NDVI is a direction a lot of drone people are going, and probably a good place to start to get a sense of techniques and work being done in plant monitoring:
https://agribotix.com/blog/2014/06/10/misconceptions-about-u...
https://en.wikipedia.org/wiki/Normalized_difference_vegetati...
More ideas: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4600171/