Which is why along with your food you need to stock up on garden seed. I prefer heirloom seeds because they haven’t been tinkered with- but most seeds are fine anyway. If seeds are kept in a dark, cool, dry place many will be good for years in case you don’t grow a garden every year- but I do recommend replacing your seed stash every couple of years. I usually have a garden and seeds I don’t use I pass on to friends and buy fresh ones to keep seeds on hand that will be good if things go to crap. Remember some civilizations were built on corn, beans, and squash so you can use that for the core of your seed saving and add other things that you like.
To really be prepared you need a short term plan- in case you are evacuated or lose power for a few days. You need a medium term plan in case things fall apart for weeks, and you should have a plan in case things fall apart permanently and you have to start over from scratch.
Being raised in the southwest, priorities for survival were always crystal clear. You have to have water, and you have to have electricity. But beyond that, everything is negotiable. And that is a good philosophy to have in a survival situation, because the idea of living “as an island” has some serious flaws.
I am dubious about home gardening in most circumstances, because except under very strict limits, it cannot support you. Fortunately, in most of the US people can raise potatoes, but that is one of the few plants that can provide substantial nutrition in a small crop area, and with limited fertilizer, pesticide, and protection from birds and animals.
Were there to be such a catastrophe that most people would have to leave the southwest, it would be wise to leave with them.
There is a functional limit to how much society can collapse if there are people still around. Collectively people have enormous resources in the US, and are practiced in organizing for mutual benefit.