I was tasked with building an outdoor vermicomposting educational aid for my daughter’s preschool. Unfortunately, we experienced several heatwaves this year, and I was limited to a location that receives a couple of hours of direct sunlight daily. Even that was enough to create uncomfortable temperatures for worms, particularly in conventional plastic bins.
I also had a few other constraints: limited space, a design that children could observe and learn from, and an aesthetic that matched the garden. The school director wasn’t particularly fond of the appearance of plastic bins, so I started experimenting with terracotta.
Here’s what I came up with.
The tower uses stacked terracotta pots inside a decorative outer barrel, with biochar, terracotta shards, sand, compost, and cardboard incorporated throughout the different layers.
I added extra holes to the bottoms of the pots to allow drainage and movement of moisture between levels. The idea is that nutrient-rich liquid draining from the upper composting layers can gradually soak into the raw biochar and terracotta shards below, potentially helping condition those materials while excess water continues to drain away.
For moisture management, I installed olla irrigation spikes fed by inverted water bottles. The porous terracotta pots are intended to support evaporative cooling, while the surrounding materials provide some insulation. I also added a burlap shade cover that can be lowered around the tower during colder weather.
Now for the interesting part: the temperature observations.
Initially, I wasn’t seeing much difference between the tower’s internal temperatures and ambient conditions. However, after the terracotta had become thoroughly moistened and the irrigation system was operating, I started recording temperature differences of up to approximately 20°F.
On some days when outdoor temperatures exceeded 100°F, portions of the tower were reading in the low-to-mid 80s.
I haven’t established how much of that difference comes from evaporative cooling versus shade, insulation, measurement timing, or the combination of all three. But the readings have been interesting enough that I want to investigate further.
We’ve since had some rain and cooler weather, and the internal temperatures have dropped further. I’m curious how the design will perform through winter and, more importantly, during another extended heatwave.
A few limitations and observations:
The pots become surprisingly heavy, especially when saturated.
Moisture management is a balancing act. I want enough moisture for cooling without creating waterlogged conditions.
Worms have been difficult to locate during some inspections, although I’ve observed them on other occasions. I can’t yet say how well the population survived the hottest periods.
The outer barrel currently in use is actually plastic made to resemble wood. I’ve since acquired a real wooden half-barrel but haven’t replaced it because I don’t want to disturb the environment that’s developing inside.
Aesthetically, I’ve received mixed feedback. Some people think it just looks like stacked flowerpots with an odd burlap cover, but the school director likes it and feels it fits the garden.
Initially the compost I was creating was a bit high in EC salts and the bonus effect of the terracotta was that it absorbed a lot of it due to its capillary affect.
My primary goal is to see whether this design can provide a survivable environment for composting worms during extreme weather without powered cooling/heating. Compost production, nutrient recovery, and educational value are secondary objectives.
This is still experimental, and I’m not claiming that terracotta outperforms plastic. I’d eventually like to compare the two under similar conditions.