Volume 63 Number 2 Article 5
Year 2009 Month 4
Title: Rest Completion of Eastern Black Walnut
Authors: M.R. Warmund, M.V. Coggeshall and W. Terrell Stamps
Abstract:
The time of rest completion of buds of eastern black walnut (Juglans nigra 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.
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