The study of variation in nuclear genome size, especially when combined with common garden experiments, significantly contributes to disentangling interspecies relationships within taxonomically complicated plant groups. The Chenopodium album aggregate is among the morphologically most variable groups and consists of many weakly differentiated cosmopolitan entities. We analysed nuclear genome size variation in diploid and polyploid species of the aggregate from Iran using flow cytometry of 282 accessions from 88 populations of 7 species. To this end, we also determined chromosome numbers and performed a morphometric study to reveal the extent of intraspecific morphological variation. We found that Iranian species are exclusively diploid (C. vulvaria), tetraploid (C. novopokrovskyanum, C. strictum, C. sosnowskyi and C. chaldoranicum) or hexaploid (C. album subsp. album, C. album subsp. iranicum and C. opulifolium). Six homogeneous relative genome size groups were distinguished among the species studied. Our morphometric study surprisingly revealed that under similar ecological conditions Chenopodium species are morphologically stable and well distinguishable, exhibited very little morphological variation. Hence, immense variation in leaf shapes, branching and inflorescence organization seen in the field has not been repeated under greenhouse conditions. The only exception was C. album s. str. which exhibited numerous morphotypes, covering the variation of remaining species.