An important but largely overlooked point in studies on annual plant species with heteromorphic diaspores is that the phenotypes can be affected by the maternal environmental conditions. We performed a 2-year pot experiment to evaluate transgenerational plasticity in response to heterogeneity of environmental conditions in the seed-dimorphic species Suaeda corniculata subsp. mongolica. We grew plants from dimorphic seeds (brown and black) of S. corniculata in controlled favorable vs. stressful conditions (early vs. late germination timing or low vs. high soil salinity) for two generations, producing replicate individuals with all permutations of maternal (F0) and progeny (F1) conditions and seed morphs. We then measured the effects of two-generational conditions and seed morph histories on traits of plants (F1) and their offspring (F2). F1 progeny plants from late-germinating mothers allocated more biomass to seeds and produced fewer brown seeds than those from early-germinating mothers. F1 progeny plants that grew under the same salinity as their mothers had higher reproductive allocation than those that grew in the contrasting conditions. When progeny plants were grown in favorable conditions (early germination or low salinity), the stressful maternal conditions (late germination or high salinity) increased germination of black seeds. Moreover, the transgenerational effects of seed heteromorphism interacted with experimental conditions in determining performance of progeny generations. Thus, transgenerational plasticity provided ecological diversity in the regenerative strategy for a seed heteromorphic species, and it may contribute to population maintenance via benefiting offspring responses to temporal and spatial variations.