No, radon is more an issue in a basement with no air flow, where the gas can accumulate. A greenhouse will have plenty of air flow so radon would be no more an issue than if you were standing outside.
I'm not quite a lumberjack, but that's OK, I sleep all night and I dream all day; I'll coppice trees, I'll grow my food, and compost poo and pee! With a well and off-grid solar, it's a permies life for me! https://www.youtube.com/watch?v=FshU58nI0Ts
Not looking good. I think this might be the end. Wait! Is that a tiny ad?