Updated: Jul 24, 2019
One thing I learned this year is the importance of monitoring temperature, humidity, and soil moisture. It's tedious work to have to test the soil moisture level of every single plant in my small greenhouse almost daily during this summer, but as you can see it's paying off. I discovered that just because one plant registers a 7 (ideal for Calendulas) in a box that contain two other plants, does not mean they will all have an equal moisture reading of 7. Some are dryer or wetter than others and it's important to either water or skip each plant based on their reading. That's why I recommend starting with a small batch of plants so you don't get overwhelmed.
As for temperature, a good rule is ensure the humidity is around 50% than the temperature. The digital thermostat shown here reads 56%, which is pretty good. Anything consistently higher attracts pests and if you've read or seen my other posts on pests, you know what a great time I've had. Both readers cost me about $45 combined. Both are a great investment and I highly recommend getting both if you have a greenhouse.
Since links change all the time, use the keywords: "battery digital thermostat for greenhouse" and "soil reader" to shop around.