custom foliage placement - need tips to increase performance

Hi,
i wrote a script which renders some foliage on a mesh terrain. The problem is, it drops the framerate from about 260 down to 90. I really dont like that..

I'm looking for ways to improve it or maybe write a new and better one.

I hope the following code can explain what it does because if i put that in words, it would be even longer:
(in short: Raycast from a grid around the player down to ground, get pixel, if pixel.green>0.5 draw a mesh with color from that pixel)

 void DrawFoliage () {
         if (draw == true) {
             Collider[] cols = Physics.OverlapSphere(transform.position, radius, groundLayer);
             if (cols.Length == 0) {
                 return;
             }
             
             foreach( Collider col in cols) {
                 if (col.renderer == null || col.renderer.material == null) {
                     return;
                 }
                 FoliageHolder fol = col.GetComponent<FoliageHolder>();
                 Texture2D tex = col.renderer.material.GetTexture("_MixTex") as Texture2D;
                 float strength = col.renderer.material.GetFloat("_LightMix");
                 
                 if (tex != null) {
                     for (int a=0;a<radius;a++) {
                         for (int b=0;b<radius;b++) {
                             Vector3 gpoint = transform.position+new Vector3((-radius/2)+b, 0,  (-radius/2)+a);
                             if (Vector3.Distance(gpoint, transform.position)<radius/2) {
                                 gpoint = new Vector3(Mathf.Round(gpoint.x),gpoint.y, Mathf.Round(gpoint.z));
                                 if (Physics.Raycast(gpoint+Vector3.up*(radius/2),Vector3.down, out hit, radius*2, groundLayer)) {
                                     Color fcol = tex.GetPixel(Mathf.RoundToInt(hit.textureCoord.x*tex.width), Mathf.RoundToInt(hit.textureCoord.y*tex.height));
                                     Debug.DrawRay(gpoint+Vector3.up*(radius/2), Vector3.down*(radius*2), fcol); 
                                     if (fol != null && fol.foliageMesh != null) {
                                         if (fcol.g>0.5f) {
                                             MaterialPropertyBlock propblock = new MaterialPropertyBlock();
                                             propblock.AddColor("_Emission", new Color(fcol.r*strength/2, fcol.g*strength/2, fcol.b*strength/2, 1.0f));
                                             Graphics.DrawMesh(fol.foliageMesh, hit.point, Quaternion.FromToRotation (transform.up, hit.normal), fol.foliageMaterial, 0,null,0,propblock);
                                         }
                                     }
                                 } else {
                                     Debug.DrawRay(gpoint+Vector3.up*(radius/2), Vector3.down*(radius*2), Color.red);
                                 }
                             }
                         }
                     }
                 }
             }
         }
     }

Thank you.