Rest Completion of Eastern Black Walnut

Abstract

The time of rest completion of buds of eastern black walnut (<italic>Juglans nigra</italic> L.) cultivars was compared from 2004 to 2008 and various models for estimating chilling were evaluated. The chilling model that best accounted for the variation in days to budbreak among cultivars and temperatures during dormant periods had the following two components: 1) a chilling inception temperature of -2.2°C and 2) weighted chilling hours that accumulated after the chilling inception temperature. Chilling hours in this model were weighted as follows: <0°C = 0; 0 to 9.1°C = 1; 9.2 to 12.4 °C = 0.5; 12.5 to 15.9°C = 0; 16 to 18°C = -0.5; >18°C = -1. Based upon this chilling model, Schessler, Davidson, and South Fork ranked among the cultivars with the shortest chilling requirements (< 1400 chill units). Jackson also had a relatively short chilling requirement (≈ 1400 units). In contrast, Sparks 147 and Thomas Myers, did not complete rest during the experimental period (i.e., break bud within 14 days after exposure to 21°C) and required > 1625 chill units. This study also elucidated that a black walnut model with a chilling inception temperature of -2.2°C estimated chilling more accurately than one with chilling inception just after the maximum negative accumulation of chill units as used in the Utah chilling model. Also, temperatures between 0 and 2.4°C must be weighted more heavily in a black walnut model than in the Utah peach model to accurately estimate chilling and rest completion.