London Mayor Boris Johnson out-greened Mayor Bloomberg yesterday by suggesting there’s plenty of space here to foster homegrown foods.

“Basically, we are too dense,” Bloomberg said, responding to a question during a conference at Columbia University.

But Johnson said his city has discovered 2,000 growing spaces, and so can New York.

“There are roofs in New York,” he pointed out.

Bloomberg retorted that his foundation’s new building will have an environmentally friendly roof and perhaps he’d “talk to them about planting corn and rhubarb.”