[ed seeley]

You can get heater cables that are designed to snake along a bench in a deep trough that is then filled with material you can keep wet like Hortag (Baked clay granules) that will keep the humidity very high. Beware of increasing your humidity too much in winter though as it will encourage moulds if your climate is anything like mine!

I tend to get around the problem by using water trays with hortag in for the plants that like more humid conditions and keep those trays damp. The plants in the rest of the greenhouse stay drier then.