It is certainly useful to learn about the native food resources in your area, but the bottom line for survival preparations - not a very big or dependable factor.

You need to look at natural indicators rather than a calendar - in the areas with which I am familiar (California and the Southwest), the amount and distribution of rain has much more to do with plant productivity than a calendar.

There are native walnuts growing in riparian areas near my home. I can go out and collect the nuts, shell, and store them, acquiring a useful and nutritious food. (Right now is the time to do this.) For a fraction of the effort, time, and expense, I can hit the local big box and get as many bags of processed, ready to eat walnuts as I desire. I make regular trips to the BB and I am comfortable with the amount of food I have laid by.

Still, it is worth learning about the natural products - miners lettuce, collected at lunch time, has improved my crusty sandwiches. I have spent a lot of time on the Channel Islands and I have thought how easy survival would be in that situation. Do like the Chumash did for many millennia - hit the tide pools and get your fill of limpets, mussels, abalone (currently on the wane), and other crustaceans - just look out for the red tide during the summer months. Seals and sea lions are easy to harvest on land - all you need is a club.

Gotta go out and pick up a few avocados and oranges from the backyard. The blackberries have just about finished for the year. I love nature's bounty but I am not about to abandon Trader Joe's - Mormon tea is not really a good substitute for a good Earl Grey.

A real good guide for SoCal environments is "Chumash Ethnobotany" by Jan Timbrook. It also covers nonfood applications of the local flora - a good many of these could be quite useful - weaving baskets, cordage, etc.

If you do come to depend on native foods, you will put in some real long days, spending a lot of time and effort collecting, processing, and storing the fruits of your labor. It is really remarkable how much of a typical prehistoric site is devoted to storage.