Although a coarse resolution (1km), this location provides some nice maps comparing "greenness" across the US, with cutouts for the wheat and corn belts.  These maps are derived from NDVI values using NOAA AVHRR data.

GreenReport  

Then select the year of interest and time period.  At that point you can look at current "greenness" and differences from last time period, last time period 1 year previous and difference from average at current time period.  The database includes data since about 1989, so is pretty complete for drawing out a good average (minus differences in sensor calibrations and multiple platforms.

Drop me a line if you have any problems accessing the site or have any questions about the data and how it is derived.

 Adios,

 Diet