Denver Zoning Code Text Amendment 15 for Residential Growing of Marijuana under Colorado Amendment 64
I think what they are trying to do is prohibit outdoor, backyard growing of your six plants (or would that be 12 plants, 6 medical plus 6 recreational). Original A64's "enclosed, locked space" (your backyard with a six-foot fence) becomes "completely enclosed, lockable space" (their garage.)
"Assistant City Attorney David Broadwell says unequivocally (at 1:00) that outdoor home grows are against the law, but when Councilwoman Kneich asks where it is so stated (4:39), Broadwell admits that Amendment 64 doesn't make it illegal to grow outside. Broadwell eventually calls for NEW legislation to make it illegal to grow outside."
This is the implementation of that amendment.