ask for Cheap shawdow sulotion for lots of foliage

recently, I make a mobile game which has hundreds foliage tree in screen, And The foliage tree can be removed. I can’t use lightmap shadow,because after i remove the foliage,It’s shadow still exist.And i can’t use dynamic shadow too because of it’s great performance hit.How can i add shadow to my foliage tree.The shadow doesn’t need to be realistic.Thanks

up up,thank you.

Decals maybe ?