Temperature, humidity levels, air circulation and light are all important to a healthy greenhouse environment. You'll find many tools and supplies to help you monitor and control your greenhouse climate at greenhouse manufacturers and garden centres.
Wall-mountable or tabletop battery operated digital thermometers might also indicate highs and lows and their time of occurrence. Having a minimum/maximum monitoring feature on your greenhouse thermometer helps monitor what has happened while you've been away, and will help you make adjustments to your climate controls. There are also greenhouse thermometers that have a humidity indicator, measuring relative humidity, which is one key to good plant health.
A digital indoor/outdoor thermometer will cost as little as $10, while a combination thermometer and humidity indicator will cost about $60. A soil thermometer for checking temperature of propagator seed beds and compost is $5.
For more information about greenhouse heaters, check out the shopping links at the top of this page.