Phenological research is engaged in monitoring the influence of climate change on the natural environment. The International Phenological Gardens (IPG) network provides a valuable dataset of standardized tree phenology records dating back to the mid-20th century. To make best use of this actively growing record, it is important to investigate how network data can be applied to predict the timing of phenological events in natural populations. This study compared clonally propagated IPG downy birch (Betula pubescens Ehrh.) and hazel (Corylus avellana L.) specimens of central European provenance to nearby wild populations at the western-most margin of the IPG network, in the south-west of Ireland. In addition to monitoring by trained scientists, observations by citizen scientists were included. The order of the timing of phenological events among sites was consistent across 2 years, confirming reproducibility of the results. IPG trees had the earliest B. pubescens leaf unfolding and C. avellana flowering dates of the sites studied. In addition, leaf unfolding occurred later in the wild populations than expected from the temperature responses of the B. pubescens and C. avellana IPG clones. Natural variation in phenology also exceeded the historical change observed at the IPG site, suggesting a potential genetic basis for climate adaptation. Trunk circumference, reflecting the age-dependent increase in tree size, was found to influence C. avellana phenology, with earlier timing of phenological events in larger trees. This finding highlights tree size as an important consideration in the management of phenological gardens and tree phenology research in general.