Now that many people will be thinking about digging ponds, it’s time to start looking more closely at design advice.
It’s commonly suggested that wildlife ponds should be at least 60 cm (about 2 feet) deep. The reason usually put forward will be something or other to do with protecting animals from cold weather (or sometimes warm). Here are some typical quotes you can find on the Internet, including some very recently published:
– ‘You need at least 60 cm depth for frogs to hibernate successfully, and ideally 90 cm or more.’
– ‘A pond should ideally have a section at least 60cm deep to protect certain animals (like frogs) from weather extremes (particularly in winter).’
– ‘Depth profile is important. The deepest point should be at least 75cm, this will allow hibernating amphibians and invertebrates to survive the coldest winters when the pond is frozen over.’
But is any of this true?
Well, the first thing to say is that nobody has ever actually looked carefully at a variety of different garden ponds and their wildlife to know for sure. But we’ve just had the coldest weather in the UK for about 20 years which might give us some clues about the need for a pond to be, say, ¾ of a metre deep (75 cm – that’s a pretty deep pond!) for animals to survive freezing over.
In my pond, there’s little sign that the cold snap, when we had ice cover for about 2 weeks, had much effect on the dragonflies, mayflies, beetles, bugs and the like – the invertebrates. The dip yesterday showed a full complement of the beasts that were there before the cold weather. I don’t known whether frogs hibernated in the pond – but I certainly haven’t had any dead ones, and there was so much oxygen in the pond that there was no danger of frogs suffocating.
But quite a few other people have had frogs which presumably ran out of oxygen. I’m guessing these were ponds where the dissolved oxygen was much lower anyway – my neighbours Sally and Adrian, for example, have a pond where there was only just enough oxygen to keep things going under the ice. Frogs in their pond could have been in a much more precarious situation (luckily they didn’t have any casualties).
The theory is that a deeper pond is, the more ‘stable’ it is: so it won’t freeze solid, and will also have enough oxygen to tide animals over. Does this happen in practice? Well, my own pond shows that you don’t need a particularly deep pond to keep the dissolved oxygen up, or to avoid compete freezing.
In my case I suspect the shallowness of the water may have helped with the amazingly high oxygen concentrations by allowing oxygen producing algae to grow on the pond bottom. The very clear water and large amount of moss growing under the water probably also helped.
And as for freezing solid – in the south of England after the coldest weather for years there was a couple of inches of ice on the water – there was no chance the pond would freeze completely solid.
My pond is 25 cm deep in the deepest point, about the depth of a Wellington boot – this looks a pretty good to depth me.
For the size of pond this is also much more like the depth of many naturally formed pools – always a good guide to what’s the right design for wildlife.