I figured out the #beekeeper tag! It works just like with Twitter.
When you write a post about beekeeping, just add #beekeeper to the end of the post.
When you want to search for beekeeping, go to the top of your home page and enter #beekeeping into the search box. Posts about beekeeping will be shown on your screen.
I originally thought that I could add the #beekeeping tag to my profile, but it doesn't look like Google+ works that way.