Spatial and temporal variation in water availability is known to play a large role in shaping plant phenotypes within species. For ecologically and phenotypically diverse species complexes such as seep monkeyflowers (Erythranthe guttata (DC.) G.L.Nesom complex, Phrymaceae), it is not always clear how acclimation to local soil moisture environments contributes to morphological, physiological and phenological variation. Here we investigate phenotypic variation and plasticity in annual and perennial seep monkeyflowers where they co-occur within meters of each other on the Central Coast of California. Although the annual form tends to occur in drier rock outcrops and the perennial form occurs in adjacent perennial seeps, there appears to be a gradient of indistinguishable phenotypes between these microhabitats. We first tested and confirmed that key morphological, physiological, and phenological life history traits varied with soil moisture along the natural gradient from rock outcrops to seeps and with experimental water additions in the rock outcrops. To determine the role of plasticity in maintaining trait differences in the field, we collected seeds from individuals at the extremes of the soil moisture gradient to grow under common conditions. In the common garden, petal size and flowering time differences were partially maintained, while vegetative and physiological trait differences were not maintained. This indicates that much of the phenotypic variation between microhabitats is environmentally induced, although other factors may be at play. These results improve our understanding of how plants respond to fine-scale variability in soil moisture and suggest that plasticity plays a key role in the phenotypic diversity observed in seep monkeyflowers.