Phosphorus Requirement of Rhododendron ‘Victor’ and Cotoneaster adpressa praecox Grown in a Perlite-Peat Medium

Rooted cuttings of Rhododendron ‘Victor’ (a PJM cultivar) and Cotoneaster adpressa praecox were grown in the greenhouse with phosphorus (P) levels ranging from 0 to 15 ppm in a perlite-peat medium. Shoot growth of Rhododendron was equal at P levels from 2.5 to 15 ppm. Maximum growth of shoots and roots of Cotoneaster was at 10 ppm. Maximum Rhododendron leaf P content occurred at 5 ppm in the medium and remained the same at higher P levels. The P content of Cotoneaster leaves increased throughout the range of P levels in the growing medium. A P level of 10 ppm in soilless media solution is suggested for most nursery crops.