It's really getting to be the time of the year when the combination of a greenhouse and overnight temperatures can lead to sleepless nights.
I have a remote, RF alarm system linked to a Sensaphone, but I wanted to be able to remotely monitor the conditions in the greenhouse too. LaCrosse Technologies now sells a system called "LaCross Alerts" that includes an RF transmitter (range 200' / 60m), and receiver that connects to your internet router. You can monitor the Temp and RH via any browser connecting to their website, and if the conditions reach user-configurable limits, it will send you a text alert.
I installed one yesterday, and I really like it. My greenhouse is about 100 feet from the house, which has aluminum siding on it, so I was very pleased to see that the measured "link quality" was between 94% & 100%.
Here is a screen shot of the LaCrosse Alerts display on their website:
The graph on the right is excellent. I have mine set up to "report in" every 5 minutes (2 is the minimum), and you can see the cycling of the heater, and the effect of the little bit of sunlight we've gotten.
Due to the wind, and branches hitting the power lines, we had a short power outage, and I received an alert telling me that no data had been sent for a period of time, so that's good too, warning of outages.
The <$100 purchase price (I carry them for $90) includes 1 year of alerts and 5 years of online monitoring. If you want alerts to continue after year one (and they will let you know that part of the subscription is running out), it's $0.99/month, or another $11.88/year - well worth it in my opinion.