Although planting seagrass is not technically complex, the ability to plant large areas is limited by the time-consuming nature of manual methods. Additionally, manual methods use small, spatially isolated planting units (PUs; shoot bundles or plugs/cores) that are often highly susceptible to disturbance. The likelihood for harvesting intact apical meristems may be higher with large sods compared to smaller units, thus increasing survival and expansion rates. Here, we examined the survival and expansion of large units (1.5 x 1.2 m) of seagrass transplanted using a mechanized planting boat (Giga Unit Transplant System; GUTS). Twenty-seven units of seagrass (18 Halodule wrightii and 9 Thalassia testudinum) were transplanted and monitored for survival, shoot density, and expansion. After 3 years, 74.1% of the units had survived (66.7% H. wrightii and 88.9% T. testudinum) with 12 H. wrightii units having expanded substantially beyond the bounds of the original PU, merging with adjacent units to form spatially continuous patches of seagrass. High survival rates for T. testudinum should be interpreted in light of concomitant declines in density and lack of significant expansion after 3 years. In its tested configuration, the GUTS was a viable method for transplanting H. wrightii where donor and receiver sites were in close proximity (<2 km; a current limitation of the GUTS design used here). However, based on the reduced density and lack of significant expansion of T. testudinum that has persisted 3 years post-transplant, the GUTS cannot yet be fully recommended for transplanting this species.