Allelopathy is the chemical interaction between plant species whereby one organism may promote or interfere with another. Accordingly, allelopathic plant species play significant roles in shaping natural ecosystems such as affecting species distribution and diversity. While a high degree of inter-species variation in allelopathic capacities is commonly reported, variation across subspecies and between dioecious sexes is limited. Here we use the ecologically important, allelopathic dwarf shrub Empetrum nigrum to assess how allelopathic capacity varies between subspecies (ssp. nigrum and ssp. hermaphroditum) and sex (ssp. nigrum male and female plants). Specifically, we test Icelandic Empetrum, as its allelopathic nature may be an influencing factor in long-term land degradation across Icelandic highlands. Allelopathic capacity of Empetrum foliar tissues was assessed as root elongation and seed germination inhibition of the palatable grass species, Festuca richardsonii. We observed a very strong allelopathic response of Empetrum, whereby the different subspecies and sexes severely inhibited Festuca root elongation, even at very low leaf density levels. Sex and subspecies related differences in allelopathic capacity was also observed, with ssp. hermaphroditum affecting Festuca seed germination more than ssp. nigrum, and male ssp. nigrum possessing a greater germination inhibitory effect compared to female plants. Our results indicate that Empetrum may differentially impact plant interactions and ecosystem processes depending on the relative abundance of Empetrum subspecies and sexes within populations.