You certainly could be correct that it means a salad in a poke (bag). When I was a kid a big plant with huge leaves grew wild on the farm, and we called it "polk greens". My mother would pull leaves off the plant, which looked a lot like a big weed with thick leaves, and make a salad which we called polk salad. As I recall the taste was about like what you would expect cardboard to taste like. And it really was the only thing in the barnyard that the cows wouldn't eat.