Change to Prototype - Solar Modular Design

I met today with Bob, and we decided to make a few changes to the system. Here are the changes:

1. The sensor stations will be powered via a rechargeable 9V battery, and will be hooked up to a small solar panel that will charge the battery (don't have this yet). We think this will give us the advantage of not having to string a bunch of wires around the garden, and if the configuration of the garden changes (which it might), then we can easily relocate the sensor stations.
2. The sensor stations will use wireless communication via XBee modems (shown in the photo) to communicate with a central irrigation control unit. This will raise the price of the sensor stations considerably, but we think that the added benefit of having the students learn to program a wireless module will be great! This was something we were planning on doing next year, so it looks like we have a great way to integrate that into the project.
3. The irrigation system will be regulated via solenoids (shown in the photo) that will open/close a valve. These solenoids are 24V so we might be able to use two 12V car batteries to power these, and then again, use a small solar array to recharge these.
4. The sensor stations will transmit their sensor readings to the control unit which will identify the valve that needs to be turned on/off based on an identifier associated with the sensor station.
5. The sensor stations will be constructed using an Arduino, a 9V Battery and holder, and a circuit for charging the battery. These will all reside in a water-proof container and then mounted to the bottom of a PVC pole. The PVC pole will have the small solar module attached to the top and the wires will run down the pipe to the sensor station weather-proof container that will be at the base.

So, I need to modify my prototype so that two Arduino's are talking to each other via XBee modules, and then have one sense the soil humidity and send that reading over to the other Arduino which will turn on the solenoid.

I also need to check out how to charge a 9V battery with a solar panel. This tutorial on Instructables.com looks like its going to be handy.