By reputation and referral. We started with a number of the local people we knew (like Walter Anderson) and expanded from there.
Each time we get someone new to come aboard we ask them who they consider gardening experts and we contact them - and so on, and so on...
We also have a small peer group which reviews qualifications. We're always looking for more qualified experts. It's always nice to hear recommendations from others, so if you know of a gardening expert you'd like to learn more about, let us know and we'll track them down.
We have no set definition of what "an expert" is. If we had to define it, it would be that they've dedicated a large portion of their life to the gardening industry or to teaching others about gardening. But like all things, there are always exceptions.
Interestingly enough most of the people we approach don't like to consider themselves an expert, they're just not like that, the word "unassuming" comes to mind.
The Mulch is about inclusion and good information - so please contact us with your recommendations!