Vegetative propagation of plants for forestry, agriculture, and horticulture relies on the successful induction of adventitious root formation (ARF). In our earlier reviews, we documented the successful induction of ARF through manipulation of plant growth media and growth regulators, and discussed the complex network of hormone signaling behind this process. Here, we review the molecular biology of hormone action as a pivotal factor behind ARF. We summarize what is known based on ample studies of model organisms, as well as ornamentals. Conserved pathways of hormone signaling are emphasized, as our knowledge of these from different plant taxa can often be integrated. The central role of the auxin response pathway is described, as well as cross-talk between cytokinin, ethylene, and sucrose. This final review will bridge current genetic and molecular research approaches with what we know about the ontogeny and physiology of ARF in ornamentals.