So I had a spot of bad luck the other day, that I initially thought was good luck because I stumbled across a turnip plant fairly early on in my playthrough and went and planted it back at home. Unfortunately I was then attacked by trolls and one of them smashed my garden to pieces killing all but a couple carrot plants, destroyed my turnips before they ever grew.

I decided today to go back to the swamp and attempt to find more, I of course couldn't find any and decided to google search and found posts made by other players mentioning how rare it is that they find Turnips too. I returned to my game and not 10 seconds later I rounded a tree and spotted a plant, what luck! Then a stupid damn skeleton ran up to the plant and destroyed it before turning and charging at me.

So it appears to me that maybe Turnip plants aren't quite as rare as they seem, enemies are likely aggroing on the plants before players even notice them and destroying them. After killing the skeleton I couldn't find any seeds laying anywhere around where the turnip plant was.