Multiple low cost low voltage temperature monitoring devices are great. See 2nd page of this download. I searched high and low for good insurance and freeze prevention for myself. Mine has worked for years as one web-600 monitors records 6 areas in the winter, then in other seasons I monitor other things not related to freezups. Mine alert my cellphone when one is below any range, but yours can  be set to control whatever heater needs to be turned on instead of an alert.

Another form of heat control that I have used in recent years to save a temporary above ground sewer line from freezing is a propane water heater. I set it outside with exposed 1/2" pex piping of 100 feet with a mist jet of water that emitted all Alaska winter long at the far end. In your case you would just loop and return the end (closed loop hydronic heating) back to the water heater using a light duty pump. 

I have been involved for years setting up system controls in Alaska, and I love plants. I would think a system like it, even though I have not done greenhouses, should have application with web-600 or closed loop anywhere. I could even program mine to your secifications one time to get you started by submitting over the internet your sequence or method you wish to impliment then even your new login credentials will be there.

For the closed loop I would go 3/4" for longer than a 100' loop and purchase the pex at Furguson because a slippery interior layer is important when it comes to the fabrication of pex piping used for long looped systems.

A forced air system such as unit heaters treat the whole greenhouse into a large comfort zone instead of each flat of plants, where they want it. And now made possible with the advantage of better manufacturing methods while at the same time more a contol of heat distribution.

Link:
pex heaters