PeterH:
You will need a driver circuit between the Arduino and the fans. What circuit you need will depend on the electrical characteristics of the fans you're using.
Thank you very much for the feedback Peter. I will need some sort of Driver circuit yes, to "drive" the fans due to voltage differences between the Arduino and fans? Arduino wont' be able to output 12V required by the fans for example right?
I am still looking into which fans I will be using, my main concern is, fans alone won't really lower the temperature inside the greenhouse. My budget however won't allow any other fancier methods though. Manual air vent and controlled fans are what I have at the moment.