Determination of Optimal Controlled Release Fertilizer Rates for Container Nursery Crop Production in Cold Climates

Region specific trials examining optimum controlled release fertilizer (CRF) rates for the Canadian climate are limited. Most studies are conducted in temperate to warm regions of the United States and use a limited number of plant species and growing substrates. Accordingly, this study was conducted to evaluate the effect of CRF application rates on the release of nutrients and on the growth of 17 economically important container-grown ornamental shrubs, using three growing substrates and fertilizer types at three southern Ontario nurseries. Five different fertilizer rates, ranging from N at 0.60 kg∙m^-3 to 1.95 kg∙m^-3 for each fertilizer type, were incorporated during potting of plug-rooted liners between 29 June and 10 July 2012. Plant performance (i.e., shrub height and growth index) and leachate EC and pH were evaluated once every three to four weeks during the growing season. There were significant differences (P < 0.05) in shoot dry weight, leaf area, and the concentration of nitrogen (N), phosphorus (P), and potassium (K) in the shoots and root ball between the different fertilizer rates and substrate types. Of the 17 species observed in the trial, none performed best at the highest or second highest CRF rate (i.e., N at 1.95 kg∙m^-3 and 1.80 kg∙m^-3, respectively). Buxus ‘Green Velvet’ achieved optimum growth at the third highest CRF rate (i.e., N at 1.65 kg∙m^-3) and Hydrangea macrophylla, traditionally classified as a heavy feeder, performed best at a fertilizer rate with a low level of N at 0.75 kg·m^-3. Both species were grown in a substrate containing no compost (i.e., 3 parts aged pine bark, 1 part Canadian sphagnum peat moss, and 16%–17% perlite). The remaining 15 trial species had optimum fertilizer rates at or below N at 1.35 kg∙m^-3 .  Given a growing substrate with 15% compost, there was no statistical difference in growth between different fertilizer rates for Thuja plicata 'Whipcord', Euonymus forunei  ‘Harlequin’,  Forsythia x intermedia 'Fiesta', Weigela florida ‘Variegata’, Cornus sericea 'Kelsey', and Cotoneaster dammeri ‘Coral Beauty’.  Significant differences (P < 0.05) in growth occurred between fertilizer rates for Salix purpurea ‘Nana’, Cornus sericea ‘Cardinal’, and Hibiscus syriacus ‘Ardens’ grown in a substrate containing 60% composted pine bark, 10% compost, and 30% peat moss.  Nitrogen balances for all species were also calculated.