Although informal learning environments have been studied extensively, ours is one of the first studies to quantitatively assess the impact of learning in botanical gardens on students' cognitive achievement. We observed a group of 10th graders participating in a one-day educational intervention on climate change implemented in a botanical garden. The students completed multiple-choice questionnaires in a pre-post-retention test design. Comparing the test scores revealed a significant short-term knowledge gain as well as a long-term knowledge gain. Consequently, our results show the potentials of botanical gardens as effective learning environments, and for complementing formal school-based learning settings regarding climate change education.