use the classic shadow map cascade. or vsm. that is for directional lighting. the latter seems to have improved the memory footprint. so… you should need raytraced shadows only for very specific shadow scenarios (like overcast lighting. still gotta deal with shadow flicker in that case tho. or find alternative ways to move the foliage and update the bvh. like skeletal mesh trees and procedural animation. this is a lil expensive, tho. : )
for regular instanced static meshes you should have the…
r.RayTracing.Geometry.InstancedStaticMeshes.EvaluateWPO 1
…cvar. it does not work for explicitly painted foliage, tho. i don’t have a test case for this and hism right now. sry.