16 Green Media / August 2014 www.greenmediaonline.com
B y D r . K e n C a r e y
T
urfgrass managers can tell a lot about turf just by look-
ing at it — and the more experienced they are, the
better their judgment. Nutrient status, pest damage,
abiotic stresses (drought, traffi c, etc.) are all visible to the
trained eye. Sometimes, however, it's good to have some
tools to help; the highly trained manager may not be avail-
able to see everything, or the problem may produce very
subtle effects. This article discusses some recent innovations
in assessing turfgrass, developed and widely used in turf
research, which might be useful to the turfgrass manager.
What we see when we look at a turfgrass sward could be
termed "canopy refl ectance"; it's just the ambient sunlight re-
fl ected off the leaves in the full visible light spectrum. A trained
researcher or turfgrass manager learns to record and interpret
the details of what they see, whether it's the off color of nutri-
ent defi ciency or spray damage, or the darkening of drought
stress. However, both in research and in practical management
situations, we work with less well-trained helpers, and will
benefi t from techniques that remove the subjectivity and ob-
server bias, and reduce the need for training.
One very familiar tool is a camera, and with improved
digital cameras this is a very useful adjunct to assessing prob-
lems. However, even though they can form an important
permanent record, the digital photos still need to be inter-
preted. Researchers are working on improving software to
analyze digital images to document and quantify turf charac-
teristics (weed and disease infestation, drought and nutrient
stress), but these full-spectrum techniques are still relatively
early in development for widespread turf use.
A more mature, and somewhat simpler, technology for
assessing turf involves restricting the wavelengths observed
to ones that we have learned through experience are indica-
tive of turfgrass problems. Photosynthesis in plants involves
chlorophyll absorbing light to power the plant, and the wave-
lengths that chlorophyll absorbs are a subset of the sunlight
hitting the plant (Fig. 1). Light that chlorophyll absorbs is not
refl ected, and the light hitting the plant looks different from
that refl ected. Of the visible wavelengths, chlorophyll absorbs
red light, generally, so the light refl ected is white minus red
= green. The wavelengths that chlorophyll absorbs are often
termed photosynthetically active radiation or PAR.
Various sensors have been developed that all function
TURFGRASS MANAGEMENT TURFGRASS MANAGEMENT
USING CANOPY
REFLECTANCE TOOLS
IN TURFGRASS
MANAGEMENT
Figure 1. The peaks show the wavelengths of visible light that are
absorbed by chlorophyll (Photosynthetically active radiation).
Figure 3. Measurement (660 nm) and reference (770 nm) wave-
lengths used by the Greenseeker to calculate NDVI.
Figure 2. Measurement (ρVIS) and reference (ρNIR) waves of light
refl ecting off bare soil and turf.
