In order to grow Rhododendrens in a greenhouse, you will need the following: climate control, timers, misters, and thermostats. Temperature, moisture, and light must be kept in balance. Any changes in one factor, means that all of the others will have to be adjusted. These need to be in a soilless culture. So, it takes adaptation, in order to allow them to grow in a pot. They grow best between 55 and 80 degrees. They also need 60% or greater humidity. Humidity is necessary in order to encourage blooms. Air circulation is also a necessity. This will retard the growth of funguses and molds.