It used to be that horticultural fluorescents yielded little more than a seedling’s true leaves and some early growth from cuttings. When kept in close proximity to the plant canopy, standard fluorescents have also been known to cultivate leafy greens and please a few houseplants, but now there are high-output fluorescent garden lights that can easily raise plants to maturity and even force flowering and fruiting. These new fluorescent tubes produce almost twice as much light as stan•dard types and often run cooler, last longer, and operate more efficiently than point-source high intensity discharge lamps.

Here’s how to make sense of the different types and determine which fluorescent is right for your space and growing application.

Many fluorescent tubes and corresponding fixtures (equipped with a ballast necessary to power the tube) are prized for their minimal space requirements. Most systems are only slightly longer than the tubes they illuminate and can mount practically anywhere, in any position. The naming conventions of some of the tubes will give you an indication of their size. With T5, T8 and T12 tubes for example, the numbers signify the diameter of the tube, represented in eighths of an inch. So a T5 has a diameter of 5/8”; T8s have a one-inch diameter (8/8”) and so on. Keep in mind that a smaller diameter does not mean the tube is less powerful (T5s are producing some of the most impressive growing results of all tubes on the market). The “T” that precedes the number simply references the shape, in this case, a linear tube shape.

There are also TT96, TT105 and other similarly-named tube types. These have the same linear shape but are
comprised of two tubes joined together at one end for increased output while maintaining a single connection
point to the fixture. With these tubes, the numbers that follow indicate wattage rather than diameter. This means a dual TT96 tube is a 96-watt tube and a TT105 is a 105 watt tube.

Regardless of whether it’s a single or dual tube, two- and particularly four-foot lengths are the norm. If you need to cover an extended distance, consider a fixture with “daisy-chaining” capabilities so you can link multiple systems together. Fixture widths will vary depending on the manufacturer and number of tubes: a single T5 fixture can have an insignificant 7/8” width while eight-tube T5 fixtures can be more than 25” wide.

It’s worth mentioning that all fluorescent tubes are not compatible with all fixtures. There are exceptions to the rule (i.e. some four-foot T8 systems can also receive T12 tubes), but most fixtures have a ballast designed to power a specific type of tube. What’s more, tube prongs and fixture receptacles will also vary. If you see a fixture you like, investigate the included or recommended tubes to make sure they’ll meet your growing needs. You also want to verify that replacement tubes are accessible.

Lumen ratings have long been a standard point of reference for light source comparisons. Remember, if you light a candle, step one foot away and assess the amount of light in that one-square-foot section, the measurable light would equal one footcandle or one lumen. To thrive, leafy greens need to receive at least 2,500 lumens per square foot and flowering plants need 10,000 lumens per square foot. You can also look at wattage (50-75 watts per square foot is considered optimal) and even gauge a tube’s efficiency by calculating lumens per watt (divide lumens by watts).

Because lumens are truly a measure of the perceived power of light (or how we, as humans, see light), be sure to note the other variables that identify a tube’s output. Color temperature, for one, is represented in degrees Kelvin and tells you how visually cool (blue) or warm (red) the light source is. A 6,500-degree tube appears “cooler” and is more characteristic of the sun’s mid-day light. This is also why fluorescent tubes with higher color temperatures are sometimes referred to as “daylight” tubes. These types are preferred for promoting vegetative growth. Alternately, a lower 2,900-degree tube appears “warmer” and is more representative of a rosy-red sunrise or sunset. Fluorescent tubes with these lower temperature ratings are just the ticket for growing full-sized flowers and produce.

Furthermore, plants only respond to a limited portion of the visible light spectrum known as photosynthetically active radiation, or PAR. This light energy is measured in wavelengths and represented as tiny units called nanometers (nm). Plants primarily use wavelengths between 400 and 700 nm with peak responses at 440 nm (for blue light and vegetative growth) and 660 nm (for red light and flowering). PAR ratings are less common than lumens, watts and color temperatures, but they are worth understanding and give you yet another way to compare the different fluorescent types and brands. GG

Emphasizing environmentally-friendly gardening practices indoors and out, Kari Bayne is the Senior Staff Writer for The Worm’s Way Group, member of the Garden Writers Association and life-long gardening enthusiast. Get inspired about organics, hydroponics and more at www.wormsway.com or call 1-800-274-9676 to request a catalog.