It'd be a matter for the Community Standards people. But, I can report an apparent "raised" garden near me. It is very visible, so I must assume that it has been approved. I know, I know, assuming such stuff may not be safe, but, the visibility tells me that complainers could not help but complain - and the Community Watch drivers can certainly see it!
Aside for the "standards" issue, I suspect the lack of gardens is mostly because folks are saying, "I'm too busy doing fun things to be worrying about whether my beans will ripen!"
SWR
