In an effort to keep pace with an expanding knowledge-based economy, K-12 schools across the U.S. are increasingly shifting their STEM teacher professional learning programs to help teachers foster student experiences with skills such as problem solving, adaptability, and creativity. This study utilized quantitative and qualitative data to examine the impact of school culture and teacher professional learning factors on teachers' self-reported instructional practices in a sample of Texas middle grades STEM teachers (n = 115). Triangulation of quantitative and qualitative survey data revealed connections between the influence of predictors on teaching practice and teachers' reported perceptions of barriers to effective instruction. Building teachers' professional capacity and the provision of adequate time and/or funding significantly and positively predicted teachers' use of effective STEM practices, such as whole-group discussion and small group collaboration. Qualitative analysis of teachers' perceptions of barriers to STEM instruction showed that teachers perceived that a lack of time for collaboration, external expectations, and teacher motivation were among the greatest barriers to high-quality STEM instruction. Building teachers' professional capacity by strengthening teachers' feelings of instructional preparedness, providing opportunities for collaboration, and building teacher motivation were identified as key areas of focus for encouraging teacher use of reform-based instructional practices.