Effect of Fall Fertilization on Freeze Resistance of Deciduous Versus Evergreen Azaleas

Abstract

Abstract Plants that maintain their leaves throughout winter may respond differently to fall fertilization than deciduous plants. The effects of fall fertilization on cold hardiness, nutrient uptake, growth and flower production of evergreen versus deciduous azaleas were studied. Rhododendron canescens (Michx.) Sweet and R. × satsuki ‘Wakaebisu’ were grown in containers, outdoors in Athens, GA, under three fall fertigation regimes applied daily as 0.5 liter (0.13 gal) solutions containing: 1) 75 mg·liter−1 N from August 1 through September 29, 2) 75 mg·liter−1 N from August 1 through November 28, and 3) 125 mg·liter−1 N from August 1 through November 28. Stem freeze resistance was analyzed monthly November through March. Growth of azaleas that received 120 days of extended fertigation (August 1 through November 28) was not increased compared to azaleas that received 60 days of extended fertigation (August 1 through September 29). Growth of the two taxa did not differ in their response to fertilization treatments. The high rate of extended fertilization 125 mg·liter−1 N (from August 1 through November 28) reduced stem freeze resistance November through February, while the moderate rate of extended fertilization (75 mg·liter−1 N from August 1 through November 28) reduced azalea freeze resistance in December. Fall fertilization regimes did not produce differences in the timing of cold acclimation, or deacclimation of R. canescens and R. × satsuki. The high rate of extended fertilization promoted early budbreak of R. × satsuki and postponed flower budbreak of R. canescens. Flower production of R. canescens was not affected by fall fertilization, but the high rate of extended fertilization increased flower production of R. × satsuki compared to plants that received the moderate rate of fertilization 75 mg·liter−1 N from August 1 through September 29.