I don't believe this 100% true, unless you use an effective ad blocker. Data Management Platforms [0] (such as Krux, Bluekai, etc.) exist to keep track of the different sites you visit so that they can place you into various audience segments. It's unlikely they have a "bonsai" segment but you probably fall into a "gardener" segment, as the sites you visit to read about bonsai trees are most likely classified as a gardening website. Any site can signup with a DMP and offer advertisers the ability to target specific users (i.e. gardeners).

Advertisers are always looking for better ways to target a specific audience, publishers are looking for ways to charge higher CPMs, and DMPs are helping both achieve their goal while tracking you across numerous sites on the internet.

[0] http://digiday.com/platforms/what-is-a-dmp-data-management-p...