Tall Ninebark, Pacific Ninebark: Physocarpus capitatus (Synonyms: Neillia malvacea, Neillia monogyna, Neillia monogyna var. malvacea, Neillia torreyi, Opulaster cordatus, Opulaster pauciflorus, Opulaster pubescens, Physocarpus pauciflorus, Spiraea opulifolia var. pauciflora, Spiraea pauciflora) - Shrub to 12 feet. Flowers small, white, and arranged into spherical clumps. Pistils generally 3-5. Leaves simple, and palmately lobed (typically with 3 lobes).
Mallow Ninebark: Physocarpus malvaceus (Synonyms: Neillia capitata, Neillia opulifolia, Neillia opulifolia var. mollis, Opulaster capitatus, Opulaster opulifolius var. capitatus, Physocarpa tomentosa, Physocarpus opulifolius, Physocarpus opulifolius var. tomentellus, Spiraea capitata, Spiraea opulifolia var. mollis, Spiraea opulifolia var. tomentella) - An erect, loosely branched shrub up to 6 feet tall. The alternate leaves are maple-like with 3 palmate lobes, the margins double toothed. The flowers are small, white, and borne in terminal corymbs. Pistils generally 1-2. Similar to Physocarpus capitatus but with smaller leaves and corymbs, and smaller growth habit.