What is the problem with urban gardening? On a rooftop, greenery would help with the heat, and I find it difficult to believe that greenery in a space which otherwise has none would be an "ecological catastrophe".
There isn't anything wrong with urban gardening itself. The problem comes when anti-development groups impose policies to prevent any building from casting a shadow on anything. Greenery and open space are both good and important, and gardening is a fine hobby. But urban gardens don't feed anyone and if there is a choice between housing people in an existing city, or protecting an existing garden from shadows, we should choose the housing every time.
Same thing goes for rooftop solar power: it's fine, but protecting it is not important.
Just going from my local experience. The build-nothing approach to housing in Berkeley, championed by zucchini-hugging (google it) hippie morons, combined with the solar access ordinance that prevents development even along major transit corridors. This has caused an explosion of car-dependent exurban sprawl in former wilderness, wetlands, and farms.
Same thing goes for rooftop solar power: it's fine, but protecting it is not important.