The Rose Kingdom, which is introduced in Tik-Tok of Oz, has some definite similarities to the subterranean Vegetable Kingdom. The inhabitants, who are live rosebushes, are similarly heartless. Also, Betsy Bobbin and the Shaggy Man follow Dorothy and the Wizard's lead in picking a princess. In this case, however, the roses don't accept Princess Ozga at all, and she's exiled from the kingdom with Betsy and Shaggy. The royalty in the Rose Kingdom aren't exactly like their subjects, instead being human-shaped flower fairies, who become fully human upon leaving the country.
A mysterious lost chapter from Patchwork Girl was also going to feature vegetable people. The text has not been found, but the surviving illustrations show anthropomorphic vegetables growing human-headed flowers in a garden. The publisher deemed this episode too disturbing for children (quite likely because of the possibility that the vegetables were growing people for food), and it was removed.
Ruth Plumly Thompson introduced some of her own plant people in her books. I've already covered Carter Green in this post, but I don't believe I've written anything in my journal about Urtha from Grampa. Like Ozga, she's a flower fairy who serves as the love interest for a male protagonist. She's made of flowers, and her personality is similar to Polychrome's, whom she meets on a brief visit to the sky. Without giving too much of the plot away, I suppose I can say that Urtha is someone whom a wizard planned to turn into dirt, but her inherent sweetness made her into a girl of flowers instead. Prince Tatters and Grampa use water from the wizard's golden can to animate Urtha, and she joins them on their journey.
In his Runaway, which wasn't published until many years after his death, John R. Neill introduced his own plant protagonist in Popla the Power Plant. This incredibly strong bush with the face of a beautiful girl originally grew alongside the road on the Weather Witch's mountain, but the Patchwork Girl transplanted her into a collapsible flowerpot from High Faluting City, and the two of them became close friends and companions. The only problem was that both of these girls enjoyed making up verses, but neither could stand to listen to the ones that the other wrote. At the end of the tale, Popla is given a home in Ozma's palace. And while I'm on the subject of Neill's plant characters, I might as well also mention the patch of sentient potatoes from Scalawagons, led by the tyrannical Dick Tater.
I'll end this post by saying a little about one of the most successful apocryphal Oz characters, the sorcerer-botanist Zim Greenleaf. Introduced in Melody Grandy's Seven Blue Mountains trilogy, the Flying Sorcerer has made his way into other books as well. He and his sister Fern were built out of plants by the legendary wizard Wam (whose exploits had been mentioned in Cowardly Lion and Wishing Horse), and animated with magic powder. When they began to wilt, Wam gave up his own humanity in order to grant life to his plant children. In exchange for his sacrifice, Lurline herself turned Zim and Fern into flesh-and-blood people, and Zim began using his own powers to undo as many of his father's magical mistakes as possible. Melody herself has described his personality as being at least partially based on Sherlock Holmes and Spock, and Zim not only performs experiments in his hidden arboretum and serves the Munchkins as their official wizard, but also has several different alternate identities, each one with its own personality, that he uses for adventures and specimen collecting throughout Oz and beyond. At the end of the SBM trilogy, he marries his old compatriot, the good witch Maggie. As for Fern, there are strong hints that she's the same person as Su-Posy, a butterfly-winged girl with a brief appearance in Yankee, whose job is distributing flowers to the people of Upandup Mountain in the Gillikin Country.