Inefficient fluorescents

Dear Ed,
High Pressure Sodium (HPS) and Metal Halide (MH) lights might be good for some growers but for me I like to save my money and use fluorescents. I have two four-foot lights in a 4 x 3 foot grow space. Is this enough light? (5 plants total) How tall should I grow the plants before flowering?

Yello Thumb,
Las Vegas, Nevada

Fluorescent lights do not save you money. Although they are cheaper to buy, they are much less efficient at converting electricity to light. The full amount that you spend on light is the total of the cost of the equipment plus the cost of power. On this basis, fluorescent lights are much more expensive than HPS or MH lamps.

Fluorescents can be used to grow a crop, but they don’t produce a good yield because the plants don’t receive intense light. Many years ago, I recommended a minimum of 20 watts of fluorescent per square foot (psf) for budding. Later, I said 30w fluorescent or 20w HPS psf. Now I recommend 40-60 watts HPS psf.

The more intense the light, the more energy it delivers. Plants are fueled by light energy, which they use for photosynthesis. With more light, the plant is able to produce more sugar, which it uses both for energy and tissue building.

If the garden is to be lit by fluorescents, it will need two more 4-tube fluorescents fixtures for adequate results. The lights will have a total input of 40 watts psf. Since there are only five plants in the garden, they should be encouraged to bush out, so they have a wider canopy. On the other hand, too much branching produces spindly buds.

The lights should be kept about 12 inches from the top of the canopy. This can be a little awkward if plants are different heights. One way to solve the problem is to raise the containers of the shorter plants.

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