Almost all leaves are a pretty similar shade of green, and from the altitude of a satellite a single pixel must be bigger than a whole bush. Yet satellite imagery has become central to detecting illegal drug crops - even though farmers take extensive steps to hide the crops by interspersing the plants among legal crops and planting small, irregular fields.
The World Drug Report 2010 explains a little, but also makes it sound extremely hard:
Satellite images contain more information than a normal photo, since the satellites have extra sensors to register infrared colours. Precisely these infrared colours help to distinguish vegetation types. [From footnote:] Coca is a bush and the leaves can be harvested throughout the year. Therefore, the crop stage can vary by field but also within a field. At the same time a satellite image is taken, a field can contain different crops in different growing stages, which give a different appearance in the satellite image.
So it has something to do with infrared information, but how does that give such specific information that an individual kind of plant can be distinguished with any confidence at all, even despite efforts to hide it, and confounding factors such as weather, slopes, and a wide range of mixed developmental stages?