S3·E23 The Great Vegetable Rebellion

Doctor Smith uses the Space Pod to land on a planet that the Jupiter II is passing by. It turns out that the planet has evolved in such a way that plants are the highest form of life, and some of them are quite intelligent. One of these plants, a large carrot, captures Smith and tries to change him into a plant. The Jupiter II lands and the Robinsons and West come to Smith's rescue, only to be captured themselves. Eventually they thwart the plant creature and make their escape.