University of Maryland Extension

Grow Lights

The natural light from a window is seldom enough for good, strong seedling growth. They will usually stretch and lean towards the light and will not produce sturdy plants. Sowing seeds indoors under fluorescent lights is the easiest way for the home gardener to control growing conditions and grow healthy transplants. Incandescent bulbs give off more heat and less light (mostly red) than fluorescent tubes and will not produce good transplants by themselves.

Many gardeners use inexpensive and widely available “shop lights”-- 4-ft. long lightweight metal fluorescent fixtures that take two T-8 fluorescent tubes (multiply the T-number X 1/8 inches to get tube diameter; in this case 8/8 or 1 inch). T-8 fixtures and tubes have largely replaced and are more energy efficient than T-12 fixtures and tubes. You can hang these fixtures by removing the strip or panel above the tubes and inserting chain or wire through the hole(s) in the top of the fixture.

T-5 fluorescent fixtures and tubes are another option. They are relatively expensive, but typically allow for more rapid and robust plant growth. Unlike T-8 tubes, the T-5 tubes can’t be used in a T-12 fixture (common shop light).

Lightstand

Light Stands

Chlorophyll absorbs most of its energy from the violet-blue and orange-red wavelengths. Cool, white tubes (40 watts) produce light in the blue and yellow-green segments of the light spectrum. They are the least expensive and the mostly blue light can produce healthy, stocky salad greens and vegetable transplants. The more expensive full-spectrum fluorescent tubes (“grow lights”) produce a balance of warm (red) and cool (blue) light. “Grow lights” enhance foliar growth and produce thicker stems than cool white tubes, and are needed for producing flowers on indoor plants. Some gardeners insert one warm and one cool tube into a fixture to gain the same effect.

See also: DIY PVC table-top light stand (photos and step-by-step instructions) and growing transplants indoors.

Comparing Fluorescent Lights

Type

Diameter of tubes

Light output 
(initial lumens*
 per tube)

Tube Life (hrs.)**

Notes

T-12 (40w)

1 1/2 inches  

1500 - 3200

10,000 -20,000

Until recently, the industry standard

T-8(32w)

1 inch

2800

30,000 -40,000

20-25% more efficient than T-12

T-5 (54w)

5/8 inch

5000

30,000

9% more efficient than T-8

T-12 (Wide Spectrum)

1 1/2 inches

1700

20,000

Type of “Gro-Light” (adds red and far-red to blue light)

*Lumens measures light intensity over the visual spectrum- the brightness that we see. 
** Over time the bulbs begin to lose their intensity. T-8 bulbs have a slower period of decrease,losing about  10% of initial brightness after 7,000 hours. T-12 bulbs can lose 20% after the same number of hours.

 

 

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