Tree Segmentation (SetPropertyBlock)

Hello ! I'm kinda new at Unity and i'm trying to do a segmentation on images coming from the main camera. I got a code which works absolutely fine ... except with trees.

 public void OnSceneChange(bool grayscale=false)
 {
     var renderers = Object.FindObjectsOfType<GameObject>();
     var mpb = new MaterialPropertyBlock();
     foreach (var r in renderers)
     {
         var id = r.gameObject.GetInstanceID();
         var layer = r.gameObject.layer;
         var tag = r.gameObject.tag;
         if(r.GetComponent<Renderer>()!=null)
         {
             Debug.Log(layer);
             mpb.SetColor("_ObjectColor", ColorEncoding.EncodeIDAsColor(id));
             mpb.SetColor("_CategoryColor", ColorEncoding.EncodeLayerAsColor(layer, grayscale));
             r.GetComponent<Renderer>().SetPropertyBlock(mpb);
         }
     }
 }

The code uses the layer of the gameobject. The code print the layer i gave to my trees in the console. There is no errors in the console so i guess it gets a renderer for my trees prefabs but my thoughts is that it doesn't setPropertyBlock(mpb) on the right component. Do anyone have a solution to this problem ?