Measuring Light for Growing- How Much Light Does my Garden Need?

Plants need light to live, but sometimes you need to measure the light to be sure there’s enough. (For example, we don’t have a window sunny enough in our house to raise seedlings.)

A footcandle is a measurement of light based on the light in a 1-foot-square area illuminated by a single candle known as an international candle. This method isn’t totally accurate, but will give you a good idea of your footcandle measurement. (For example, the windows I tried to raise seeds in my first year here are measuring at about 32 fc. No wonder those buggers didn’t grow!)

To give you an approximate idea of the fc power of various light sources-
Starlight: .00011 fc
Moonlight: .02 fc
Overcast daylight: 1,000 fc
Direct sun: 10,000 fc

Here’s a neat trick to measure the light where you’re growing- you will need a manual camera, set on Shutter Value (that’s TV on a Canon) so that when you set the shutter speed and ISO, the camera will set the aperture. Don’t worry, this is EASY.

1) Set the ISO at 200
2) Set the shutter speed at 1/125.
3) Point the camera at your light source– for example, a window or greenhouse roof– and take a picture.

When you’re looking through the lens you’ll see numbers along the bottom- check your aperture or F-Stop

Veggies need light about 1,000 fc, give or take for each variety, to grow well.  (And they need it about 8 hours a day.)

If your aperture is:
2.8, that’s 32 fc
4, that’s 64 fc
5.6, that’s 125 fc
8, that’s 250 fc
15, that’s 1,000 fc
22, that’s 2,000 fc

I’m considering adding flourescent lighting to my backyard greenhouse, this is a handy way to see how much extra I need!

All information from Greenouse Gardeners Companion, written by Shane Smith.  Copyright 1992.

Here is the revised version on Amazon.