.

Here’s a few typical quotes:

‘During autumn, falling leaves should be skimmed off the pond surface before they sink and overload the nutrient balance of the pond.’

‘Net floating leaves before they sink in autumn’

‘Falling leaves in large numbers could clog up ponds and cause a nutrient overload in autumn.’

But is this right?

As I have written before, leaves are a natural part of the pond ecosystem providing food, shelter and case building materials for animals like the larvae of caddis flies. Leaves falling in the water is entirely natural, so you might expect plants and animals to have adpated to exploit this process over the millenia.

In fact it makes no more sense to keep out all leaves than it would do to keep all plants out. In streams and rivers, it is a commonplace amongst biologists that leaves are a good thing – in fact a vital part of the food supply for invertebrates and, therefore, also for fish. Something similar is probably true of ponds.

One of the things that people say about leaves is that they add nutrients to ponds. So, given that you want to keep nutrient levels low in ponds, doesn’t it make sense to keep leaves out?

There have never been any proper surveys which tell us the real answer to this question: no-one has carefully measured nutrient concentrations in garden ponds with and without added leaves. So in the absence of definitive information I thought I’d have a quick look around at what research is available about the nutrient content of leaves to get a rough handle on the problem.

American research shows a dead leaf falling into a pond contains about 125 micrograms (that is, 125 millionths of a gram) of phosphorus, the most important plant nutrient. Phosphorus is an essential element for plant growth – the problem is in many freshwaters it is now in huge excess because of things people do: spread fertilisers like there was no tomorrow, discharge sewage effluent into rivers, keep cows – all these things release huge quantities of phosphorus into the environment.

Assuming that all the phosphorus in the leaf that falls in a pond is available to the plants and animals (this is quite a big assumption), I estimate that if you added 100 leaves to a 750 litre pond, roughly the size of mine, this would be equivalent to a phosphorus concentration of 17 micrograms of phosphorus in each litre of water. This is at the low end of the range you see naturally in ponds and lakes – so a perfectly acceptable value.

If you added 1000 leaves, then you would theoretically have added 10 times as much phosphorus to the pond – 170 micrograms of phosphorus in each litre of water. This is getting to the level where you might start to see some unwanted effects.

In my own pond, where there’s a constant moderate leaf fall, I think there’s little sign of detrimental effects so far. But the problem is this is just one pond, and the calculations above are entirely theoretical. There have never been any careful experiments on the effects of leaves in ponds.

And at the extreme end of leaf fall – like the pond below – the effect of the leaves is obviosuly overwelming (though even this pond had one or two smooth newts in this spring, and yellow irises – but it doesn’t look very appealing!).

This school pond is directly under several trees: the branches are no more than a couple of metres above the pond

This school pond is directly under several trees: the branches are no more than a couple of metres above the pond

Yellow Irises are one of the plants that can grow in dense shade with accumulated leaves; the naturally inhabitat swampy fen woodland

Yellow Irises (you can just see them at the back of the pond in the picture above) are one of the plants that can grow in dense shade with accumulated leaves; they naturally inhabitat swampy fen woodland

So should you remove the leaves from your pond? Almost certainly not if the leaf fall is modest; but if you’re destined to get a blizzard of leaves every autumn – well, maybe then’s the time to take some action.

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