Running a Cost-effective Greenhouse

The trick to making a hydroponic greenhouse cost-effective is how well you control the lighting. During the summer months, sunlight has to be your chief source of light, and the supplemental lighting system—which in our case consists of 45 600W High Pressure Sodium lights—will only get turned on when needed.

In the case of Cornell University’s Controlled Environmental Agricultural facility (CEA) in upstate New York, professor Louis Albright designed their lighting system. His research indicated that lettuce plants require precisely 17 mols of photosynthetically active radiation (PAR) per square meter, per day.

Since they use a bank of 144 lights, they opted for water-cooled light fixtures, to reduce the emitted infrared radiation, otherwise known as heat. Our space is smaller and we use fewer lights, so we achieve the cooling with fans and an evaporative cooling pad, when the sunlight gets too hot.

Since we know that a 600W High Pressure Sodium bulb emits 90,000 lumens 1 foot away, 22,500 lumens 2 feet away, 9,999 lumens 3 feet away, and 6,428 lumens 4 feet away, the computer does the appropriate calculations to figure out how high the lights have to be situated above the Nutrient Pond surface in order to provide the optimum level of PAR per day.

Given enough fans, we can cool the air between the lights and the lettuce plants adequately, without having to water cool the lights. Also, our lights are on tracks so they can be raised or lowered, or moved to the side on intensely sunny days, to keep them from casting a shadow on our growing lettuce crop.

Photo sensitive cells measure the sunlight as well as the artificial light and just like a good digital camera, the computer figures out how much fill light is necessary to bring the intensity of light to the desired level each hour.

The same precise measurements are taken each hour in our nutrient solution, which primarily consists of Advanced Nutrients Grow, Micro, and Bloom, as well as other ingredients to help produce healthy, robust vegetables, such as Sensi Cal, Piranha, Tarantula, and Voodoo Juice.

The first one mentioned boosts the level of calcium required by lettuce, while the latter three strengthen the roots by colonizing them with beneficial fungi, bacteria, and other microbes. Voodoo Juice stimulates root growth and it contains Gibberellins, which create natural growth-enhancing hormones.

If the pH of the solution deviates from the desired 6.5 to 7.0, the system automatically adds the required amount of Advanced Nutrients pH Up or pH Down, depending on the circumstance. The temperature of the solution should be between 60º to 65º F (15º to 18º C) at all times, since lettuce grows best at these temperatures.

If the temperature of the solution drops below these numbers, a system of pipes circulate warm water to bring it up. If it dips below these numbers, the pipes circulate cold water to bring it down.

The crop of lettuce constantly moves from one end of the Nutrient Pond to the other, with the young seedlings at one end, and the ready to harvest mature plants at the other. In this way our hydroponic greenhouse can produce 350 heads of butterhead or Boston lettuce each and every day.

Aside from Chuck and I, we only have two other part time employees. Since most every step is computerized, our need to pay salaries has been minimized, thus making the whole operation that much more cost-effective.