Overgrown (Portal 2)
The overgrown theme is displayed near the beginning of the single player campaign, in the chambers closest to the earth's surface. It is characterized by rusted, decayed, and moldy areas filled with debris, swaying vegetation and cables. The lighting is harsher than other areas in the game, having bright ceiling lights above plants and wires that cast shadows onto the chamber's playable area. Soundscapes include noises by crickets, cicadas and birds.
After your encounter with GLaDOS, you are dropped much deeper into the facility and the sounds of animals and the presence of plants is significantly diminished.
In able to have swaying vegetation in your map you must have an active env_wind entity and your plant models must be prop_static with the Ignore surface normal for computing vertex lighting set to 'yes' or '1'