1. Cuscuta species are rootless, leafless and branchless stems without meaningful photosynthesis. Hence, sink-free and source-free mechanistic growth models are possible yet remain unexplored. Furthermore, phytohormone expression has not yet been studied in the absence of hosts. 2. We use mass conservation in second-order differential equations to build mechanistic models for Cuscuta growth dynamics and UPLC-MS/MS to identify and mathematically score phytohormone expression. 3. We identified four sequential stages of growth – exponential, linear, parabolic deceleration and terminal stages and, in Cuscuta chinensis, the phases are discernable by eye. 4. Analytical solutions to the differential equations fit the growth data well and the model also predicts faster growth in Cuscuta species with smaller seeds, in agreement with the fact that Cuscuta chinensis attains terminal stage faster than Cuscuta japonica. 5. We found evidence for stage-specific phytohormone expression and for the existence of stem-wide phytohormone gradients, especially for the Auxin components MEIAA and ICAId in Cuscuta japonica. 6. We have built models for the roles of phytohormones in Cuscuta growth dynamics, and performed ad hoc calculations which suggest a continuously-increasing Cytokinin/Auxin ratio in growing Cuscuta seedlings, thereby implying a maximum value beyond-which growth slow-down begins. 7. Synthesis: We have created the first-ever source-free and sink-free plant growth models using mass conservation in second-order differential equations and in so-doing, uncovered four growth stages in Cuscuta, observable by eye in Cuscuta chinensis. We found stage-specific phytohormone expression and a likelihood of stem-wide phytohormone expression gradients. We then brought it all together by building models for the roles of phytohormones in Cuscuta growth dynamics and by performing ad hoc calculations which have suggested that the Cytokinin to Auxin ratio increases continuously in growing Cuscuta seedlings, to perhaps attain a maximum value beyond-which slow-down in growth begins.