Whoa! Be careful - locally grown isn't always greener.

If you're set on making a positive impact on the environment by only buying locally grown produce, heads up. Know the source and how your foods are grown. Even if they come from just down the street, if they are produced in an indoor "plant factory" there may actually be a negative environmental impact. And that's compared to foods grown at a great distance.

Here are some of the highlights of Dr. Lou Albright's research that originally ran in GrowerTalks magazine.

Warehouse growing faces the big issue of carbon footprint. This was spelled out quite clearly by Dr. Lou Albright, professor emeritus from Cornell, who gave a talk on these new indoor “plant factories.”

Lou showed in calculation after calculation that the production costs for growing without sunlight costs more in electricity than the crop can possibly support. For instance, a single butterhead lettuce plant would cost 20 to 30 cents in electricity just for lighting it (when grown with zero sunlight).

And how much of a carbon footprint does that electricity have? He calculates 8 lbs. of CO2 emitted for every 1 lb. of lettuce produced. That’s three 55-gallon drums of CO2. Tomatoes would be worse, needing 18 lbs. of CO2 per lb. of tomato.

Urban plant factory proponents would argue, “But they’re produced locally, so you don’t have to ship them.” Yes, but to ship that lettuce from California would require just 0.7 lbs. of CO2. Meaning those dreaded “food miles” are a lot more sustainable than one might think.

And if you think you’re going to run those lights with photovoltaic panels, think again: To light one acre of indoor growing space you’d need 9.3 acres of photovoltaic panels.