Seeing as the portfolio is the hardest part of the admissions process (even for those of us who have been working for five years on creative projects), I'd say your best bet is to wait a year or two and work on your own projects and then re-apply. If you neglect the portfolio, I don't think the committee will even consider you. 

check some out on issuu do a search for masters of landscape architecture I bet you will find some and get an idea what to include. Overall you want to show you have creative potential and understand how to carry out a design concept. And that you have some sort of artistic ability such as paintings, photography, painting and sculpture.

thank you :)

One way to get a leg up on the portfolio is to do a pre-architecture/pre-landscape summer design program - or take art classes. I've heard that the GSD has a very popular summer institute - plus you get to see if you really like it there. There are other programs around the US/world if spending a summer in Cambridge . 

Also ask yourself again, why do you want to go to the GSD? There are many other MLA programs that are just as good and perhaps a better value for you. Choose the GSD if you really want to study with their faculty, or if there is a specific series of classes that you can't find elsewhere. Don't apply just to the GSD just for their name on your diploma, as that makes for a boring application essay.

Oh, there are a bunch of threads about MLA programs and portfolios in general on archinect - if you have more questions after reading all of them, I'd be happy to try and answer them.

Hey Barry, how come you no longer teach at UMN?

byen01 - got a tenure-track gig back in LA and it was time to escape the cold.

Don't have experience from "non-design" to "MLA", only have experience from "MLA" to "MArch". But similar procedure, I have a lot of mates who did not have a design background at all when I was in a 3-year March1. So this is what I saw:

Portfolio is the most important thing to be done. Even no design background, you need to create some "projects" by yourself, to show your inspirations, creativity and sensitivity of design (of landscape/architecture). Someone was showing hand-drawing in whole portfolio, some did put a lot model photos with different material.colors.lights... 3 year program application is tricky, because jury may want to see your potential instead how much you already know about landscape/architecture.

But, on the other hand, no harm to start to understand landscape/architecture, by doing competition, self-assigned design project, and polish them become your "design part" in your portfolio besides of "non-design-show".

FYI, there are more and more "organizations" out there, help and guide students who does not have design background to apply mla/march, help to finish portfolio as well. All the software problem will be solved by their tutorials and can feed you some interesting project to dig into. Absolutely, it's not free. Most of them are constituted by graduate students from GSD/UPENN/GSAPP/SCI-ARC/WUSTL/UMICH/UCB...And even some retired professor or previous lecturers. If you do not have enough time to prepare your application/portfolio, you can look around for these guys.