Purpose. To reveal the peculiarities of the seasonal rhythm of growth and development of Itoh Group peony cultivars in the conditions of the M. M. Hryshko National Botanical Garden National Academy of Sciences of Ukraine. Methods. The objects of research were plants of 24 cultivars of Itoh Group peonies. The research was conducted on the experimental field of the Department of Flowering and Ornamental Plants of the M. M. Hryshko National Botanical Garden National Academy of Sciences of Ukraine during 2017–2020. The sum of effective temperatures was calculated by summing the daily air temperatures reduced by the biological zero. Results. The phenological phases of Ito Group cultivars growth were determined. Phenological spectra for different groups are presented. It was determined that the flowering of early cultivars: ‘First Arrival’, ‘Hillary’, ‘Julia Rose’, ‘Morning Lilac’, ‘Old Rose Dandy’, ‘Sonoma Apricot’ starts at the sum of effective temperatures ≥ 400 °С. The middle group hybrids bloom when the sum of temperatures reaches 450 °С. This group includes: ‘Bartzella’, ‘Callie’s Memory’, ‘Cora Louise’, ‘Lollipop’, ‘Scarlet Heaven’, ‘Sonoma Velvet Ruby’, ‘Yellow Dream’, ‘Yellow Emperor’, ‘Yellow Heaven’, ‘Yellow Waterlily’. Late flowering group includes: ‘Border Charm’, ‘Garden Treasure’, ‘Kopper Kettle’, ‘Pastel Splendor’, ‘Prairie Charm’, ‘Viking Full Moon’, ‘White Emperor’, ‘Yankee Doodle Dandy’; accumulation of effective temperatures above 500 °С is an essential requirement for their flowering. Conclusions. Itoh Group cultivars successfully pass all phases of seasonal development and manage to complete the growing season. Cultivars belong to the spring-summer-autumn-green phenorhythmotype. The onset of the corresponding phenological phases in peonies of the studied group of cultivars requires a certain sum of effective temperatures. Plant outgrowth begins on March 23 – April 2, when the sum of effective temperatures ranges from 20–40 °С. The flowering of varieties characterized as late spring, lasts 6–9 days ± 3–4 days, depending on the varietal characteristics and the year of cultivation. A rapid increase in the sum of effective temperatures up to 700 °C shortens the flowering phase by 4–5 days. An assortment of early (May 22–25 ± 2–3 days), medium (May 26–28 ± 3–5 days) and late-flowering (May 29–31 ± 4–6 days) cultivars has been selected, what ensures the continuity of peony flowering during two months.