I have a lemon tree that lives in a large pot so it can be brought inside during winter. It produces flowers every few months.
I’ve tried “playing bee” with a small paintbrush, but have had no luck getting the tree to set fruit from the flowers that bloom while the tree is indoors. It does produce fruit from the flowers that appear in the summer, when it is outside. Several lemons ripened this winter from last summer’s flowers; two are still on the tree. We “pick” them when they fall off the tree.