The leaves of wild sweetwilliam are dark, shiny green and occur in opposite pairs. They are sessile, measuring 2 to 4 inches (5 to 10 cm) in length and 0.5 to 1 inch (1.25 to 2.5 cm) in width, with a lanceolate shape and either smooth or finely toothed margins. The leaves have wide bases, tapering to slender, sharply pointed tips, and lack petioles. The texture is smooth, and the veins are not prominently raised, contributing to their sleek appearance.

The 1/2 inch (1.27 cm) tubular, 5-lobed, pinkish-purple flowers of wild sweetwilliam feature long corolla tubes and are densely packed in large, cylindrical, terminal clusters up to 8 inches (20 cm) long. These flowers exhibit a vibrant coloration, forming prominent, upright inflorescences. The blossoms are known for their striking appearance and are key for identifying wild sweetwilliam, blooming profusely during the summer months. Their arrangement and hue make them unmistakable in a garden setting.

The stem of wild sweetwilliam is smooth, round, and green, distinguished by numerous red/purple spots. Typically unbranched, the stem stands upright with a medium thickness, usually around 0.2-0.4 inches (5-10 mm) in diameter. This characteristic spotting provides a unique visual cue crucial for the identification of wild sweetwilliam. The stem's texture is generally consistent, lacking additional ridges or hairs, contributing to its smooth appearance.

The fruit of wild sweetwilliam manifests as oval seed capsules roughly 1/3 inch in size. Each capsule is divided into three distinct cells, with each cell harboring one or more seeds. These capsules, typically containing multiple seeds, contribute to the successful propagation of wild sweetwilliam. Their structure and the presence of multiple cells is a distinguishing factor helpful in identifying this species' fruit. The size and compartmentalized nature of wild sweetwilliam's fruit are noteworthy features that stand out during the identification process.