Interesting observation.

I’m starting some seedlings under a makeshift grow light system I built, basically consisting of 2 clamp lights with 105W Daylight fluorescent CFLs in them. For the single 72-cell Jiffy tray it seems to work fine. Right now I’m starting onions, Rudbeckia, broccoli, and Echinecea seedlings.

The Jiffy tray comes with a “greenhouse” cover (a clear plastic cover) that I am using just until the seedlings emerge, then I will slowly remove it, as I’m concerned about too much humidity contributing to issues at that point.

The ambient temperature of the room is about 65F. The first day, I had a 17.5W cheapo heat mat under the tray. With the clear cover on, the lights on, and the heating mat on, the temperature inside reached 97F!!!

So I turned off the heat mat. It now reaches about 85 inside with the lights on, and slowly cools down to about 68 by morning after the lights shut off at night.

It actually seems as if the plastic cover is creating a “solar” heating effect with the CFL light/heat, is that possible? There’s zero direct sun in that room.