I have combined information from several different sources, but this is what I have read. There are actually trees in Australia and Africa called bottle trees.
The slaves brought the idea of the traditional bottle trees, live trees with colored bottles on the ends of branches, into this country. It has been told that bottle trees protected homes from evil spirits by trapping spirits inside the bottle, where they could do no harm.
Some say that bottles were hung from trees by strings, in hopes that any devilish spirits lurking near the house would be attracted to the colorful bottles shining in the dusk. The spirits would become mesmerized by pretty colored bottles and become trapped inside the bottles. The bottles were periodically corked and thrown into the river to wash away the unwanted spirits. Another swing to the story is that the spirits become trapped in the bottles, and are then destroyed when the sun comes up.
Sometimes the bottles, then made of hand-blown glass, were hung by strings. But mostly they were put on the tips of a tree. Although bottles of many colors were used, blue was thought to be an especially potent spirit repellant. It is still common to see homes in the south with their doors and window frames painted blue. It was thought that the color blue has the power to repel or overcome evil.
We ordered some bottle trees for the shop. Needless to say, I brought one home today. I already have a cobalt blue bottle to put on it first. Please feel free to let me know if you have any other information about bottle trees!