Ragweed was introduced to Ukraine at around 100 years ago and significantly spread through the country since. Its pollination patterns based on photoperiodism were seen in Vinnitsa at the third ten-day period of August and were clearly predictable in Vinnisa until the year 2010 when the temperature increase changed the peak timing for two weeks early. The new pollination mode can be described as a “three-maximum” season including pollen concentration increase at the first half of August, at the last ten-day period of August and on September. The appearance of the first pollen maximum and a shift of the last one toward later time is observed due to temperature increase. It’s shown droughts can decrease the Ambrosia pollen count significantly.