Heating outdoor greenhouse potential issues?

temperature below ~18c is going to cause nutrient uptake issue in plant. roots? chemistry in plant? i don'tknow the details of causality, but it will turn leaves fall colors and reduce photosynthesis etc.. other nutrient deficiency symptoms in leaves etc etc... so, i'd say the heater is a great idea... if propane, it'll be a nice boost to co2 during the light hours too. may also want to get co2/co detectors? depending on how you are heating it. condensaton is quite normal for a greenhouse, so just make sure you don't incubate a bunch of microbes and you'll be fine.. i.e. cleaning it occasionally should avoid any problems.