People hear radiation and lose rationality because of the fear mongering in the 50s and beyond.
Probably not, but only because "That's silly, bananas are just fruit!" rather than because of any scientific study I've seen or done. So you really shouldn't listen to me.
Just because of where the article is listed, I've always just assumed it to be crackpottery.
All potassium is always radioactive. Unless your have a gas centrifuge facility next to your greenhouse, all your bananas are always radioactive.
While practically true, this is sufficiently imprecise to bother the more pedantic variety of nerd; for accuracy I therefore quote the relevant section of Wikipedia (a resource we should give daily thanks for):
"Naturally occurring potassium is composed of three isotopes, of which potassium-40 is radioactive. Traces of potassium-40 are found in all potassium, and it is the most common radioisotope in the human body."