Love the toxic colour filter; a bit of vignetting at the top might look good.
About the buttercups, I made some large dense fields for a BF2 mod (hence it needed to draw fast & work across specs) & the best solution was an irregular grid of very long polys (the width of the field & chest-high, 0.5m cells) textured with something like a 2048x256 1-bit alpha map. The key to getting nice transparency was to custom edit each mipmap level in photoshop, using things like unsharpmask followed by threshold to get the alphatest cutout perfect for each progressive mipmap.
To edit, there's an option in the nvidia dds p/shop loader to open mipmaps, & for saving there are more options where you can define how many levels of mips you want (try saving just 5 & see how sharp it looks).