You can try to spreads in social media such as Facebook, Twitter, and invite all of your friend's lists to join in your forums. So your forum is about plants and animal, I think that is a good idea since people still stay at home, they can start to have their own yard
Introduce a forum to people is not easy, and that needs an effort to do. But if you are serious with your project, I think nothing is impossible. Good luck @OP