Herbaceous perennial species may exhibit winter or summer dormancy, or they may be spring ephemerals with dormancy extending from summer through winter. Herbaceous perennials, and spring ephemerals in particular, are most common in strongly seasonal but relatively mesic temperate environments. Warm deserts are typically dominated by shrubs and annuals that are better adapted to withstand or avoid severe summer heat. Astragalus holmgreniorum Barneby (Fabaceae) is a perennial spring ephemeral that has a restricted distribution at the northeastern edge of the Mojave Desert. We explored factors that regulate its phenological development in an observational study at 2 sites over 2 growing seasons. We examined consequences of weather variation in terms of phenological patterns and their effect on survival, growth, and reproductive output. Both seedling emergence and reinitiation of shoot growth occurred in early spring, with timing varying by site and year. Flowering peaked in early April each year regardless of weather variation. Plants ceased aboveground activity as daily maximum temperatures exceeded 30 °C in May. Between-year variation in winter-spring precipitation timing had significant effects on growth, survival, and reproductive output in 2 years with similar above-average total precipitation. Early-season precipitation favored high seedling emergence, whereas midseason precipitation favored increased growth and flowering of adult plants, high fruit set, and high seedling survival, and late-season precipitation favored high seed set. Our study showed that variation in precipitation timing can have major impacts on plant success in a desert environment, even in years with similar above-average total precipitation.