Tree habit, Illabarook Rail Line Nature Conservation Reserve
Banksia marginata is a highly variable species, usually ranging from a small shrub around a metre (3 ft) tall to a 12-metre-high (39 ft) tree. Unusually large trees of 15 to possibly 30 m (50–100 ft) have been reported near Beeac in Victoria's Western District as well as several locations in Tasmania, while compact shrubs limited to 20 cm (7.9 in) high have been recorded on coastal heathland in Tasmania (such as at Rocky Cape National Park). Shrubs reach only 2 m (6.6 ft) high in Gibraltar Range National Park. The bark is pale grey and initially smooth before becoming finely tessellated with age. The new branchlets are hairy at first but lose their hairs as they mature, with new growth a pale or pinkish brown. The leaves are alternately arranged on the stems on 2–5 mm long petioles, and characteristically toothed in juvenile or younger leaves (3–7 cm [1.2–2.8 in] long). The narrow adult leaves are dull green in colour and generally linear, oblong or wedge-shaped (cuneate) and measure 1.5–6 cm (0.6–2.4 in) long and 0.3–1.3 cm (0.1–0.5 in) wide. The margins become entire with age, and the tip is most commonly truncate or emarginate, but can be acute or mucronate. The cellular makeup of the leaves shows evidence of lignification, and the leaves themselves are somewhat stiff. Leaves also have sunken stomates. The leaf undersurface is white with a prominent midrib covered in brownish hairs.
The complex flower spikes, known as inflorescences, appear generally from late summer to early winter (February to June) in New South Wales and Victoria, although flowering occurs in late autumn and winter in the Gibraltar Range. Cylindrical in shape, they are composed of a central woody spike or axis, perpendicularly from which a large number of compact floral units arise, which measure 5–10 cm (2–4 in) tall and 4–6 cm (1.6–2.4 in) wide. Pale yellow in colour, they are composed of up to 1,000 individual flowers (784 recorded in the Gibraltar Range) and arise from nodes on branchlets that are at least three years old. Sometimes two may grow from successive nodes in the same flowering season. They can have a grey or golden tinge in late bud. As with most banksias, anthesis is acropetal; the opening of the individual buds proceeds up the flower spike from the base to the top. Over time the flower spikes fade to brown and then grey, and the old flowers generally persist on the cone. The woody follicles grow in the six months after flowering, with up to 150 developing on a single flower spike. In many populations, only a few follicles develop. Small and elliptic, they measure 0.7–1.7 cm (0.3–0.7 in) long, 0.2–0.5 cm (0.1–0.2 in) high, and 0.2–0.4 cm (0.1–0.2 in) wide. In coastal and floodplain populations, these usually open spontaneously and release seed, while they generally remain sealed until burnt by fire in plants from heathland and montane habitats. Each follicle contains one or two fertile seeds, between which lies a woody dark brown separator of similar shape to the seeds. Measuring 0.9–1.5 cm (0.4–0.6 in) in length, the seed is egg- to wedge-shaped (obovate to cuneate) and composed of a dark brown 0.8–1.1 cm (0.3–0.4 in) wide membranous "wing" and wedge- or sickle-shaped (cuneate–falcate) seed proper which measures 0.5–0.8 cm (0.2–0.3 in) long by 0.3–0.4 cm (0.1–0.2 in) wide. The seed surface can be smooth or covered in tiny ridges, and often glistens. The resulting seedling first grows two obovate cotyledon leaves, which may remain for several months as several more leaves appear. The cotyledons of Banksia marginata, B. paludosa and B. integrifolia are very similar in appearance.