"...... Is it feasible to control the heat with a dimmer switch? ....."

Should work after a fashion, but would not be able to take into account
changes in the ambient air temperature caused by sunshine etc. so you would
not be able to be very accurate.
I should think that unless you are going to use a very short cable then the
extra cost of a thermostat would pay for itself in time.

What I cant understand is why a Mechanical time clock which will switch
lights(or other appliances) on or off wont work with florescent lights.



--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk