This may be old news to you and many other seasoned growies but I just found out this year that you can get 4 or so harvests from lettuce just by leaving several inches of core and lower leaves intact (not taking the whole head).
I know you specifically asked for "root crop" an I don't think lettuce is that, sorry - but thought it might be valuable info for you to consider
It just keeps regrowing, need to give it some time to regenerate but it does. Quality will start to diminish and after 4 cuts the farm I'm at had us take them out...I think it still tasted fine and makes me wonder how many other plants can produce more/longer if only partially harvested.