The leaves of willowleaf meadowsweet are simple and narrowly lanceolate, reaching up to 4 inches (10 cm) in length. They have a double-toothed margin and a pointed tip. The upper surface is smooth and free of hairs, while the underside may exhibit a slightly lighter hue. The leaves are arranged alternately along the stem, featuring a prominent midrib and delicate vein pattern. These characteristics provide a clear and distinctive appearance, aiding in the identification of willowleaf meadowsweet.

Willowleaf meadowsweet flowers are dense, forming 4 to 5 inch (10 to 13 cm) panicles that bloom all summer. The flowers are small and delicate, typically pink to white in color, with five petals each. The arrangement of the petals creates a frothy, plume-like appearance on the panicles. The blooms are mildly fragrant and attract a variety of pollinating insects. These flowers are a characteristic feature, providing a distinct and identifiable look to the plant during its long blooming period.

Willowleaf meadowsweet features stems that are predominantly straight and typically do not branch. The stems range in color from yellow-brown to yellow-gray, providing a characteristic hue for identification. They are slender and smooth to the touch, with a thickness usually between 0.1 and 0.4 inches (2.5 to 10 mm). These stems maintain a mostly vertical growth habit, distinguishing them from other shrubs. Unique to willowleaf meadowsweet, the stem's uniform color and texture play pivotal roles in recognizing this plant.

The fruit of willowleaf meadowsweet is a small, dry follicle which opens on one side when mature to release seeds. The pods are typically brown and bear 2-winged seeds that aid in their dispersal by wind. These fruits form in clusters, with each follicle being slender and elongated, resembling miniature capsules. The texture is coarse and papery. When mature, the seeds inside the follicles become visible, which can be a distinctive feature for identification.