Spectral Discrimination of Cannabis sativa L.
Leaves and Canopies
C. S. T. Daughtry* and C. L. Walthall*
T
he growing of marijuana (Cannabis sativa L.) on pub- on measured leaf spectra and simulated canopy reflec-
lic lands poses problems to the environment and the pub-
tance spectra, we would choose several relatively narrow
lic. Remote sensing offers a potential way of monitoring
(i.e., 30 nm or less) spectral bands in the green (550 nm),
public lands for the production of marijuana. However,
red (670 nm), “red edge” (720 nm), and the near-infrared
very little information on the spectral properties of mari-
(800 nm) to discriminate marijuana leaves and canopies
juana is available in the scientific literature. Our objec-
from other species. Much of the leaf spectral information
tives were to 1) characterize the spectral properties of the
is also available in the canopy reflectance data. Pub-
leaves of marijuana and various other plants that occur
lished by Elsevier Science Inc., 1998
where marijuana is grown in the eastern United States,
2) simulate canopy reflectance, and 3) identify wave-
bands for discriminating marijuana from other plants. In
INTRODUCTION
a series of replicated field experiments, the basic factors
The growing of cannabis or marijuana (Cannabis sativa
affecting marijuana growth and reflectance, including
L.) on public lands poses problems to the environment
planting date, plant density, and N-fertilization were var-
and the public. Not only are unauthorized disturbances
ied. Leaf optical properties were measured periodically
to the environment created, more seriously, growers of-
during the growing season with a spectroradiometer and
ten set booby traps or post armed guards to protect their
integrating sphere. As N-fertilization rate decreased, the
plots. This is especially important with respect to the use
marijuana plants produced leaves with lower chlorophyll
of our national forests by the public. Remote sensing of-
concentrations and higher reflectance values in the visi-
fers a potential way of monitoring public lands for the
ble wavelength region, particularly at 550 nm. The re-
production of cannabis. However, very little information
flectance spectra of the herbaceous dicot species exam-
on the spectral properties of marijuana is available in the
ined were very similar to the spectrum of marijuana. The
scientific literature.
reflectance spectra of the monocots and the trees differed
The spectral properties of vegetation and soils must
significantly from the spectrum of marijuana, particu-
be understood to identify plant species and to estimate
larly in the green and near-infrared wavelength regions.
plant productivity from remotely sensed data. When
Canopy reflectance spectra of marijuana and several rep-
dealing with remote sensing of specific plants, as in agri-
resentative species were simulated for a wide range of
culture and forestry, the problem is interpreting the re-
LAI and background reflectances. Major differences in
flected signal produced by the soil–plant–atmosphere
canopy reflectance of marijuana and other plants were
system. The vegetation of interest, the underlying strata
observed near 550 nm, 720 nm, and 800 nm. Dense can-
(such as soil, plant litter, other types of vegetation, or
opies of marijuana were more spectrally discriminable
water, etc.), and the intervening atmosphere between the
from other vegetation than sparse canopies. Thus, based
target and the sensor contribute to the sensor response.
However, because plant leaves contribute most of the
signal from vegetation, the spectral reflectance and trans-
* USDA/ARS, Remote Sensing and Modeling Laboratory, Belts-
ville, Maryland
mittance of leaves are primary factors in understanding
Address correspondence to C. S. T. Daughtry, Remote Sensing
the reflectance of the full plant canopy.
and Modeling Lab., USDA/AR, Bldg. 007, BARC-West, 10300 Balti-
For more than 3 decades, scientists have examined
more Ave., Beltsville, MD 20705-2350.
Received 17 October 1996; revised 9 December 1997.
the biological and physical factors that affect leaf reflec-
REMOTE SENS. ENVIRON. 64:192–201 (1998)
Published by Elsevier Science Inc., 1998 0034-4257/98/$00.00
655 Avenue of the Americas, New York, NY 10010 PII S0034-4257(98)00002-9