Forsythia sage features gray-green, heart-shaped leaves that are approximately 4-6 inches (10-15 cm) long and 2-4 inches (5-10 cm) wide. The leaves display a slightly serrated margin and a velvety texture to the touch. Prominent veins create a net-like pattern, contributing to their distinctive appearance. The upper surface of the leaves is often a muted grayish-green, while the underside is a lighter shade. These characteristics make them easily recognizable for plant enthusiasts.

The flowers of forsythia sage appear in whorls along 12-inch (30 cm) spikes. These yellow flowers are somewhat unusual for their genus and can feel sticky to the touch. The calyces are the same bright yellow as the flowers themselves. The blooms are often tubular in shape, forming a dense, eye-catching display. While exact blooming periods can vary, they typically flower in late summer to fall. Their distinct coloring and arrangement make them a unique identifier for forsythia sage.

The stem of forsythia sage is thick and rigid, providing robust structural support. It exhibits a green to light brown hue, sometimes with a slightly woody texture as it matures. Measuring around 0.5 to 1 inch (1.25 to 2.5 cm) in diameter, the stem has a sturdy construction. The branching pattern is irregular, with new growth often emerging from axillary buds. This stout stem is a key identifying feature, setting forsythia sage apart from other species with its solid and stiff characteristics.

The fruit of forsythia sage is a schizocarp, distinguishing itself by splitting into four separate chambers when mature. Each chamber houses a single nutlet. These nutlets are typically small and hard-textured, possibly exhibiting a brown or tan color upon maturity. The schizocarps themselves are only a few millimeters in length, generally not exceeding 0.2 inches (5 mm). The distinctive multi-chambered characteristic of the fruit is a significant identification feature for those studying the plant's reproductive structures.