Phenology of Heracleum mantegazzianum Sommier & Levier (Apiaceae) Plants in the Middle Taiga Subzone of the Komi Republic

Long-term observations of seasonal development of H. mantegazzianum in the Middle Taiga subzone of the Komi Republic revealed that phenorhythm types belonging to different phenotypic classes were present in the species populations. Juvenile plants developed according to the spring–autumn-green phenorhythm type with the periods of summer and winter dormancy. Immature and virginal plants were characterized by the spring–summer–autumn-green phenological type with the period of winter dormancy. Generative plants showed the spring–summer-green phenorhythm type, with the shortest vegetation season compared to other phenotypic classes. The general spring–summer–autumn-green phenological seasonal development type with the winter dormancy period typical for the self-sustaining populations of H. mantegazzianum ensured the species dominance over other herbaceous plants during the vegetation season. Dispersal of H. mantegazzianum seeds in different parts of the invasive range started when environmental heat amounts were at a relatively constant level. The sum of active air temperatures ≥ 5°C (SAT5) averaging 1662 ± 176°C and 113 ± 12 SAT5 days can be recommended as a predictor of the start of invasive hogweed mericarpia dispersal period. In urban locations, phenological phases of H. mantegazzianum came 1–2 weeks earlier than in the suburbs, which should be taken into account when implementing plant eradication measures.