The latest Mastodon update lets you follow hashtags. When you follow, posts with a tag appear in your timeline automatically.

Try following #Gardening to make posts about gardening appear in your timeline.

This can be REALLY useful for discovering interesting accounts.

To follow a hashtag:
1. Log in through your server's website
2. Search for hashtag
3. Click it to see its results
4. Click on 👤+ icon in top right (click again to unfollow)

Posts from followed tags are visible in the apps too.

@feditips I love this, but I would like an indication in the timeline that reminds me that I follow a hashtag (and which one). Just a little icon would do.
@mattgriffin @feditips yes. A pop up list of followed-hashtags showing number of new posts next to each one, would be the logical next requirement to simulating a groups feature.