I think it is an excellent opportunity. An LA studio has people from all sorts of backgrounds!

My BSLA is from Cornell. I'd say the MLA programme here is worth checking out. We do a lot of projects with urban communities in the local area. Also, it may be a little more affordable than some other big programs.

For portfolio, try to show your ability to draw. Maybe some polished perspective drawings and sketches. Experiments and volunteer/research are also very cool to include. Not everyone is great at drawing at my school, but you definitely want to show some potential.

Look on issuu for portfolio layout inspiration. Make sure it is neat with minimal text.

Maybe having your portfolio reflect key points in your statement of purpose might be a nice touch. GPA seems fine. Not the most important thing for LA.

LMK if you have any other questions. Love to meet people interested in LA.

You need to do your research. 

"Green space in urban communities" is a very vague phrase and most programs will address that somehow. You could go to a place like Cornell where it's most recognizable in service and community work (although with new faculty that is changing ), or a place like the GSD where the Core 1 studios are are centered around public space of different scale in the Cambridge/Boston. This doesn't take into account other programs where "urban" is also addressed in international studios (granted that has also changed). 

I'm not familiar with any program that focus on urban agriculture, but that does not mean you could not make it into a thesis or capstone project-  Just don't expect to talk intensely about food as part of your core curriculum. In approx. three years you will cover projects as different scales- from gardens to counties- in studio. You'll also learn theory/history, site engineering, and woody plants. This doesn't include graphic representation/autocad, and other technical courses. 

Bottom line- if you want to do food- find a program that is allied with others interested in Urban Ag. The one person that comes to mind is CL Bohannon at Va. Tech. But you'll also need to consider other departments/programs that might benefit you as well. Naomi Sachs at U of Maryland does work with health and well being in urban environments as well.

RE: the portfolio. That varies based on the program and the review committee. Honestly, some programs are wowed by graphic styles that are very polished, but also very copied. Other programs will consider your effort to draw and show some commitment to making. A portfolio need not be only drawings as well- three dimensional objects count. Avoid photography that has not been recognized/awarded. Imagine yourself as one of 70 students that went out and took pictures with their phone, and jammed them in the back. Those images often don't help to you to stand out.

Research people and the places. Good luck.

First off, check out other locations for the job market status... never been busier.  Secondly the addition of landscape architecture will only make you more employable and and speaking on a personal note I would certainly recommend going in that direction.