It is caused by urban light. Light controls flowering initiation through the action of the photoreceptors that perceive an increase in day length and light quality :)
Thermal pollution in addition to light pollution is present in the city, for example, in the center of Krasnoyarsk snow melts earlier than outside the city in spring, and in autumn a stable snow cover forms later.
Just to add some detail to Andrew and Natalya’s answers:
Most night lighting affects the seasonal rhythms of plants such as growth and dormancy, as well as flowering, that are regulated by daylength. The growing season may be lengthened, and in Bristol UK it is said that some night-lighted London plane trees remain in leaf throughout the winter.
An urban area is warmer than the surrounding rural areas, and is called an ‘urban heat island’ (UHI), mainly because: (i) The buildings have a higher surface area than the ground, increasing heat absorption. (ii) The building materials and other urban fabric (including vehicles) store heat. (iii) Heat is emitted by vehicular traffic, space heating and cooling, people and animals. (iv) Heat tends to be trapped by air pollution. (v) There is less cooling from evapotranspiration. (vi) Air movement close to the ground may be reduced. Other effects of the UHI are that elevated CO2 may promote growth, hot sunshine can reduce air quality and excessive heat can give rise to physiological stress.
As an addition to everyone's comments. The day, night temperature of a large city is noticeably more in the spring and summer. And the evening cooling of the same city is slower because much of it behaves like a -heat-sink. However, it's likely this will change due to environmental (climate et al) changes. The insect population in a city is less than 25 years ago and the pollinating insects are even less. In cold winters the chemicals used to melt ice and snow is increasing, with that health of inner-city plant life is less.
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