That's actually a very good point. Unless we can successfully green our own deserts, in the presence of a full atmosphere and 1G gravity, how can we realistically hope to green other worlds without either?
In reality, we do not need to green our deserts, in general. As alluded to by Vertigo, we have a global food surplus, not a deficit. The reson people starve in a world with a food surplus is mainly to do with inequality, and greed. Even in places hit by drought, or famine, a major factor for the starvation is hoarding by rich elites, and/or (para)military factions damaging the food supply chain, by burning farms, killing livestock (including draught animals), and targeting people.
Add to that the greed of corporations and governments who continue to deny that the global climate is in crisis through their actions, and who would rather see people starve than share their patents at a price which might reap a lower dividend for their shareholders. And, of course, any localised greening of desert environments would require the level of investment, which would not necessarily benefit the people who could afford to be investors.
I think I should stop there, rather than to get on to a (rightly) banned topic. Basically, we can (and, indeed, have in places) created green spaces in Earth's deserts, but they are, by and large not required for global food security, and the set-up costs are probably not affordable for the local residents who could benefit from it. A Martian oasis creation (if successful) would be funded by the organisation creating the colony, which would not be interested in a short-term return, as it is a long-term project.
It is a fair point, though, in that if we do not change our short-sighted demand for quick personal profit and self-gratification, we will screw up any other world we try to colonise, and complete our present headlong course towards self-extinction.
Edit: Sorry, Serendipity, for derailing the thread.