As a landscaper I've been involoved in the construction of several large parks in the GTA, as well as a couple of rooftop gardens. They are rewarding, but very formulaic: stick to varied combinations of plant material from within your climate zone; add forced irrigation then let nature and nursery warranties do the rest. In other words, there is very little risk.

Having just watched Greening the Desert for a second time I am amazed at what they accomplished. Truly inspiring and significant.