We 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 us ideas for what needs changing in our site tree.

In the design phase, we should "go wide" by creating several alternative site trees incorporating the ideas from our research.

Online testing tools are cheap and fast, allowing us to "go deep" (run several quick rounds of tests), winnowing and refining our 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.