Effect of Fall Fertilization on Freeze Hardiness of Deciduous versus Evergreen Azaleas

Abstract

This experiment compared the effect of fall fertilization on freeze hardiness of evergreen vs. deciduous azaleas (Rhododendron). Beginning in Spring 2003, a 2 × 3 factorial experiment was conducted in Athens, Ga., on container plants grown outdoors under nursery conditions involving two taxa (R. canescens and R. ×satsuki `Wakaebisu') and three fall fertigation regimes (Aug.–Sept., 75 mg·L-1 of N; Aug.–Nov., 75 mg·L-1 of N; and Aug.–Nov., 125 mg·L-1 of N). On 15 Nov. and 17 Dec. 2003 and 16 Jan., 18 Feb., and 19 Mar. 2004, plant stem tissue was harvested and exposed to 10 progressively lower temperature intervals between –3 °C and –30 °C under laboratory conditions in order to estimate azalea freeze hardiness. Freeze hardiness was affected by fertilizer and taxa treatments, but there were no significant interaction effects in this study. The timing of freeze hardening was not significantly different among the two species over time, and the fall fertilizer treatments did not affect the timing of hardening. Compared to the industry standard (75 mg·L-1 of N, Aug.–Sept.), R. canescens that received extended fertilization at the high rate (125 mg·L-1 of N, Aug.–Nov.) was less freeze hardy in November, December, and January, and R. ×satsuki was less freeze hardy in December. However, when compared to the industry standard, the low rate of extended fertilization (75 mg·L-1 of N, Aug.–Nov.) did not affect azalea freeze hardiness.