Teachers' experiences and attitudes play a critical role in outdoor education. In this study, we aim to explore how 5th-10th-grade (10-15 years-old students) teachers use the natural environment in their teaching practices, with special attention to the link to sustainable development (SD). In total, 42 in-service teachers were invited to participate in the study. The data collection was based on the participating teachers' individual written texts concerning their previous outdoor teaching practices, the link to SD and the related assessments. Based on content analysis, three main themes emerged from the data, including (1) teaching biology/ecology concepts, (2) exploring visible pollution and (3) applying a context- and inquiry-based approach. However, we found that outdoor teaching was scarcely linked to SD. The research results' implications for outdoor education and teachers' professional development are discussed in this paper.