Evaporative Cooling from Potting Media

The Issue with Evaporative Cooling

One of the biggest issues folks face with s/h culture is growing their plants too cool. Phalaenopsis, for example, can be particularly sensitive to cold, wet roots. Room temperature alone isn’t the whole picture, as open, airy media can lead to a lot of evaporative cooling, especially in dry, winter air.

Living in southeastern NC, that is not a particular issue for me, as our RH stays pretty solid year round. Over the last couple of days, however, we saw interior RH of 35%, so I though I’d run an experiment.

The Evaporative Cooling Experiment

I used three 3.5″ pots. Two filled with LECA, the other with 50/50 LECA and rock wool cubes. One of the LECA pots was set aside dry, to be my “control”. The other two pots were placed in a tub of tepid water for several hours to become saturated, then allowed to drain and equilibrate with household temperature.

After sitting around for a half of a day, I stuck a digital thermometer probe into the middle of each pot. As a baseline, the ambient conditions in the room were 74.1°F and 35% RH, with a ceiling fan on low, gently moving air around the room. Here are the results:

So… With the room temperature at a very comfortable 74°, evaporative cooling from the media drove the pot temperature more than three degrees lower. If the air temperature – and the pot – were even cooler, the evaporation would chill the potting medium even more. (I think the reason the 50/50 mix was warmer is because there were a lot of saturated cubes on the surface, sacrificing their water before the “core” had to. In the photo, you can see that some of the surface LECA pellets are already drying out.)

The Typical Scenario

To save energy, we push the thermostat down at night, allowing the temperature to drop to 62°. That means the potting medium and the roots it supports could be several degrees cooler, maybe even down into the 50’s. A cattleya or oncidium is not likely to have any issues with that, but a phalaenopsis – particularly those that prefer “hot” conditions, like Phal. bellina or Phal. tetraspis our hybrids made with them – will suffer in those cool, damp conditions.

The Solution

The obvious and best solution to evaporative cooling is to raise the humidity, as that lowers the evaporation driving force. That may not be practical however, so we might want to consider barriers to evaporation instead.

A commonly-used barrier is a layer of wet sphagnum moss on top of the medium. Like the layer of rock wool cubes in my experiment above, it provides the “sacrificial” moisture, allowing the medium below to avoid evaporative cooling and remain moist. Personally, I’m not a fan of sphagnum, as it can crumble, infiltrate the medium and suffocate the roots. I believe a layer of rock wool is a better choice, as it won’t decompose.

Another alternative is a semi-rigid, plastic barrier, as described in my Evaporation Reducing Barrier article.