Intense research is being carried out on climate variability and change and the estimation and detection of anthropogenic effects. In addition to statistical methods, the use of plants, as biological indicators is becoming more popular as they are sensitive to environmental conditions. In this article we compare maps of the flowering dates of the locust tree (Robinia pseudoacacia L.) for three different time intervals between 1851 and 1994. The maps revealed noticeable shifts of dates, of approximately 3–8 days, towards earlier flowering. This change is related to the average temperature of spring (15 March–15 May), via a simple statistical model that is accurate enough to be able to quantify phenological changes and to calculate the corresponding warming. The model developed can estimate spring mean temperature using phenological data from R. pseudoacacia L. with an accuracy of 0.2° C. Estimates of mean temperature based on phenological changes are compared to climatic series. This comparison emphasizes the possibility of using R. pseudoacacia. L. as a bio-indicator. Estimates of temperature changes are also given.