Why isn't the conversation about floating greenhouses just off the coast of a city. Transportation problems would be solved, nearly all the same benefits as mentioned in the Economist article, while you would get enough energy from the sun most of the year round.
I'm pretty sure it's because boats are a whole lot more expensive than land surface. Unless you can get a lot more productivity per square foot than a field, transportation costs are still going to be low in comparison.
I don't see why this wouldn't work for high-profit produce like tomatoes or peppers, though.
Afterthought: it could work for high-end produce like ripe tomatoes and things that can't be transported very far. Although there, I'd think a penthouse farm would be even better. Except that the return on a penthouse in a major city for residential use will probably remain higher than any possible use for produce even after oil tops $10. Unless the produce is a prohibited drug, in which case you still probably don't want to use the penthouse, being better off with grow lights in the subbasement.
People are already making money growing tasty, ripe hydroponically grown tomatoes in the city to sell to high-end restaurants. In New York, no less. This is only going to work for high-end produce, though.
