The neighborhood kids think Gloria Dump is a witch. And why not? She's got a big gnarly tree in her backyard with clanking whiskey, beer, and wine bottles hanging off it.
It's Gloria's mistake tree, meant to scare away "the ghosts of all the things I done wrong" (14.22). Why the bottles? She's an alcoholic. Hanging the bottles also reminds her never to use them again.
The tree also teaches Opal loads of lessons.
As Gloria says, you can't judge people "by the things they done. You got to judge them by what they are doing now" (14.40). Opal keeps this in mind with ex-con Otis, who is kind and good to the animals in the pet store.
Opal's mom made mistakes (yeah, she left!) that probably haunt her. Plus, Amanda Wilkinson's brother was her ghost, resulting in Amanda acting snotty and rude to others.
Opal decides to read to Gloria "loud enough to keep the ghosts away," "those ghosts chattering about the things she had done wrong" (15.4). And the love that Opal gets back helps her deal with the "ghosts" of her mother's absence.
And when Opal finally gets rid of her ghosts? Yup, she heads back to the mistake tree to say her goodbyes and finally move on.