All of our cereal crops are planted and harvested every year. They won't have time to accumulate the damage that we need to shield humans from. Also, plants seem to handle radiation very well - see the growth around Chernobyl[1].

(Note: The danger of eating Chernobyl's foliage is not that it has been irradiated, but that it has ingested radioactive material. That shouldn't happen with cosmic rays.)

[1]https://www.acs.org/content/acs/en/pressroom/presspacs/2010/...