How long can Gilfeather turnips stay in ground?

I'd say you'll be right for another month. I didn't even know what a root cellar was till you mentioned it. The way I grow turnips is to broadcast the seeds in early Autumn(March). Then when the largest bulbs get to be about one inch in diameter I start picking. That thins them out and gives space for the smaller ones to grow. From September onwards they start to go woody. My trick to deal with that is to peel off the outer layers, when that doesn't work I just let them go for a little longer to collect the seeds. When that happens about November, it's time to transplant the corn, tomatoes and cucumbers etc. into the spots where they used to grow. This upcoming summer I'm going to plant a small plot of turnips because I noticed some escaped weeds growing like wildfire during the summer. Hope you enjoy all that turnipy goodness...