This is a very popular question in late winter/early spring. Depending on the size of your greenhouse we have a few options for you.
The TEMPERATUREALERTWRKIT is a good solution for small operations where the owner/operators live on-site. Simply place the Temperature Alert Transmitter inside your greenhouse and set your high and low alarm points. If the temperature goes into an alarm state the sensor will send a signal inside to a chime which will continue to sound until the temperature goes back into an OK range.
If your greenhouse has WiFi our TEMPSPOT is another easy to use temperature sensor that will email and/or text message you at any time with the temperature alert.
We have several other solutions such as the Sensaphone WSG-30 for large scale operations. For more information check out our Greenhouse Alarm pages full of good ideas.
Canadian Greenhouse Alarms – http://www.absoluteautomation.ca/greenhouse-temperature-alarms-s/1856.htm
USA Greenhouse Alarms – http://www.absoluteautomation.com/applications/greenhouse/index.html