I used Pixelmator Pro to select the plant, then added a mask to reveal only the plant, and the added a black layer underneath. It took me forever to learn how to do this, but now I can do it in a few minutes. Now Pixelmator Pro has a "remove background" feature that accomplishes it easily. You still need to put a black layer underneath, but it is very fast. Pixelmator Pro requires a computer, and I prefer to work on my iPad. Affinity Photo can do the same thing on my iPad, but I have been very slow to learn it.
I am glad to know about the other ways of doing the same thing that several of you mentioned in this thread.
