When should we do a tree test?

Tree testing is designed to give us rough answers early in the design process:

Initial research
Along with the other up-front research we would normally do (surveys, interviews, card sorting, and so on), it’s useful to do a baseline tree test of the existing site. This shows us problem areas to investigate, and provides a benchmark score that we can compare our new site structure to.
Early design phase
It’s common to start drafting our site structure while doing the first conceptual round of interaction design. As soon as we have one or two tree ideas roughed out, we can run tree tests to see how well they work (compared to each other and the existing structure), and what needs to change.
NOT late in the project
Once we have a working prototype or a real site constructed, it’s better to do traditional usability testing because it combines everything that affects our IA - not just organisation and labelling, but also navigation, page design, visual design, and content.