Spectral discrimination of Cannabis sativa L. leaves and canopies

The growing of marijuana (Cannabis sativa L.) on public lands poses problems to the environment and the public. Remote sensing offers a potential way of monitoring public lands for the production of marijuana. However, very little information on the spectral properties of marijuana is available in the scientific literature. Our objectives were to 1) characterize the spectral properties of the leaves of marijuana and various other plants that occur where marijuana is grown in the eastern United States, 2) simulate canopy reflectance, and 3) identify wavebands for discriminating marijuana from other plants. In a series of replicated field experiments, the basic factors affecting marijuana growth and reflectance, including planting data, plant density, and N-fertilization were varied. Leaf optical properties were measured periodically during the growing season with a spectroradiometer and integrating sphere. As N-fertilization rate decreased, the marijuana plants produced leaves with lower chlorophyll concentrations and higher reflectance values in the visible wavelength region, particularly at 550 nm. The reflectance spectra of the herbaceous dicot species examined were very similar to the spectrum of marijuana. The reflectance spectra of the monocots and the trees differed significantly from the spectrum of marijuana, particularly in the green and near-infrared wavelength regions. Canopy reflectance spectra of marijuana and several representative species were simulated for a wide range of LAI and background reflectances. Major differences in canopy reflectance of marijuana and other plants were observed near 550 nm, 720 nm, and 800 nm. Dense canopies of marijuana were more spectrally discriminable from other vegetation than sparse canopies. Thus, based on measured leaf spectra and simulated canopy reflectance spectra, we would choose several relatively narrow (i.e., 30 nm or less) spectral bands in the green (550 nm), red (670 nm), "red edge" (720 nm), and the near-infrared (800 nm) to discriminate marijuana leaves and canopies from other species. Much of the leaf spectral information is also available in the canopy reflectance data. Published by Elsevier Science Inc., 1998.