Looking like an ordinary tree

[Kromm]

Baseline appearance really is just a 0-point feature if it doesn't generate any reaction modifiers.

Example: One person is a serial killer who looks like an ordinary Joe and doesn't have disadvantages that require him to foam at the mouth and brandish knives. Another person is an ordinary Joe who doesn't kill people and maybe even has Pacifism (Reluctant Killer). You can't tell the apart at a glance. To do that, you need to speak with them a bit or observe them for a while.

It's the same with plant monsters: Two willow trees, one of them murderous and animate, the other just a tree, are indistinguishable until somebody applies suitable observational skills to the puzzle. To ensure that the murderous tree isn't singled out at random for a spot check, effectively giving the adventurers a passive skill check to notice trouble, Honest Face should suffice.