Annual or perennial herbs, monoecious or dioecious, with stinging hairs. Stems ridged or 4-angled. Leaves opposite, dentate to incised-lobed; stipules free or connate. Inflorescences uni- or bisexual, axillary, spike-like (catkins) or glomerate with clustered cymes. Flowers unisexual, ebracteate, with 4-merous green perianth. Staminate flowers with 4 stamens, opposite the equal tepals and with rudimentary pistil. Pistillate flowers with outer tepals much smaller than inner ones; pistil 1, stigma sessile. Achenes somewhat compressed, oblong, enclosed in the persistent, hairy perianth.