A lot of great things have been said here, so I don’t think I have much to add in the plant department. However:
BEES. There would really need to be beekeping.
Bees are the major pollenators on this planet. Wasps and flies also contribute, but not nearly enough to rely on them. The sad thing is: The bees that exist in the western world mostly do need beekeepers to even keep them alive. Parasites and illnesses are a real issue. My father is a beekeeper and he once lost 29 of 30 hives in a winter because he hadn’t looked after them enough. (Mind you, he HAD done treatments, just not enough of them.) That’s how bad it can get. So I do think your survivor’s community should be keeping bees to not only sustain their crops but even make them come into existence.
The major parasite those beekeepers need to fight is (I just discovered) aptly named the varroa destructor in English. If you’re still looking for a villain - this name would work They can be fought with things such as essential oils and powdered sugar if you’re in a pinch, but also with various acids - that need to be food safe if you want to eat the honey in the end.
There is no need for fancy beekeeping gear. Those weird suits are for people who are afraid to get stung - if you’ve gotten stung a few times (and are not allergic), you get used to it and barely notice it anymore. If you want to harvest the honey, you might need equipment - or you could always eat it with the comb, unless you want to use the wax for something else. (Which you could. You could use it to make wax cloths, which are basically nature’s cling film. Or you could use it to make salves.)
Other things you could make/harvest if you have bees: Propolis (disinfectant), pollen (source of vitamins and protein), mead (alcohol, doesn’t need much processing or skill), larvae (the male larvae are usually removed if the queen bee is already fertilized as they are a drain on resources and a health risk. as gross as that may sound, they are another source of protein).
Things that honey could be used for: treating wounds, washing hair, making mead, a treat, cosmetic purposes, making medicine (such as cough syrup, works well with radishes). It is also SAID to help with a whole bunch of other things such as hemmorhoids, allergies and insomnia, but I have no experience with those things (and immediately get suspicious if any list contains the phrase “can help in curing cancer”), so I can’t really speak on that.
Bees can easily be kept on top of roofs where they’re safe. You’d just need to be careful with storms.