Well, I have been doing a bit of research at the moment - hardly out of the ordinary for me, I admit, but plants aren't my field of primary training, so learning more is essential. Restate that - learning more is always essential, but building up the basics in additional fields is often necessary.
The twist to it is that I'm looking beyond growing plants for food - yes, essential to do, but easy to forget that they're not only needed for sustenance. The other applications are (as not-subtly suggested above) industrial chemistry and pharmacology. Well, if we don't count fuel, building materials, cotton or hemp for clothes...
Plants are their own little chemical factories, and in a long-term survival aspect we might as well be able to take advantage of what we have at our disposal.
Want to have clothes be other than dust coloured? Grow and use indigo, woad, or any number of other plants. Remember to find suitable mordants, used to fix the dye to the fabric, ideally ones that are easily obtained in your area.
Want to create plastics? Caseinite, created from milk and the right acids, is a possibility. Henry Ford (well, at least his company) apparently created a plastic from Hemp. A number of plant oils can be used as mechanical lubricants...
Consider also the fact that many medicines come from plants - willow, menthol... so many, many others. Paw-paws contain papain, useful for helping people with digestive problems digest proteins. Menthol, a very useful plant... Aloe Vera a hippy favourite.
As bad luck would have it, though, camphor is a proscribed plant in this area - meaning that it's recognised as a pest species, invasive and difficult to get rid of once it's in. So, despite its usefulness, a problem - both to get and to keep under control.
The problem is, though, that there are a number of chemicals still required... Sulfuric acid can do a lot, in an industrial environment, but if you are short of it, you may be reduced to making it yourself - which is fine, if you have a source of sulfur or iron sulfide; if not, you've few options.
At least I'm about twenty kilometres from the ocean - sodium chloride should be easy enough to source... That's something... I guess... Plenty of uses for it...