Predicting broccoli development II. Comparison and validation of thermal time models D.K.Y. Tan a , C.J. Birch a , A.H. Wearing a,* , K.G. Rickert b a School of Agriculture and Horticulture, The University of Queensland, Gatton, Qld 4343, Australia b School of Natural and Rural Systems Management, The University of Queensland, Gatton, Qld 4343, Australia Accepted 8 November 1999 Abstract Models predicting broccoli ontogeny and maturity should ideally be precise and readily adopted by farmers and researchers. The objective of this study was to compare the predictive accuracy of thermal time models for three broccoli (Brassica oleracea L. var. italica Plenck) cultivars (`Fiesta', `Greenbelt' and `Marathon') from emergence to harvest maturity (Model 1), from emergence to ¯oral initiation (Model 2), and from ¯oral initiation to harvest maturity (Model 3). Comparisons were also made between Model 1 and Model 4 (Models 2 and 3 combined). Model 1 is useful when the timing of ¯oral initiation is not known. When Model 1 was tested using independent data from 1983 to 1984 sowings of three cultivars (`Premium Crop', `Selection 160' and `Selection 165A'), it predicted harvest maturity well. Prediction of ¯oral initiation using Model 2 is useful for timing cultural practices, frost and heat avoidance. Where timing of ¯oral initiation was recorded, predictions of harvest maturity were most precise using Model 3, since the variation which occurred from emergence to ¯oral initiation was removed. The good predictions for Model 4 suggests that it would best predict the chronological duration from emergence to harvest maturity. # 2000 Published by Elsevier Science B.V. Keywords: Broccoli; Modelling; Temperature; Thermal time; Brassica oleracea 1. Introduction Some thermal time models for broccoli use the same base temperature from sowing to harvest (Titley, 1987; Marshall and Thompson, 1987) while others use Scientia Horticulturae 86 (2000) 89±101 * Corresponding author. Tel.: 61-7-5460-1230; fax: 61-7-5460-1112. E-mail address: alan.wearing@mailbox.uq.edu.au (A.H. Wearing). 0304-4238/00/$ ± see front matter # 2000 Published by Elsevier Science B.V. PII:S0304-4238(00)00203-X