Flowering often is a very drought-sensitive phase in the life cycle of a plant. Seed set of summer- flowering tarweeds, however, appears to be insensitive to both atmospheric and soil drought. I therefore examined the physiological mechanisms that enable tarweed annuals to be buffered from the summer drought and explored whether these water-limited taxa can exploit unpredictable sources of water such as dew. These questions were addressed primarily for the most common tarweed taxon, Hemizonia luzulifolia DC ssp rudis (Benth) Keck. Summer-flowering tarweeds cope with drought by maintaining tissue hydration by continual root growth into deep soil water reserves and by water storage in extracellular polysaccharides. Analysis of the water release kinetics of these polysaccharides indicates that they contribute significantly to tissue water storage and effectively buffer living cells from rapid and detrimental changes in plant water status. Tissue water storage (capacitance) is not constant throughout ontogeny for summer-flowering tarweeds. Capacitance of above-ground shoots increases from small values in the wet spring to very high values in the hot, dry summer. Greenhouse studies show that although buffering capacity increases through the dry season, the ontogenetic shift of capacitance is an inherent trait, not induced by a decrease in water availability. In the population studied, seed production by Hemizonia luzulifolia ssp rudis is primarily limited by herbivore destruction and consumption of capitula. Seed set also is limited by pollinator visitation because of early capitulum closure in response to tissue water deficits. Moisture on foliage in the form of natural and artificial dew partially ameliorates the effects of summer drought on the diurnal course of plant water potentials. Wetted plants extend the period of photosynthetic activity and capitulum opening. These results suggest that the small incremental changes in these two parameters compounded over the season may translate into significant effects on both longevity and fecundity.