I've amalgamated the low-posting volume Arbutus forum with the low- to medium-posting volume Rhododendron forum into a broader focus Ericaceae forum, with hopefully a bit more traffic (or maybe it will backfire?) The use of the thread prefixes when starting new threads will make it trivial to split off specific groups of the heath family into their own forums if/when the day comes that it is necessary to do so. I think a few people are subscribed to this forum specifically for rhodos (i.e. receive notification of new posts) -- I hope that the slight increase in traffic volume from rhododendron-relatives won't be a reason to unsubscribe.
You might want to title the forum is such as way that all the people coming here with comments and questions in future who do not know botanical family names will then have a greater likelihood of picking up on where to place their posts.
Yes, I've wavered back-and-forth on this -- I'm trying to consider approachability, science and length of forum name, and only ever finding solutions that satisfy me in 2 of the 3 areas. My compromise right now is to use the additional forum description option to help, but you're probably correct that this won't be suitable. (updated 15 minutes later: let's try this -- added a subset of possibilities in association with the family name as a thread title)