Spreading orach is an annual weed that colonizes roadsides, field edges, and increasingly, no-till agricultural fields. It produces dimorphic seeds with different levels of physiological dormancy, but little is known about the germination ecology of the two seed types. Field and controlled-environment studies were conducted to determine seed responses to light and stratification, the pattern of seedling emergence in the field, and the effect of soil water content on the length of cold stratification required to break dormancy for each seed type. The large, brown seeds have three times the mass of the smaller, black seeds, primarily because of a larger embryo, but have a thinner seed coat. Germination of brown and black seeds in petri dishes was 98 and 90%, respectively, after stratification for 3 mo at 5 C, whereas germination of unstratified seeds was 19 and 12%, respectively. Light stimulated germination of both stratified and unstratified black seeds but did not increase germination in stratified brown seeds. Up to 40% of brown seeds germinated in situ during stratification, compared with only 2% for black seeds. Germination in petri dishes and emergence in the field were more rapid for brown seeds than for black seeds. Maximum germination of black seeds occurred after stratification for 2 or 3 mo at 5 C on soil that was waterlogged (pore-water matric potential, ψ = 0 kPa), wet (ψ = −0.38 kPa), or at field capacity (ψ = −10 kPa). For shorter periods of stratification, total germination and germination rate of black seeds declined as soil water content decreased from waterlogged to dry (ψ = −500 kPa). Seed dimorphism in spreading orach may provide a mechanism to enhance survival in uncertain or variable habitats such as disturbed agricultural fields.