This study quantified the relationship between vegetative development and temperature of ‘Old New Zealand’ faba bean, ‘Milton’ oats, and ‘Feast II’ Italian ryegrass using thermal time (Tt, degree-days) calculations. Each species was sown on five dates in autumn and winter 2008 and three dates in autumn 2009. The linear model for rate of development calculated the Tt requirement of faba bean for 75% emergence as 217 degree-days (base temperature (T_b) = 1.2°C), compared with 132 (T_b = 1.6°C) for oats and 132 (T_b = 1.8°C) for Italian ryegrass. Leaf appearance had a T_b of 2.4°C for faba bean, 3.0°C for oats, and 0.7°C for Italian ryegrass. The mean phyllochron (degree-days leaf^–1) was 66 ± 1 for faba bean, 123 ± 3.90 for oats, and 120 ± 4.21 for Italian ryegrass. Soil temperature at 20 mm depth was the most accurate predictor of T_b and the Tt requirements to reach 75% emergence. Conversely, air temperature on-site was required to predict the phyllochron for faba bean because of its elevated growing point. Either air or soil temperature at the experimental site or at a nearby meteorological station could be used to define the phyllochron for oats and Italian ryegrass. These results highlight the importance of both soil and air temperatures to accurately define vegetative development before the processes are included in simulation models for these winter annual forage crops.