Specifically, it appears that a 2B Pyro application is needed (see Gauge Unit Theory/Talents) or applying multiple 1A attacks is needed to burn grass.
After some time, the burning will stop. The duration may be related to the theory on gauge decay.
Burnt grass will appear darker than non-burnt grass. After some time, the burnt grass will respawn as non-burnt.
Grass Burning Talents
The following Pyro attacks can immediately cause grass to burn:
- Amber's Charged Shot
- Bennett's Elemental Skill
These are 2B Pyro applications.
The following may require multiple uses to burn grass:
- Diluc's Elemental Skill
- Yoimiya's infused arrows and charged shots
These are 1A Pyro applications.