iOS - shadows not appearing for double-sided panes

I've got a script which procedurally creates trees, and each branch segment consists of a double-sided plane (4 vertices with 4 tris). The material contains a transparent texture for the leaves, and a custom shader:

Code (CSharp):

1. Shader "Unlit/Transparent Cutout (Shadow)"
2. {
3.     Properties
4.     {
5.         _Color   ("Main Color", Color) = (1,1,1,1)
6.         _MainTex ("Base (RGB) Trans (A)", 2D) = "white" {}
7.         _Cutoff  ("Alpha cutoff", Range(0,1)) = 0.5
8.     }
9.     SubShader
10.     {
11.         Tags
12.         {
13.             "Queue" = "AlphaTest"
14.             "IgnoreProjector" = "True"
15.             "RenderType" = "TransparentCutout"
16.  
17.         }
18.         LOD      100
19.         Lighting Off
20.         CGPROGRAM
21.         #pragma surface surf Lambert alphatest:_Cutoff
22.         sampler2D _MainTex;
23.         fixed4 _Color;
24.         struct Input
25.         {
26.             float2 uv_MainTex;
27.         };
28.         void surf (Input IN, inout SurfaceOutput o)
29.         {
30.             fixed4 c = tex2D(_MainTex, IN.uv_MainTex) * _Color;
31.             o.Emission = c.rgb;
32.             o.Alpha = c.a;
33.         }
34.         ENDCG
35.     }
36.     Fallback "Transparent/Cutout/VertexLit"
37.     //FallBack "Diffuse"
38. }

My problem is that while shadows appear to be fine on my mac, on iOS they don't appear. I think it's down to the fact that the planes are double-sided. If I add the material to a cube, the shadows appear fine on iOS. As there are many branches, I'd like to keep the vertex count low. Any ideas? Thanks.