Hi ThisGuy,
I am away from my computer at the moment so I can’t be too technical in my answer but I would make a blueprint out of your mesh that will be on fire and make a blueprint out of the foliage and add a sphere component A bit bigger than the meshes. Then I would add a blueprint interface to blueprints and have a collision between two spheres (actor on fire and a shrub for example) trigger an event that will set the shrub on fire. You can pass the event to the actor by getting the other actor from the collision event and using that as the target for the event. Let me know if you need more help and hopefully I’ll be on my computer to better help you. Good luck!