© CSIRO 2015. The purpose of this research was to examine the effect of the seed coat on seed dormancy in Grevillea (Proteaceae) species, and to further investigate the existence of germination inhibitors in Grevillea seed coat extracts. Seed dormancy of 18 Grevillea accessions involving 17 species was investigated: results indicated that removal of seed coat increased seed germination from 0-6% (intact seeds) to 83-100% for the Grevillea accessions and removal of half seed coat resulted in no increase in seed germination. Grevillea seed coat extracts reduced germination of barley, canola, lupin and ryegrass seeds by 48, 57, 10 and 38% respectively. The extracts also reduced seedling growth of the above four species. Ryegrass seeds showed no germination on the 3rd day after imbibition in the presence of Grevillea seed coat extracts compared with 88% germination for the control. Thus, our results showed that seed coat is a major factor determining Grevillea seed dormancy and removal of seed coat dramatically increased seed germination. Grevillea seed coat extracts reduced seed germination and seedling growth of other species. We conclude that there is exogenous seed dormancy in Grevillea species and the chemical(s) in the seed coat is a major factor inhibiting seed germination.