I don't think it should be legal, with a few small exceptions.

Patents are for novel processes and inventions. You can't patent ideas, you can't patent natural processes, and you can't patent things that are obvious to make. An example of 'obvious to make' is something like a hammer. You can't patent a basic hammer. You can patent a novel way to make hammers, or patent hammers that do an entirely new thing like self right a nail, but not just plain ordinary hammers.

Similarly you can't patent things that grow themselves. You can't patent a tree. You can patent the process to create it, but if it also grows from saplings and someone gets a hold of them, you can't patent that because they can grow naturally and it doesn't use your process to grow new trees.