How Do Frost Cloth Covers Trap Heat?

Garden Science

The leafy greens in the raised bed are thriving under the frost cloth tent. They've been planted out about a month and I am harvesting leaves. Harvest one leaf per plant and I can decorate a sandwich. Salads will come later.

One Month's Growth of Bok Choi

I always assumed that frost cloth worked by trapping heat under it, with stored heat from the ground keeping the air temperature above freezing. But I got curious and when I had the plants uncovered to water and weed them, I put a wireless remote thermometer under the tent.

Temperature? I recorded the temperature under the cloth and from an online personal weather station a block away periodically from 2PM until about 1PM the next day.

Temperatures (F) Under Frost Cloth (Remote) vs Open Air (PWS)

The "because it traps heat" hypothesis is clearly disproven.

While the frost cover is in sunlight, the temperature under it is above the outdoor temperature by as much as 20 degrees. That drops almost immediately as soon as the fence shades the area. Overnight air temperatures under the cover are very close to the local weather station's temperatures.

Humidity? Then I repeated the measurements with a sensor that reports temperature and relative humidity.  At any time of day or night the air inside the tent was more humid than the air outside the tent.

Relative Humidity Under Frost Cloth vs Open Air

Plants like humidity. This might be the main answer, with a boost from the warmth of mid-day under the tent.

Something else? The frost cloth tent alters two other factors that are harder to measure: wind and "night sky cooling".

Plants aren't subject to "wind chill" the same way mammals are, but dry winter winds will dry out the soil and if it's cold enough, freeze-dry the foliage. The frost cloth is protecting the tender leaves from the wind and helping the soil stay moist.

Night sky cooling is probably important, but to test it I would have to remove the frost cloth on a night that will be below 20F, and I don't want to risk killing my salad greens. I've made a note in my gardening calendar for next year to plant some research subjects for this.