Yep, my solenoid is on a timer. At 3wpg...you could try without co2 if you start with alot of plants. If you plant a little at first and plan to slowly increase the plant mass, I would definitely get the co2 going because otherwise you'll be inviting algae. More plants will leave less nutrients (and light) for the algae to use. Or if you drop to 2wpg, you'd be good without co2 IMO....just look for algae (more than the normal diatoms) to form and plant health to be your deciding factors.
Jimmy, what size tank are you wanting live plants for? If its over 20 gallons, I would either do low-medium light plants or look into pressurized co2. DIY co2 can be alot more work on larger tanks...to get the right amount of co2 into the tank. Some people feel its worth it...I personally don't. I'd rather spend a bit more money and have it be less time consuming.