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Issue link: http://gcmdigital.gcsaa.org/i/178551

research Spring canopy density 0.7 Primo Maxx + 0.61 pound nitrogen/1,000 square feet 0.6 0 Spring GNDVI 0.5 Full 0.4 Split 0.3 0.2 0.1 0 2010 2011 Figure 2. Mean spring canopy density (GNDVI) by fall treatment and year of study (pooled fall application dates). Treatment means with overlapping error bars are not signifcantly different. last two application dates of 2009 showed signifcantly lower tissue-nitrogen concentrations (3.2%-3.3%). The drastic drop in tissue nitrogen for the latter two application dates is attributed to lower temperatures and a reduced rate of physiological assimilation by plant leaf and root tissue following the frst hard frost. The same effect was not observed in fall 2010, when the frst hard frost occurred after all treatments were applied (Nov. 2). Thus, the plants remained more physiologically active throughout the fall and showed a slight linear decline in nitrogen assimilation over the application timings (Figure 1). Regardless, leaf tissue-nitrogen concentration was greater in fall 2009 than in 2010; this is likely the residual effect of yearly variation in summer fertilization practices. Spring canopy density and growth Green normalized differential vegetative index (GNDVI) is a measurement of chlorophyll content and thus quantifes turfgrass canopy density. Figure 2 shows the mean spring canopy density by treatment and year (Figure 2). Over all fall 2009 application timings, Primo Maxx treatments yielded greater spring GNDVI relative to nitrogen alone (Figure 2). In spring 2011, differences in canopy density by treatment were less pronounced, perhaps due to lower winter temperatures and a comparatively delayed spring green-up. Early spring vigor is analogous to shoot growth and is a sure sign that mowing is about to resume. Canopy refectance was measured to calculate the green normalized differential vegetation index (GNDVI). A radiometer was held at waist height and carried the length of the center of each plot. The device measured canopy refectance of 590- and 880-nanometer wavelength radiation. Photo by Max Schlossberg October 2013 GCM 85

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