Tomatoes and footcandles

Asked August 8, 2013, 11:29 AM EDT

I am currently living in montana and I would like to grow determinite tomatoes like legend, celebrity or gold nugget indoors during the winter hydroponicly under a 600w HID grow light, and my question is how many footcandles of light per square meter does my tomato plants need to veg and fruit properly?

Cascade County Montana greenhouse

1 Response

Home grown tomatoes are always a challenge in Montana. While growing up in Bozeman I remember my parents attempts for many years. But it can be done and hydroponics indoors during the winter can be fun and the fresh tomatoes will be a welcome treat.

Most light intensity requirements for plants is determined by scientists as photosynthetic active radiation or PAR. The units for this value (photon flux rate also known as quantum flux) is in micromoles of photons per meter squared per second of light between the wave lengths of 400 to 700 nm. Plants "see" or use mostly blue and red light while reflecting green. A foot candle is a unit of measure that is based on the visual perception of the human eye, which is most receptive to green light. What does this mean to you? Well, foot candles and PAR light do not easily convert. No light is created the same and when comparing HID to sunlight, or to fluorescent, or to anything, it is a challenge.

Your lamp is 600 watts and I am assuming that it is a high pressure sodium bulb (HPS). A 600 watt lamp generates a photon flux rate at about 4.98 moles of photons per square meter per second. For your 600 watt lamp, that is 2,988 micromoles per meter squared per second (0.003 moles per meter squared per second).

Now for your question. How much light do you need for your tomatoes, you need to understand the concept of daily light integral or DLI. Rather than a long dialogue, this link to a publication at Purdue University will help.

The DLI for tomatoes is between 14 and 20 moles per day. To get 14 moles of light from your lamp, it would have to run your lamp for 38.89 hours per day. Oops, that doesn't work. Remember that these data are representative of a greenhouse scenario. You could more closely satisfy the plant demands for good photosynthesis with two 600 watt lamps and get away with 14 hours of light.

Another way to look at this is to note that tomatoes require 14 moles of light per day. Converting this down to a photon flux rate, that equals 162 micro moles per meter squared per second. Using a conversion tool used by growth chamber manufacturers, you can determine that you will need 1,234.4 foot candles of light to grow tomatoes.

With good management of your plants and good nutrition, you can squeak by with lower light, just understand that your yield may not be what the magazines quote.

Also, HID lights are very warm, make sure that you have adequate ventilation.