The control of carbon allocation, storage and usage is critical for plant growth and development and is exploited for both crop food production and CO2 capture. Potato tubers are natural carbon reserves in the form of starch that have evolved to allow propagation and survival over winter. They form from stolons, below ground, where they are protected from cold temperatures and animal foraging. We show that BRANCHED1b (BRC1b) acts as a tuberisation repressor in aerial axillary buds, which prevents buds from competing in sink strength with stolons. BRC1b loss of function leads to ectopic production of aerial tubers and reduced underground tuberisation. In buds, BRC1b promotes dormancy, ABA signalling and downregulation of plasmodesmata gene expression. This limits sucrose unloading and access of the tuberigen factor SP6A to axillary buds. Moreover, BRC1b directly interacts with SP6A and blocks its tuber-forming activity in aerial nodes. Altogether these actions help promote tuberisation underground.