I got a greenhouse last summer, and I noticed that my VFT soil was hard to keep moist with a water table at around an inch/two inch depth. I currently grow my VFTs in six inch pots and was considering getting deep trays to solve this problem. Would it be a bad idea to get deep trays with a water level of around four inches high, and allow the water to evaporate between waterings? I just don't want to over water the plants (the dreaded root rot), and I'm afraid that some of my newly potted plants do not have a root system deep enough to access water only at an inch depth during summer (I don't want the soil to dry out). Thoughts?
Try the upside-down 2 liter bottle solution. Cut a hole in the bottle's neck at the level you want the water level to be at if you placed the bottle upside-down. Fill the bottle with water and place it in the catch tray upside down. The bottle will dispense water when the level in the catch tray goes below the level where the extra hole is in the bottle.
I don't think it would be a problem to put 4 inches of water into your trays assuming that it evaporates rather quickly. In the summer in Boulder, I usually put between 2 and 3 inches of water into my trays that have 4 inch pots in them and the plants do fine, however the humidity in Boulder is less than it is in Minnesota. If your trays dry out in a day or two (that's about how long it takes here in Boulder), then I'd say they should be fine.