Growth Response of Ilex and Hydrangea using Low-phosphorus Controlled-release Fertilizers

Containerized nursery crops require relatively low substrate pore-water phosphorus (P) concentrations (i.e., <5 mg·L^-1) to achieve and sustain optimal growth. Industry-standard controlled-release fertilizers (CRFs) may supply higher P concentrations than what are necessary for optimal growth, resulting in low P use efficiencies (amount of applied P used by the plant) and losses of excess P via leaching. Adopting low-P CRFs could improve crop P use efficiency and assist with compliance of future nutrient environmental regulations. This study was conducted to determine if CRFs with decreased P content can be utilized to sustain optimal growth of two common Virginia nursery crops for ≈4 months in over-head irrigated open-air nurseries set in two climatically different Virginia locations: Blacksburg (USDA Hardiness Zone 6b; 104 cm avg. annual rainfall) or Virginia Beach (USDA Hardiness Zone 8a; 122 cm avg. annual rainfall). Ilex crenata Thunb. ‘Helleri’ (holly) and Hydrangea macrophylla ‘P11HM-11’ Bloomstruck™ (hydrangea) were top-dressed with 20 g of one of four low-P CRFs, each containing 18N-10K and 0.4P, 0.9P, 1.3P, 2.2P or a control (24 g of 16N-2.6P-10K; standard nursery fertilizer); all CRFs were an 8 to 9 month formulation. Average leaching fraction of 0.37 ± 0.06 for holly and 0.59 ± 0.06 for hydrangea were observed. In Blacksburg, holly maximal shoot dry weight (SDW) occurred in plants fertilized with 0.9P fertilizer or greater. In contrast, maximal SDW for holly growing in Virginia Beach occurred when fertilized with the 2.2P fertilizer, indicating intra-taxa variability in P requirement depending on geographical location. In both locations, hydrangea SDW was highest when fertilized with the control or treatments containing at least 1.3P. Thus, data indicates that P in CRF may be reduced by as much as 50% without reducing plant growth for Hydrangea macrophylla. However, additional research is needed to ensure low-P fertilizers are suitable for a variety of plant taxa and across a wider range of geographic locations.