Using transparent image planes is a standard practice for creating foliage models but there is a big flaw where the planes are lit too "flat" and create harsh shading. Fortunately there is a method of editing the vertex normals to drastically improve the overall lighting of planes in a cluster (Remember in goldsrc there is no "nocull" texture setting for double sided rendring so you must make each plane with a flipped duplicate for the opposite side).
Get "Slidenormals Thief" maxscript. Run the script and set up a UI shortcut from "customize user interface" if you want (its found under "Category: Slidetools").
Then select your object and add a "edit normals" modifier. Create a sphere or half-sphere (depending on how you want your lighting to be affected) and shape it over your plane cluster. Add an "edit normals" modifier to that too.
Open up slidetools and choose the reference and destination object and press steal. Hide or delete your reference object and now you have a model with modified normals. The normals should all be pointing outwards in a uniform way. (note that some versions of max may make the realtime lighting look wrong, but try exporting and verify your results).
Collapse the edit normals modifier and export your SMD. If you are using a more recent version of 3dsmax (2013+) use either wallworm exporter or Game Zombie SMD tools.
Compile your model, then verify your edited normals in HLMV by changing the render mode to "Smooth Shaded" it should look like a nice gradient.