We report on a school-based randomized control trial study comparing two morphological interventions with untaught controls: one focusing on direct instruction targeting print morphological decoding (direct decoding condition) and the other on inquiry-focused pedagogy using oral morphological analysis (inquiry-analysis condition). We identified 63 Grade 3 children with below-average morphological awareness following screening (from N = 163). This sub-sample showed average pseudoword decoding but poor language and word reading abilities. Following a 13-week supplemental intervention randomized within the 63 children, results showed a statistically significant main effect of intervention on standardized reading vocabulary measures at immediate post-test in the direct decoding condition. Pre-test morphological awareness moderated reading vocabulary effects for the untaught control group. Statistically significant moderation of growth in sentence comprehension at post- by pre-test morphological awareness was also evident in the inquiry-analysis condition. Universal screening for below-average morphological awareness followed by inquiry-based or direct instruction interventions focusing on the meaning dimensions of morphemes may be modestly efficacious for supporting reading vocabulary and sentence comprehension in such at risk learners, potentially aiding school-wide literacy improvement.