And maybe our weeding bots could compare spectral reflectance:
"For weed detection in cultivated crops, two interrelated general approaches have typically been used ...The first is to detect certain morphological differences between the crop and weeds...The second general approach is based on differences in spectral reflectance...Despite the different problems encountered thus far in detecting weeds, some researchers argue that the spectral characteristics of plants are sufficient to differentiate plant species without introducing geometric complexities."http://crop.scijournals.org/cgi/content/full/45/2/477
If it works - great (spectral analysis is pretty cheap, and fairly light in terms of the computing power needed). But as your article states, even among the real experts that method is debatable. I imagine (having no knowledge of their field) that their issues are the same as the ones I brought up - variation in the same species, limited variability among species, and the fact that many weed species are the wild variants of the crop.