Banksia Grossa - Description

Description

Banksia grossa grows as a bushy shrub, generally 70 cm to 1 m (28–39 in) high, or occasionally up to 1.5 m (5 ft) high. Its many stems rise from a woody lignotuber. Young stems have a coating of woolly hairs, while older stems are covered in flaky pale brown bark. Borne on 3 to 5 mm long petioles, the somewhat fleshy leaves are needle-like. Although they are between 4 to 12 cm (1.6–4.6 in) long, and only 2 to 3 mm wide, they are still thicker than those of other members of the series Abietinae. The margins of the leaves are straight with no teeth and rolled over, and the upper surfaces are sparsely hairy when young, but later hairless. New growth occurs over spring and early summer.

Flowers occur in a typical Banksia flower spike: an inflorescence made up of hundreds of small individual flowers, or florets, densely packed around (and completely obscuring) a woody cylindrical axis. In B. grossa, this axis is 5 to 7 cm (2–2.5 in) high with a diameter of 0.7–0.9 cm (0.3–0.4 in). From this, the florets radiate out laterally, giving the inflorescence a diameter of 8 or 9 cm (3.2–3.5 in). Flowers are rusty brown to golden brown, and consist of a hairy 3.4–4.5 cm (1.5–1.8 in) long tubular perianth which opens at maturity (anthesis) to release the dark red to purple style. 3.8–4.8 cm (1.6–1.9 in) long, the style extends past the perianth and is curved at the tip. The flower spikes arise from short thick branchlets running off larger stems, though some flower spikes are terminal (located at the ends of branches and more prominently displayed in the foliage).

Flowering occurs between March and September, though early flowering in December has been recorded. It takes five to eight weeks for a flower to develop from bud to the end of flowering. About three weeks before the flowers open, they develop a strong musky smell. The opening of flowers occurs sequentially, starting at the bottom of the inflorescence and sweeping through to the top over a period of around two weeks. At anthesis the flowers produce copious quantities of nectar; indeed, some flowers produce so much that it drips to the ground. After flowering, the old florets wither and curl against the spike, giving it a hairy appearance. Now known as an infructescence, it is roughly ellipsoidal, 6 to 10 cm high (2.2–4 in) and 4 to 8 cm (1.6–3.2 in) wide. Up to 25 smooth, elliptical follicles develop on the spike, each containing up to two wide wedge-shaped winged seeds. One field study revealed, on average, eight follicles for each fertile cone. Initially covered in fine fur, these are 2 to 4.5 cm (0.8–1.8 in) long, 1–1.8 cm (0.4–0.7 in) high and jut out by 1–1.8 cm (0.4–0.7 in). The fur rubs off and they become smooth with wear, and generally remain closed until opened by fire.

The seeds of Banksia grossa are the largest of all the species of the series Abietinae. Measuring 2.8–3.9 cm (1.1–1.5 in) long, they are made up of a cuneate (wedge-shaped) seed body, 1.4–1.8 cm (0.6–0.7 in) long by 0.4–0.9 cm (0.2–0.4 in) wide, and a 2.7–3.3 cm (1.1–1.3 in) wide wing. The woody separator is the same shape as the seed, with an impression where the seed body lies next to it. The bright green cotyledons are obovate and can be either convex or concave, measuring 1.6 to 2.2 cm long by 0.9 to 1.2 cm wide. These arise from a stocky seedling stem, known as the hypocotyl, which is reddish and covered in short hairs. The auricles of the cotyledons are 2 mm long. Seedling leaves arise 0.6 to 0.8 cm beyond the cotyledons and are oppositely arranged. Linear, they are 1.4 to 1.6 cm long with recurved margins and are covered in white hair. The stem remains reddish. Subsequent leaf pairs are successively longer.