Mark J. Pavek, N. Richard Knowles, and Carrie Wohleb Washington State University, Pullman, WA, USA Potato plant and tuber development are largely influenced by environment, particularly at key growth stages; however, commercial agronomists routinely manipulate production inputs and cultural management targeting a desirable production outcome. Identifying and implementing management leading to ideal partitioning of assimilates between canopy and tubers (source/sink ratio) for a particular growing region and production year is crucial to profitable potato production. The harvest index (HI) quantifies source/sink relationships allowing for seamless translation of research outcomes into cultural management recommendations. Data from numerous research trials between 2004-21, suggests the HI should fall within the range of 40-50%, favoring foliar growth over tuber growth at maximum foliar development (~92 days after planting (DAP)) for most indeterminate late maturing cultivars when grown in the Columbia Basin of WA. Building a robust canopy early is essential in optimizing grower return. If foliar growth is restricted early in the season (e.g., by lack of nutrients, water, or stress), HI at maximum foliar development will often exceed 50%, favoring tuber growth over foliar growth which, for many cultivars, will limit the total amount and duration of subsequent foliar growth. Moreover, plants with ideal foliage growth early- to mid-season achieve spectacular yields if allowed enough time to mature naturally. For some cultivars, excessive canopy growth late in the season may extend tuber bulking and delay plant maturity, which could result in reduced tuber yield and quality. On the other hand, an undersized canopy that senesces prematurely is likely to result in a loss of potential yield. Through trial and error, growers must properly manipulate potato source/sink relationships to maximize economic return.