Apple (Malus X domestica Borkh.) cultivars require a pollinizer for cross-pollination to ensure fruit set when planted in contiguous blocks found in modern apple orchards. The overlap of bloom time between pollinizing crabapples (Malus spp.) and commercial cultivars is a critical component for adequate fruit set to occur. Bloom time and duration is correlated with temperature, suggesting this relationship could be influenced by climate change. Understanding the influence temperature has on bloom phenology of these pollinizing crabapples is important to ensure proper cultivar and pollinizer combinations are made by growers. 4 commercially available pollinizing crabapple cultivars (‘Indian Summer’, ‘Mt. Blanc™’, ‘Mt. Evereste™’, ‘Snow Drift’) were replicated 5 times in completely randomized design. In spring of 2024 and 2025, pollinizers were analyzed 1-2 times per week for developmental stages from silver tip to petal fall. These two years represented one of the warmest and then coldest springs on record in Kentucky, respectively. Commercial apple cultivars were also analyzed for comparison. The stage progression data for each pollinizer cultivar was modeled using growing degrees days (GDD) base 6.6°C. In both observed years, the first crabapple to bloom was ‘Indian Summer’ and the final crabapple to bloom was ‘Mt. Blanc™’. In the first year, ‘Indian Summer’ bloomed on Julian Day (JD) 88 at 166.4 GGD and in the second year bloomed on JD 92 at 179.8 GDD. 'Mt. Blanc™’ bloomed on JD 104 at 250.1 GDD and 244.9 GDD in years one and two, respectively. In year one, ‘Gold Rush’ began bloom on JD 97 at 199.4 GDD, which overlapped with ‘Mt. Evereste™’ and ‘Snow Drift’, whereas in the second year ‘Gold Rush’ began bloom on JD 93 at 179.4 GDD and stopped blooming on JD 108 at 279.8 GDD, which overlapped with ‘Mt. Evereste™’, ‘Snow Drift’, and ‘Mt. Blanc™’. There was little bloom overlap in 2024, with the earliest crabapples blooming earlier and more rapidly than the cultivars. In 2025, bloom overlap improved but little overlap occurred with more than two pollinizers at a time. These results show bloom overlap can be influenced by different temperature patterns during winter and spring, and pollinizer selection needs to be carefully considered. It can be concluded that using growing degree-days can help predict bloom time for pollinizers. In Kentucky, limited bloom overlap of apples and pollinizers could indicate a concern for northern apple growing regions as spring temperatures become more variable with climate change.
