Much like the Barrens, I was going to add occasional oases throughout Durotar so that the idea of an oasis-like Orgrimmar would make sense. Oases can be man-made, and it would make sense for someone to create an oasis-like revenue then rather live in a desert-like city.
The idea of climate change is something that Blizzard has been stirring around since Cataclysm. They have changed entire zones to new climates, and it would make perfect sense if I were to do the same, not to mention the already dramatic effect Blizzard has made on Orgrimmar in Cataclysm, that they explained by saying that Garrosh wanted to fortify Orgrimmar. Consider this new source of vegetation a fortification by Thrall.