Growing Herbs Indoors
Q. I want to grow herbs indoors this winter. How do I provide enough light?
A. Herbs need a lot of light to grow well. Outdoors, they need 6 to 8 hours of full sunlight a day. Regular indoor lighting cannot provide the same quality (spectrum) or intensity of light that comes from the sun.
The goal of supplemental lighting is to imitate the sun as much as possible. We want a bulb that emits the full spectrum of light waves that plants use, at sufficient intensities to promote growth.
It might seem simple to go out, grab a light that says, “full spectrum,” and set up shop, but it is not as simple as that. Plants use rays from the full spectrum of visible light, but they use more of the wavelengths in the red and blue range. In addition, there is some evidence that green light may also be important for plant processes.
Not all lightbulbs emit the same spectrums of light. We know that regular incandescent bulbs produce abundant light in the red ranges and that regular fluorescent bulbs produce more light in the blue
ranges. Before the advent of LED technology, growers used combinations of these bulbs to ensure plants received the necessary spectrum of light. Modern LED lights can allow for customizing the spectrum to suit the needs of the plants you are growing.
For beginning gardeners who just want to set up a light and get growing, you can follow these general recommendations for light set-ups from University of Iowa.If you are using fixtures that are manufactured as “grow lights”:
- Purchase or install an LED or fluorescent grow light as outlined in the directions in the package and leave lights on for 14 hours a day, or as specified on the product instructions.
If you are using lights NOT manufactured as “grow lights”, such as standard fluorescent or LED tube lights:
- LED: standard, 4-foot, plug-in shop fixture with full spectrum (also called “daylight”) light, which provides at least 3,000 lumens, placed 18 inches from the foliage, with lights on for 14 hours a day.
- Fluorescent: Standard, 4-foot, plug-in, 2-bulb shop fixture with each bulb being 40-watt, T5, high output and full spectrum (A.K.A.”daylight”) placed 9 inches from the foliage with lights on for 14 hours a day. (Alternative: Instead of “daylight,” use one “cool white” and one “warm white” bulb).
Once the lights have been set up, monitor the plant’s growth and development over the next several weeks and make adjustments to the lights based on recommendations in this publication: https://yardandgarden.extension.iastate.edu/…/how-set….
You will notice that some LED grow lights shine with a pinkish/purple color. This occurs because these fixtures produce light only in the blue and red wavelengths. They are designed this way because research has shown that plants use mostly red and blue light. Many people, however, object to the way plants appear under these lights. You can get equally good results with a fixture that emits full spectrum, white light.
For gardeners who would like to delve into the world of plant lighting:
https://yardandgarden.extension.iastate.edu/…/how-can-i…
https://yardandgarden.extension.iastate.edu/…/important…
