You should do tree testing early in the design process.
In the initial research phase, an open card sort and a baseline tree test of the existing site will give you ideas for what needs changing in your site tree.
In the design phase, you should "go wide" by creating several alternative site trees incorporating the ideas from your research.
Online testing tools are cheap and fast, allowing you to "go deep" (run several quick rounds of tests), winnowing and refining your ideas down to a single high-performing structure.
For evaluating site structures early in the design process, tree testing is more effective than closed card sorts.
Next: Chapter 4 - Planning a tree test