Grow Lights in a Greenhouse

Introduction by Catherine Haug

A question has come from one of our members:

I essentially live in a forest and don’t get enough hours of light to grow a garden. Is it workable to garden in a greenhouse using artificial lighting?

This query was forwarded to Don Bates, who will be giving our March presentation on Season Extenders (Row covers, cold frames, hoophouses, and greenhouses). What follows is his response, which is right-on when it comes to sustainable food production.

The Unworkability of Grow Lights

by Don Bates

This is, essentially, not a good idea for growing plants.* Here’s why:

The whole idea of agriculture must be that we convert the energy of sunlight into edible biomass which can then provide us with energy (calories).  We have turned that model on it’s head by using cheap petroleum in agriculture, to the point that it is now estimated to require about 10 calories of petroleum energy to produce a single calorie of food energy.  As fossil fuels run out, we can only return to a state in which the net energy of food is positive.

In terms of calories (energy expended or gained), here is what would happen with an artificially lighted greenhouse:
  • You would burn fossil fuel to generate electricity, at perhaps a 40% conversion efficiency.
  • Then you would use electricity to produce light, at about 25% efficiency.
  • Then the plant would convert that light to biomass at about 3-5% efficiency (with only some small percentage of that available as food calories).

So, from both a dollar and energy standpoint, this is a very unworkable proposition. As mentioned above, this system is essentially about 10% efficient overall. That is, for every 10 calories of fossil fuel extracted, you get about 1 calorie worth of food, by using grow lights. [NOTE, this doesn’t even consider the energy cost of extracting the fossil fuel in the first place.]

Nevertheless, the Japanese are doing exactly this (growing greens under lights), because all their farmers are getting old, and the young people don’t want to work in the fields.

* (Note: this is NOT to argue against the use of grow lights for starting seedlings, in which case electric light only provides a very tiny percentage of the total light required to grow a plant, and can enable the growing of crops which would not otherwise be possible in our climate.)

Dealing with limited hours of sunlight

by Catherine Haug

What, then can be done?

  • Grow plants that don’t mind shade/partial shade. Spinach, lettuce, chard and other leafy veggies are the most shade tolerant. Mint,  peas, asparagus, nasturtiums and some strawberries may also do well. Potatoes, beets, carrots and turnips require at least a half-day of full sun.(1,2)
  • Clear an area of some of its trees (exchanging carbon-sequestering trees with carbon-sequestering garden plants may be a reasonable compromise);
  • Negotiate with a neighbor who has more sunlight, to share garden space;
  • Join a community garden;
  • Volunteer  at a CSA (Community Supported Agriculture), getting produce in exchange for labor;
  • Shop at the valley’s Farmers Markets.

I pass the question to our membership: What else can someone with limited hours of light do, when it comes to gardening? You can either add comment to this post, or send your ideas to Catherine (Cat), to be added as comments.


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