I am interested in this project. It would be a huge undertaking but it would be well worth it. My suggestion would be to have a few broad categories that can be broken into sub categories as content is added. I would start off with the broad categories of:
Hardscaping
Softscaping
Landscape Maintenance
Business Practices
I think everything will fit into one of these categories as a sub-heading and with the right software(?) it could automatically link to other articles that are related just like Wikipedia.
I have always felt that GTX has enough quality information on it to write a landscapers encyclopedia.

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Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways - Beer in one hand - Nacho's in the other - body thoroughly used up, totally worn out and screaming : Woo Hoo, what a ride!