Botanical gardens (BGs) are important agencies that enhance human knowledge and attitude towards flora conservation. By following free-choice learning model, we developed a "Discovery map" and distributed the map to visitors at the Xishuangbanna Tropical Botanical Garden in Yunnan, China. Visitors, who did and did not receive discovery maps, were evaluated through a questionnaire and behavioural observations. The map-users scored significantly higher on knowledge than non-map-users. Map-users tended to spend more time and pay more attention to plants during tours than the non-map-users. The study provides evidence to indicate using discovery maps as a free-choice learning process can improve visitors' engagement during the visit and may enhance the effectiveness of environmental education in botanical garden.