Is it too late for me to start Castor seeds in zone 6b? And would it be possible to grow them in 5 gallon containers?
I think you can still do it. You will get a good sized plant by frost. I grew one in a large pot (probably 4-5 gal) last year and it did fine. It was only 3-4 feet high. I don't know if that was the cultivar (got seeds from a friend with minimal info) or the pot.
Not too late. Of course earlier would be better...5 gallon pot would be pushing it, but it will work... they just won't get very big (tall). I had a "wild" one in a 10 gallon pot last year that was 8' tall. My others were 21'. Btw, the seeds should last for a few years so save some for an earlier start next year.