SPECTRAL DISCRIMINATION OF CANNABIS-SATIVA L. LEAVES AND CANOPIES

The growing of marijuana (Cannabis sativa L.) on public lands poses pr oblems to the environment and the public. Remote sensing offers a pote ntial way of monitoring public lands for the production of marijuana. However, very little information on the spectral properties of marijua na 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 easter n United States, 2) simulate canopy reflectance, and 3) identify waveb ands for discriminating marijuana from other plants. In a series of re plicated field experiments, the basic factors affecting marijuana grow th and reflectance, including planting data, plant density, and N-fert ilization were varied. Leaf optical properties were measured periodica lly during the growing season with a spectroradiometer and integrating sphere. As N-fertilization rate decreased, the marijuana plants produ ced leaves with lower chlorophyll concentrations and higher reflectanc e values in the visible wavelength region, particularly at 550 nm. The reflectance spectra of the herbaceous dicot species examined were ver y similar to the spectrum of marijuana. The reflectance spectra of the monocots and the trees differed significantly from the spectrum of ma rijuana, particularly in the green and near-infrared wavelength region s. Canopy reflectance spectra of marijuana and several representative species were simulated for a wide range of LAI and background reflecta nces. Major differences in canopy reflectance of marijuana and other p lants were observed near 550 nm, 720 nm, and 800 nm. Dense canopies of marijuana were more spectrally discriminable from other vegetation th an 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 discrimi nate marijuana leaves and canopies from other species. Much of the lea f spectral information is also available in the canopy reflectance dat a. Published by Elsevier Science Inc., 1998.