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.