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Lighting Can Be Tricky When Starting Seeds Indoors

Tuesday, January 23, 2024

Although much of Oklahoma has been experiencing a deep freeze recently, gardeners are gearing up for the growing season by starting seeds indoors. Adequate lighting is a must and can be tricky for gardeners who don’t have access to a greenhouse or sunroom environment. Supplemental lighting is important to get seeds off to a good start.


There are three things that gardeners should understand about lighting, including light intensity – how bright the light is; light duration – how long the plants are lit; and light quality/color – the wavelength of the light.


Plants need light in the blue to blue-green and red to far-red parts of the spectrum. Make sure the light source chosen emits in those wavelengths. Incandescent lights typically have the red to far-red range and doesn’t emit much of the blue to blue-green light, which can cause plants to stretch and have spindly stems. These lights also give off a lot of heat, resulting in injury to young seedlings if the bulb is too close to the plants.


Fluorescent lighting is a much better choice for lighting seedlings because it emits light in the blue to blue-green part of the spectrum. In addition, it puts out little heat, so there’s less chance of burning the plants. Keep in mind, however, that plants need light in both areas of the spectrum in order to grow well, so choosing a combination of cool-white and warm-white fluorescent bulbs is a must.


A plant grow light does the same thing and is a great choice for starting seeds. These fluorescent lights should be suspended about 6 to 8 inches above the seedlings and raised accordingly as the plants grow. Be sure to dust the bulbs monthly as the accumulated dust interferes with emitting light. 


LED lighting technology has increased over the years and today there are some great LED grow lights available. The Oklahoma State University Extension fact sheet LED Grow Lights for Plant Production can provide gardeners with important information about the use of this type of lighting.


Something to consider is that light intensity decreases considerably in the last 3 inches of the fluorescent bulb so it’s important to place the seedlings about 3 inches from the ends of the bulbs. Basically, gardeners lose 3 inches of light from the ends of each bulb; therefore, a 4-foot bulb is a better choice than two 2-foot bulbs if you’re planning to start a lot of transplants under the lights. 


Seedlings need to be illuminated for approximately 14 to 16 hours per day. Gardeners can ensure proper lighting timing by hooking the light fixture to an automatic timer.

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