While polyploidization is recognized as a major evolutionary driver for ferns and angiosperms, little is known about its impact in gymnosperms, where polyploidy is much less frequent. We explore Ephedra to evaluate (i) the extent of genome size diversity in the genus and the influence polyploidy has had on the evolution of nuclear DNA contents, and (ii) identify where shifts in genome size and polyploidy have occurred both temporally and spatially. A phylogenetic framework of all Ephedra species together with genome sizes and karyotypes for 87% and 67% of them respectively, were used to explore ploidy evolution and its global distribution patterns. Polyploidy was shown to be extremely common, with 41 species (83%) being polyploid (up to 8×) or having polyploid cytotypes - the highest frequency and level reported for any gymnosperm. Genome size was also diverse, with values ranging ~5-fold (8.09-38.34 pg/1C) - the largest range for any gymnosperm family - and increasing in proportion to ploidy level (i.e. no genome downsizing). Our findings provide novel data which support the view that gymnosperms have a more conserved mode of genomic evolution compared with angiosperms.