The small flowers of epilobium hornemannii
are formed of four narrow, pointed green sepals, sometimes purple along the margins, and four white to pale pink to purple petals, which are about twice as long as the sepals (up to 0.4 inches). Sepals, pedicels and the upper portion of the stems have a covering of short, glandular hairs. Stem hairs are arranged in lines. Pedicels are erect, unlike those of the similar epilobium anagallidifolium
, which tend to be nodding. Petals have a small notch at the tip, and are crossed by a few darker lines, towards the base.
Plants form small clumps, of short, ascending stems, colored green or reddish. Leaves are generally hairless, and may be lined by small teeth. They may be sessile, or attached by stalks up to 0.3 inches long. Basal leaves are wider than those higher up the stem. Stems are usually unbranched.