The giant cactus with the Spanish name is the boon companion of the eccentric retired teacher Archie Brubaker. Señor Saguaro isn't just a plant to Archie; he's a conversation partner. Though admittedly their conversations are a little one-sided.
Nevertheless, Archie is clearly one of those people who finds great wisdom through contemplating the world around them. Nature is often the source of answers to many of life's most perplexing questions, and as Archie is regularly engaged in providing answers to some of those questions for the benefit of the kids in the neighborhood, his regular consultations with the colossal cactus in his backyard are perhaps not quite as weird or as eccentric as they might first appear.
As well as providing answers, Señor Saguaro is also pretty adept at asking challenging questions that force Leo to think about his relationship to Stargirl. There's something more than a little unusual about her; it's as if she's a force of nature. That being the case, it makes sense in a funny kind of way to find out more about her by consulting a feature of the natural world—something that can relate to Stargirl in a way that no mere human can.