Answer: I can't identify for sure the plant you are talking about. There are two plants, not true turnips, but have that in their common name: Turnip-rooted chervils (Chaerophyllum spp.) are annuals or biennials, with feathery leaves; one type, C. bulbosum, grows from 3 to 6' tall and has edible roots. Another plant, Indian-Turnip (Arisaema spp.) are related to calla lilies and caladium, and are most often found in moist woods. Though they have tuberous roots, I could not find mention that they are used as food. Of the true turnips, Brassica rapa, I can't find information about a wild type.
I suggest you contact one of the following companies that specialize in native plants:
Prairie Moon Nursery, http://www.prairiemoonnursery.com
Prairie Nursery, http://www.prairienursery.com
Prairie Ridge Nursery, ph# 608-437-5245
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