Build a Bigger User Base
More users need to be pulled into the site and linger, using it as a resource and giving back by asking questions and answering them. The larger the user base the more of a hit it can take as some become disinterested in the site. You have to account for loss and find ways of having a higher retention rate on the users that have ...
I've been hesitant to jump in here because much of what I feel has already been said, and very well. This is an important issue, though, so please forgive any redundancies. (Parts of this also apply to a few meta posts, including this.)
In addition to what J. Musser mentioned, there's a significant difference in some of the privileges once the site ...
I believe many repeat users see a forum as a game. When you get a really high rep you have "won" and no longer need to participate.
The traditional way to encourage interest in users who have been around a while is:
customized avatars (everyone can do this)
some kind of badge or star visible to all users that let's you show off your accomplishment
Well I must just be weird - I don't contribute because of the votes at all, evidenced by my presence on three other sites where there is no voting system. I do it because I'm bored a lot of the time, age and arthritis meaning I can no longer do as much gardening professionally as I once did, but I still love it. But then I'm female, and I hate computer games,...
Add self-answered questions to the site on topics that are common, but not addressed here. That way more people will be funneled here by Google. That's how I got here and ended up asking as (admittedly simple) question. The more questions there are here, the more hits there will be from search engines, and the more people will come.
All the other ideas ...
Here's your info, thanks to Rebecca Chernoff:
1247 (non-employee) users on Gardening
1122 registered users
1030 users have an account on SO or SU or SF
1003 users had SO/SU/SF before Gardening
I didn't ask for users who had accounts on SE 2.0 sites prior to Gardening, but as you can see from the above, that number is going to be pretty tiny.
How about bounty?
Don't forget that we cann add bounty to great answers and questions!
Right now even though I just have 300+ reputation I am willing to give bounties on great answers that I've received.
I think more should be done to retain high reputation users. In biological systems, things decay if not fed and watered. So, users need to start suffering erosion of their points unless they keep them fed by earning a minimum in each quarter or other time period.
In other systems just as loyalty programs, you start to lose your points if you don't use ...
I believe more simple features on the site, such as a header plant ident, would allow users to learn more from the site whilst the question/answer feature is left the same.
See my answer on the question - A Gardening.SE showcase