An English reading class of 10th graders (N = 50) was asked to self-construct English word cards for unknown vocabulary incidentally encountered when completing textbook readings. The students were assigned this task to determine what linguistic and nonlinguistic elements they would self-select to include on the cards and whether the appearance of those particular self-selected elements affect English vocabulary growth. Learners’ second language (L2) vocabulary size was assessed before and after the investigation, and learners’ self-constructed word cards were subjected to qualitative analysis uncovering additional “elements” included on the cards that were not suggested by the classroom teacher. Among the uncovered “element” themes (i.e., Formulaic Language, Word Formation, Additional Language Features, Creativity, Misunderstanding, and Carelessness), Word Formation was found to correlate significantly with L2 vocabulary growth. A multiple regression model accounted for 15.8% of the variance in vocabulary growth of which word formation explained the most variance, yielding a medium effect. A critical discussion of findings is provided in connection with previous literature to elucidate the feasibility of incorporating word card construction into L2 classroom instruction while also providing teachers suggestions on implementing a word card construction component into their classes.