Dutch greenhouse from 1996 with HP sodium lamps that supplement natural daylight and add heat to the greenhouse. Although difficult to see, an insulating blanket is a few feet above the lamps. The layer of insulation holds heat generated by lamps inside the greenhouse. (Provided by Jorge Cervantes)

How many lights do you need in a greenhouse? Our expert weighs in

Cannabis Cultivation Q&A: Electricity use is much lower in a greenhouse because the lights are turned on fewer hours per day and used for only half of the year

Renowned grower Jorge Cervantes will answer readers’ questions and give advice on all things related to the cannabis plant. Got a question for Jorge? Email him at jorge@marijuanagrowing.com.

Editor’s Note: Laws for cultivating cannabis vary from state to state and city to city — before germinating any seeds or planting any clones, take care to learn what your local laws are.

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Question: We are starting a new recreational grow down in Walsenburg, Colorado. We plan on putting in greenhouses one by one.

When installing the supplemental lighting, do we use the same number of lights as an indoor grow, or a lot less because of the greenhouse setting? Others seem to think that I can get away with less supplemental lighting than would be necessary in a brick-and-mortar building.

Could you please give me your thoughts on that?

— Thanks, Marty

Answer: A greenhouse takes advantage of days of clear skies packed with natural sunlight. That’s an average of 272 days for you in southern Colorado, according to one estimate. Even when the days are short and nights long, natural sunlight will supply the lion’s share of light plants need. The best part about your 6,171-foot elevation is that you are closer to the sun and it is brighter, even in the winter.

Artificial High Pressure (HP) sodium lamps will be necessary to supplement natural light for about six months – October-March. This is when long nights not only trigger plants to flower, but also slow growth. Lights will need to be on for a maximum of eight hours during the longest nights. Electricity use is much lower in a greenhouse because the lights are turned on fewer hours per day and used for only half of the year.

Lamp spacing in a greenhouse is not as cluttered as in an indoor grow room because natural sunlight supplies most of the light and artificial light is purely supplemental. Therefore, artificial lights can be placed farther apart and farther away from plants. When placed farther away from plants, light coverage is broader and more even.

In general, growers use one-third to one-half as many lamps in a greenhouse as they would outdoors.

I suggest starting slowly if you are a small operation. If you plan on making a large capital investment, one possible contact is Gavita-Holland for a bid on your greenhouse. I have known the manager, Theo, for years and have visited their facilities in Aalsmeer, Netherlands. In my opinion, they have the best lighting engineers and products for professional greenhouse growers. Of course, there are several local resources as well.

Your opinion is valuable:
Have a grow of your own? Please tell us how you set up your greenhouse lighting in the comment section below. Our readers want to know your tips and tricks!