Recent climate warming has been observed at the global scale, but by examining developmental stages of plant species (phenology) that are dependent on local climatic conditions, climate change at the local scale can be detected. There are four gardens in Ireland belonging to the International Phenological Gardens (IPG) network, which has recorded tree phenology for more than 30 years using a common collection of clonal tree species and cultivars. In this analysis two phenological stages were investigated */the beginning of the growing season (BGS) and the end of the growing season (EGS) */ in nine tree cultivars in relation to ambient air temperature. The length of the growing season (LGS) was determined from the number of days between BGS and EGS. Structural time series analysis was used to describe trends in the data. Overall BGS was the most responsive phenophase and was shown to start earlier in more recent years for some species at all sites. It is shown that these observations are due to recent climate change in the form of spring warming, particularly in the south-west of the country. Due to the limited number of observation sites and differences between species responses, we suggest that extrapolation of the results to larger geographical areas should be performed with caution.