We had a water reservoir and attached motor lying around that was from a car's windshield sprayers. These are perfect for a project like that since they run on 12V DC and the pumps are more than powerful enough for this application. If you need one of these, simply go to a junkyard and grab one. I wouldn't expect it to cost any more than $10 for everything.

To start out I placed my tank on an old table that we had lying around the house. Conveniently this table was on wheels which was an added benefit. I mounted the reservoir + motor onto the side of the table using screws and an L-bracket.

I then attached a tube from the motor (I believe the motor used a 3/8" tube), and using a couple tube adapters we had lying around, hooked up my tubes after cutting them to the right lengths. I found that Command Strip wire holders were perfect for holding the tubes down to the tank. Find all these parts these at any hardware store, ideally Lowe's or Home Depot.

I ran the tube from the motor to a tube splitter adaptor under the soil, then each to the 360 degree sprayer. To hold the sprayers up I just used a zip tie and secured to the tube + sprayer to chopsticks sticking out of the dirt.

One wire from the motor is then connected to the "drain" on the NTE2395 transistor. I chose this transistor because of its capability to handle high current across the drain and source, and its capability of dispersing heat. I burned a couple of different transistors before learning this. The other wire from the motor is connected directly to 12V on the breadboard. The "source" on the NTE2395 is connected to ground, and the "gate" is connected to the Arduino's digital pin through a 10M resistor. When the Arduino turns on 5V across the gate, it closes the drain and source, allowing current to flow from the 12V battery and through the pump.

I attached an IN4007 diode (it's cheap), in parallel with the motor to take care of the surge from turning the motor off. The motor is an inductor after all, so a change in voltage will mean the motor wants to resist that change by creating a sharp voltage in the opposite direction which is why the diode seems to be placed backwards in the circuit at first. Don't forget this or you could damage your Arduino!

Lastly I added a button in so I can water the plants whenever I want. Just plug it into a digital pin on the Arduino with a 10K resistor.