What kind of fan are you using? A half amp 12v fan doesn't move much air so I am assuming these are smaller spaces.
A 10w solar panel in a good position should be able to power a .5 amp 12v fan.
They can all be on the same circuit. A solar panel connected to a charge controller connected to a battery. Then from the charge controller a snap switch connected to the fans. The battery is there to buffer the charge on partly cloudy days or hot days in the spring and fall. It is bad to have power constantly flow into a battery. The charge controller will stop charging a full battery and stop providing power when the battery is low.
Here is a $15 charge controller. I have had one of these running continuously in a project for two years now.
Snap switches are set to turn on at a set temp and turn off at another. They do not require power and will work on AC or DC. This one is adjustable, non-adjustable ones are cheaper.
If you have water pressure solenoid valves can be powered by the same battery. Without water pressure there are 12v DC pumps that can be powered on the same system. This is one I've used. It will pump a gallon a minute with 3 meters of head at 4.2 watts. (Still assuming this is a small space.)
I haven't looked into over the counter timers or controllers for pumps. If all you want to do is fill a tank that gravity feeds the plants that would be easy enough with a float switch. If the pump only runs occasionally a 15w panel should cover the fan and pump but a larger charge controller might be needed.