Better have more than 6 months worth of food, even (especially) if you plan on growing more.
Food doesn't grow all year round - it has growing seasons. They may be quite long in the southern parts of the US, but most areas only have one real growing season.
What happens if TSHTF right at the end of a growing season? You eat your 6-months supply of food, and it's still too early to plant anything. If you do, the seeds rot, fail to germinate, go to waste. Remember - in pre-industrial times it's not winter that is the death season, it's late spring and early summer. Thats when the hardest farming work (plowing and planting) has to be done, and the food stocks are depleted from the winter.
And that first cycle, you won't have much of a clue what you're doing, so you better plan on a couple of years worth of food, and a couple of years worth of heirloom (non-hybrid, free breeding) seeds optimized for your location. When we lived in So. Cal we had trees in our back yard - avocados, oranges, date palms, and a hybrid apple (apple tree with 6 different cultivars of apple that had been grafted on) that actually produced fruit.
Here in the eastern part of the pacific northwest, we have apple trees, and some cherries. We have a cutting from an avocado tree, a date palm and an orange tree growing in the sun room - in pots. I doubt they will ever produce (I'm sure about the avocado, it takes 2), but it's nice to see them.
You can actually get quite a lot of food (vegetables, especially) from a small garden - but you can't live on them indefinitely. You also need protein foods (wheat, beans, animal protein) and that takes a lot of room and a lot of energy (human, animal, industrial) to grow. Plus, once the vegetables are grown, they have to be preserved. Historically, the family busted their collective asses during harvest time: The men and older boys harvested and put up, the women and older girls canned (got lots, and I mean LOTS) of canning jars, rings (and most importantly, the lids which are not reuseable)? How about salt (in hundred-pounds quantities)? Sugar (ditto) and spices? Canners, and spare parts for them? How about fuel for the stove so you can repeatedly heat, and cool, and reheat, and recool, ad infinitum, the canner?
Don't forget, you have a fairly limited window of opportunity to get it done before the harvested food spoils on it's own.
More importantly, do you know how to can? Its not hard but I'd hate to think that the first time or two we tried it we were actually depending on it for our lives. It's a skill and improves with practice.
And you can practice now - go find a farmers sales stand, and buy a trunkload of whatever. Go home and get a canner, jars, lids, and recipes, and have at it. Better learn now, while it's easy and McDonalds is still serving heart attacks.
And aside from jars and lids and whatnot, you also need soap and towels and clean water, and a place to put the jars to store them thats safe. And then wash the empty jars out, and store them away (hopefully back in the original boxes) for use next year.
There are three books that are essential for this, imho.
Carla Emery's Encyclopedia of Country Living,
http://www.amazon.com/Encyclopedia-..._1?ie=UTF8&s=books&qid=1249231389&sr=8-1 Balls Blue Book of Canning
http://www.amazon.com/Ball-Blue-Boo..._1?ie=UTF8&s=books&qid=1249231481&sr=1-1 5 Acres and Independence
http://www.amazon.com/Five-Acres-In..._1?ie=UTF8&s=books&qid=1249231513&sr=1-1
