thermal mass pond

I wanted to increase the habitat in the garden this year – the obvious way to bring in more diversity was to add a pond attracting a huge wealth of species from frogs, newts, insects, birds, and other garden wildlife.

So what’s this thermal mass thing? Thermal mass is the ability of a material to absorb and store energy in the form of heat. Materials with a high thermal mass can store a lot of energy. They take a long time to heat up, and a long time to cool down, during which they radiate heat to the surrounding air and soil, making them really good at evening out temperature changes. Water has the highest thermal mass of commonly available materials, more so than granite or heavy rock, so a pond is perfect for balancing the temperature.

The principle is incredibly effective, and is used to make passive solar homes which require no heating at all. Permaculture pioneer Sepp Holzer even grows citrus in Austria using thermal mass!

The thermal mass pond idea

Combining the benefits of the pond with a raised bed, the thermal mass of the water could be used to balance out the highs and lows of temperature for the surrounding plants. The water’s surface could reflect light back up onto the foliage, and frogs from the pond at the center of the bed would be on pest patrol for the pesky slugs. I was also hoping that the surrounding bed could be maintained as ‘no dig’ from here on.

The idea of using the thermal mass of water in a garden or greenhouse isn’t a new concept, but I’ve not seen it applied quite like this before.

I planted white clover as a nitrogen fixer and an attempt at planting through living mulch, nasturtiums as a companion plant to deal with pests, along with a mix of edible annuals.

Some of it worked well, other aspects not so much, but that’s how we learn – by experimenting, building on successes and finding ways to overcome failures.

The experiment in practice

The pond got off to a great start – a friend was getting rid of a fibreglass pond liner so that was free, and we kick-started the ecology by adding pondweed, snails and mud from my parents pond.

We removed a small pond at the front of our house when we moved in, because it was in a terrible position. Sadly the following year we found frogs outside ready to lay their eggs and wondering where it had gone. This year I found a small ball of frogspawn nestled in the water-filled depression of a manhole cover, so I rescued it, and rehomed in the pond. I also found some much smaller spawn on the lawn near the old pond. Both of these hatched and we now have both frogs and newts – not bad for year one! I spotted them frequently around the pond and the raised bed so they must have contributed to keeping the slugs at bay.

I planted all sides of the pond, and although I planned the planting, there were some plants left over which I just popped into any space. Squash planted on the north side of the pond grew towards the light and into the pond which wasn’t ideal, and yes I had planned on avoiding that, but there were a few spaces left and extra plants, so I went with it anyway.

The clover spread slower than I hoped, which left the soil bare and exposed. Coupled with the raised bed, whilst a great technique to improve drainage on heavy soils, isn’t necessary on our free draining loam, meant the soil dried out quickly and needed regular watering. Drip irrigation from a soaker hose running around the center of the raised bed provided an easy way to irrigate.

Cats and foxes decided the freshly planted soil was the perfect litter tray. Now while I’m all for the free manure, I don’t really want my salad leaves covered in it, thank you. I suspect they were also walking over the bed to get a drink for the pond too.

Lessons learned

  • Add mulch around new seedlings to ensure they don’t dry out. Missing that was a schoolboy error! In doing so I’ll be able to avoid irrigation for the large part.
  • Plan the planting to ensure each plant will thrive in its position.
  • Return to planting directly in the ground – raised beds aren’t necessary in our free draining soil.

Did the thermal mass improve the growing conditions of the plants? Possibly, but it’s hard to say as there wasn’t a comparable bed without a pond, so we can’t know for sure.

Though there were mixed results, the thermal mass pond experiment taught me a lot which is the most valuable thing. The biggest lesson has been how important it is to apply broad principles with care, and tailor them to the specific circumstances. I guess that’s the permaculture principle of designing from patterns to detail in action!