The inflorescence of spiraea douglasii
is elongated, several times longer than the width, unlike the otherwise very similar spiraea splendens
which has roughly spherical flower clusters, equal in height and width. The brownish stems and the undersurfaces of the dull green leaves of spiraea douglasii are covered by short, tomentose hairs. Upper leaf surfaces are usually hairless. Leaf edges have fine teeth, mostly towards the upper edge. Plants grow large, up to six feet, and are found generally in wet or moist areas, often forested.
Flowers are attached by short, branched pedicels, and they are formed of five, broad, brownish-green sepals, and five light pink to dark pink petals, darker at the base. The pink stamens are exserted well beyond the petals. Petals become orange then brown as they wither. Sepals are reflexed, bent back against the pedicel.