Think the issue is this isn’t a typical user input since it’s not a selection within the game settings. You have to go find the files and manually edit them. Hence it’s somewhat of a hack. And even ZwiftInsider says editing the config files may cause issues with your game:
Edit: from the ZwiftInsider article further down:
Sets the level of auto-generated scenery – trees, grass, flowers, fruit, underwater kelp, etc.
This is a very effective way to manipulate the environment to suit your preferred FPS.
A number between 0 and 1 is normal.
Higher number increases scenery density / realism but decrease FPS.
Lower number decreases scenery density / realism but increases FPS.
Examples: * 0 = 0% * 0.25 = 25% * 0.5 = 50% * 0.75 = 75% * 1 – 100% * 5 = 500% – be very careful!"