Hi brandond,

Just a few questions to clarify the situational details,
1. Where are the barrels in relation to the greenhouse. Distance and elevation difference?
2. What are the sizes (square feet) of the actual beds in the greenhouse?
3. What is the volume of the barrels and how many barrels do you have?
Aloha

The greenhouse is not build yet. IN fact I havent started yet. I will in the next few weeks though. Its only going to be around 7 foot in diameter by 13 ft long. My beds may be the length of the greenhouse. Maybe half of that Im not sure,since all of my work is going into building it this year. I have 4 rain barrels, they are 55 gallon. I can get as many as I need though as they are just 10 bucks a piece. I imagine I will have to place the barrel elevated near the greenhouse. As they are now they are slight eleved from where it will be. I may just hand water starting out. Getting a fancy watering system could be too much for this year.

Hi brandond,

I am going to make some assumptions and hopefully give you some ideas to work and play with. Others may have different ideas and I invite them to pitch in. I do not have any experience with rain barrels but with irrigation in general.
First, you probably will need to have a minimum of a two foot corridor in between your two 2.5 foot by 12 feet beds. That means 70 square feet of bed. Watering will be a trial and error because seedlings take a different amount of water than grown plants. It will also will make a difference how well you can control the climate inside the greenhouse. The types of plants you grow will dictate the type of actual water delivery system. The 55-gal. drums can be linked to fill and empty as one unit. Allow where overflow from the drums will go. You will have a system that will deliver water by gravity with very low pressure. There is approx. 0.5 psi for every foot elevation difference between water level in the barrels and the outlet in the greenhouse. So the delivery system will be very different than if you could pressurize you system. A low-pressure system needs larger diameter pipes or hoses to deliver water to an automated emitter system.
Second, the amount of water you will need for fully mature plants is approximately 40 gallons per week. That is putting 1" of water on your two beds per week. That is 5 weeks of water for four barrels. You can automate your system with a battery-powered programmable controller attached to your hose outlet. If there is no rain within 5 weeks you will need to supplement the barrels from your house water supply. Also time how long it takes to fill a 5-gal. bucket from your greenhouse outlet. You can then calculate the time it will take to water your plants. As you get further along, let us know how we can help you with a more detailed low-pressure inexpensive delivery system. Aloha