The growth response of container-grown woody ornamentals to controlled-release fertilizers

A range of Australian native and exotic woody ornamentals were grown in a potting medium consisting of 50% composted hardwood sawdust, 25% milled, aged pinebark and 25% coarse sand. Plants were fertilized with two resin-coated controlled-release fertilizers (8–9-month formulations of Osmocote(R) 18N:4.8P:8.3K and Nutricote(R) 16N:4.4P:8.3K plus a basal fertilizer of 0.5 kg m−3 isobutyridene-diurea). More than half of the 16 species showed greater shoot growth with Osmocote than with Nutricote. The difference was probably due to the higher nitrogen content of Osmocote and the different nutrient release-rates of the two fertilizers. Some plants with a high rate of growth were not necessarily very responsive to fertilizer and vice-versa. Root growth showed a similar pattern to shoot growth with respect to fertilizer rate and type. Rootshoot (RS) ratio declined with increasing fertilizer rate and, at the highest shoot weight, the RS ratio for most species was approximately 0.3. All plants tested could be grown to a marketable size in 12–20 weeks using 4–5 kg m−3 of controlled-release fertilizer as the major source of nutrients.