Any recommendations for a PC program that I can use to keep up with what I've planted where?

I currently have all of my notes in a Notepad file (.txt), but it's gotten really, really, really long. And when I move something from one garden to another, there's a good chance of duplication.

I have 18 separate gardens on my property, so the ideal program would let me create unlimited tabs (one for each garden) and make whatever notes I want under each tab, structured ever how I want. I don't want anything super complicated with a million features that I have to learn, I really just need a way to organize my notes better.