When can a gift of landscaping materials be demanded back from a former neighbor?

Gifts never have to be given back unless they are conditional. The best example of that is an engagement ring where the marriage never happens.

Under contact law, there must be an offer and consideration. Here she offered the material to you. You accepted by donating your labor to remove the items. So, a valid contract was made. Also, it can be treated as a gift. She would have to show that she gave you the items for keeping and you failed to give it back. Let her take you to court.

If the materials were given to you without anything in writing requiring that you store them, pay something for them, etc. the court would have to presume them to be a gift. Your former neighbor would have a hard time proving otherwise. However, to maintain peace and because it's less hassle (even if a false claim is filed with the court, you still have to respond to it) if you still have the materials, give them back. If you no longer have them, or their used in a way that makes it difficult to return them ( fence posts that would require you to tear down the fence) then send a friendly letter stating what the reality of the situation is. If it's plants, dig them up and return them or if perennials, divide them and return as much of the original as possible.

A gift, once delivered, transferred title. The materials are yours.