The 2009 ASHS Annual Conference

Given the number of species grown as ornamentals, the appropriate fertilization rates for different species is still largely unknown. High rates of fertilization may result in increased growth, but may also contribute to leaf characteristics that make trees more attractive to insects. We tested the effects of five rates (0, 12, 24, 36, and 48 g N) of controlled release fertilizer (19-5-8, 8-9 mo.) on the growth, appearance, and nutrient concentrations of four common ornamental shade tree species (Acer rubrum ‘Red Sunset’, Acer rubrum ‘Sun Valley’, Gleditsia tricanthos ‘Skyline’, Malus hybrida ‘Prairiefire’, and Pyrus calleryana ‘Redspire’) in 15-gal containers. Relative height (RHI) and caliper (RCI) increased linearly with nitrogen rate in all species. However, RCI was not different between the 36 and 48 g N·plant^-1 in any species. The red maples displayed the largest increase in RCI (2.3 times the control), while M. hybrida and P. calleryana RCI increases were much less (1.3 and 1.7 times the control, respectively). Individual leaf area was not larger with fertilizer rate in spring or fall sampling, except in M. hybrida in the fall. Succulence and specific leaf weight (SLW) did not change with fertilizer rate in the spring, but succulence increased and SLW decreased linearly with fertilizer rate in all species. Leaf nutrient samples were collected from all trees except ‘Red Sunset’ maple. Nitrogen increased with fertilizer rate in all trees analyzed, however, there was no leaf N difference among any of the fertilized trees in P. calleryana (compared to control). This pattern of leaf N was correlated with the appearance of the trees: appearance of red maple trees increased with fertilization, whereas there was no visual difference among the 3 highest fertilizer rates in ‘Redspire’ pear. Leaf Mn concentrations differed dramatically among the species. The highest fertilizer rate resulted in a 5.2 greater concentration of Mn over the control plants in ‘Sun Valley’, while ‘Prarifire’ and ‘Redspire’ were 2.2 and 2.9-fold higher, respectively. This data suggests that fertilizer rates can be reduced significantly without a negative effect on groth or appearance in some species, but not others.