There are many ways of converting to B&W. You do not mention the software you are using. That matters. Some of the B&W conversion tools in recent software are excellent and give you a lot of control.
For example, if you have Lightroom or Adobe Camera Raw 4.0, you can do an excellent job of B&W conversion right there in the RAW processor. You have separate control over the conversion of several color ranges. With Lightroom, you can even "scrub" over the photograph to lighte or dark features, like grass.
The B&W tool in Photoshop CS3 also gives you a lot of control. It also allows you to scrub features in the photograph.
With earlier versions of Photoshop, you can apply some B&W filter effects. I have a free set of actions for doing that. Likewise, I have free actions for toning B&W photos.
There are also techniques that allow you to affect the conversion for color ranges. A popular technique is to pair two Hue/Saturation adjustment layers. One to affect the color conversion, the other to desaturate the photo. You can replace the layer for adjusting color with Curves, Selective Color, etc. The idea, as you suggest, is to affect the color of the greens, for example, so they convert in a pleasing way.
Cheers,
Mitch