This took me 3 runs through the episode to get this, so I want to make sure it's clear.
You need to find everything during the greenhouse scene.
You have to find outside of the greenhouse as well. You probably don't need to do all of this, but here's a list of everything that you can do.
Boar Head (collectible)
Inspect the glass windowYou cannot leave the greenhouse once you climb the ladder.
Mushroom (collectible; eat a second one for the Edible achievement)
Pry bar (necessary)
Trowel (in the planter bed)
ShearsYou cannot go back to the first room once you open the lab door.
Venus Flytrap (collectible)
Test tubesMake sure you get all of the above before you get both of these:
If you do all of this, you should also pop: