Momilactones from rice have allelopathic activity, the ability to inhibit growth of competing plants. Transferring momilactone production to other crops is a potential approach to combat weeds, yet a complete momilactone biosynthetic pathway remains elusive. Here, we address this challenge through rapid gene screening in Nicotiana benthamiana, a heterologous plant host. This required us to solve a central problem: diminishing intermediate and product yields remain a bottleneck for multistep diterpene pathways. We increased intermediate and product titers by rerouting diterpene biosynthesis from the chloroplast to the cytosolic, high-flux mevalonate pathway. This enabled the discovery and reconstitution of a complete route to momilactones (>10-fold yield improvement in production versus rice). Pure momilactone B isolated from N. benthamiana inhibited germination and root growth in Arabidopsis thaliana, validating allelopathic activity. We demonstrated the broad utility of this approach by applying it to forskolin, a Hedgehog inhibitor, and taxadiene, an intermediate in taxol biosynthesis (~10-fold improvement in production versus chloroplast expression).