Is this a thread on this forum?
These are subs on Reddit (or subreddits, sort of like sub-forums). either google them or use these address
HTTP://reddit.com/r/Gardening or
HTTP://reddit.com/r/Houseplants
Reddit has the ability to sort by a few criteria such as hot, top (in various time frames), new, rising, gilded (these posts got someone to spend actual money to "award" or tip (all money goes to Reddit, but posters get bragging rights and with gold awards sometimes few extra virtual rewards)
More stuff about Reddit here if you're bored:
https://www.reddit.com/wiki/faq
Reddit points matter little, but once you could get near 100k points you get a special invite to closed subs like CenturyClub where people are a bit more open about personal lives. this sub is currently at under 7k members.
Anyone could create a new sub (assuming one doesn't exist) but the key is to get people to join and stay. Basically, Reddit is so massive it has plenty of niche subs for basically everything. I've been a member for over 9 years and still, I barely know the depths of it.