Considering a grow light in my greenhouse - Ask Extension
Hello. I have an unheated greenhouse that I have been able to use for extending the growing season by a few weeks in Spring/early summer as well as in...
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Considering a grow light in my greenhouse #782399
Asked February 22, 2022, 2:30 PM EST
Hello. I have an unheated greenhouse that I have been able to use for extending the growing season by a few weeks in Spring/early summer as well as in to the fall. Over the years my neighbors tree has grown large enough to shade it for part of the day, which leaves me with just 5-6 hours of consistent sun. I was thinking about hanging a grow light in the greenhouse to fill in during the shade hours. Do you think that would work and, if so, what would you recommend in terms of wattage, type of light, number of hours, etc. It seems like there are several options
Deschutes County Oregon
Expert Response
Hi Eclipse:
There are several options depending on what you are growing and what you are trying to accomplish.
Incandescent bulbs (common old light bulbs) give off light that is mostly in the red spectrum. That is good for triggering flowers, but produces weak plant growth. They also produce heat and may require more electricity to run. The heat generated may be a good thing in your unheated greenhouse during the shoulder season. Something to consider.
You can use full-spectrum fluorescent bulbs. The drawback is they must be situated very close to the plant to be effective, 3-4 inches from the plants. That is why they are a popular choice for germinating seedlings or plants that grow a foot or two high.
In your situation, a combination of both of these may work for you.
The final option is High Intensity Discharge (HID) lighting. This is very common for general greenhouse lighting. Both metal halide (MH) and high pressure sodium (HPS) lamps are suitable for greenhouses. A combination of MH and HPS lamps in a one to one ratio is common with the MH contributing light in the blue-violet range and the HPS contributing light in the yellow-orange part of the light spectrum. A common figure many manufactures use is 25 to 50 watts of HID light per square foot of growing area. The downside to HID's is they are more expensive than the other two options.
Not sure what you are growing in your greenhouse, but general rule is vegetables like between 700-1000 foot candles of light for proper growth. You can measure this with a light meter.
Plants need 12-16 hours of light each day. Depending on the time of year, your supplemental lighting needs may be different. A timer can be set up to simplify the process. You can enhance the available light by adding reflective surfaces. Paint everything you can gloss or semi-gloss white. The raised beds, walls, benches, and so on. You could add white rock to the outside of the greenhouse (south side), which will reflect more winter light into the greenhouse. Snow works the same way.
I'm not sure I've answered your questions exactly, but hopefully given you enough information to make a decision based on your situation.
Thank you for your question.
