Numbers of bumblebees (Hymenoptera: Apidae) are decreasing in many European countries. Thirty-six species native to Poland are legally protected, and 19 are considered as threatened with extinction. A method for assessing the attractiveness of garden flowering plants for bumblebees was developed in this study. Observations were conducted through the growing seasons in 2011 and 2012 in two botanical gardens located in Wrocław city, Poland. Bumblebees were observed and recorded on flowers during 30-min observations and determined to species level. An algorithm was used to define the attractiveness (At_p) of visited flowers, including the frequency of flower visits by bumblebees during phenological seasons. The resulting At_p values were assigned to a 4-level scale of attractiveness. In the botanical garden, the most “valuable” plant species were: Rhododendron catawbiense Michaux, Lathyrus vernus Bentham, Lathyrus odoratus L., Aristolochia clematitis L., Cyanus montanus Hill, Aconitum lycoctonum L., Hosta sp., Spiraea japonica L. filius, Lavandula × intermedia Loiseleur-Deslongchamps, and Lavandula angustifolia Miller; and in the Medicinal Plants Botanical Garden there were: Geranium phaeum L., Agastache rugosa Kuntze, Glycyrrhiza glabra L., Cydonia oblonga Miller, R. catawbiense, Monarda didyma L., Aruncus dioicus Fernald, Lavandula angustifolia, Phacelia tanacetifolia Bentham, Origanum vulgare L., and Scutellaria baicalensis Georgi. The proposed algorithm may be useful in protecting bumblebees, which takes into account their nutritional needs or feeding behavior. Careful plant selection in planning green urban areas with high rates of At_p will likely to contribute to an increase in numbers of bumblebees.