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Plant Highlight: Banksia nobilis
by Brian Kemble
About the genus
The Australian genus Banksia is a large one, with about 170 species. Most of these are shrubs, though a few of the larger ones are trees. The genus was greatly enlarged in 2007, when all of the species formerly placed in Dryandra were merged into Banksia, based on DNA evidence. It was always known that Dryandra was close to Banksia, but the flower heads of the former are rounded, while the original species included in Banksia generally have cylindrical flower heads. These plants belong to the Proteaceae (Protea Family), and they have flower heads composed of many small flowers packed together, with the long pistils extending outward from the cluster and giving them a pincushion look (much like their South African relatives in the genus Leucospermum). The species formerly in Dryandra are heavily concentrated in Australia’s southwest, in the part of the country with a Mediterranean climate featuring winter rainfall and dry summers. One such species is Banksia nobilis, formerly named Dryandra nobilis and still known by the common name golden dryandra. It is found in the vicinity of Perth, Western Australia.
About the plant
Banksia nobilis is variable in height, ranging from 2 feet to 13 feet (.6 m to 4 m), with upright branches bearing narrow saw-tooth gray-green leaves. Each leaf is between 3.2 and 8.7 inches long (8 to 22 cm) and between .2 inch and 1 inch wide (5 to 25 mm), with prominent triangular lobes accounting for a considerable portion of the width, imparting a jagged look, as though the leaves had been cut with pinking shears. This leaf type, basically a strap with zig-zag edges, is found in many species of Banksia, and even those with less strap-like leaves still usually have serrated edges. As is common with most of its relatives, the undersides of the leaves of B. nobilis are markedly paler and more reflective than the upper surfaces. At the base of the leaf is the short petiole (the leaf stalk), which is between .2 inch and .6 inch in length (5 to 15 mm).
About the flowers and fruits
The flowering time for this species is in winter to early spring, from July to October in its Australian home. This would correspond to January through April in the Northern Hemisphere, but our plant at the Ruth Bancroft Garden has begun a little earlier, in early December.
Some Banksia species in the Dryandra group have their flower clusters at the tips of the branches, but often they are held close to the stem farther down the branch, and this can result in the flower display being partially hidden by the foliage, which is true of Banksia nobilis. However, a mature plant can produce an extraordinary number of flower clusters, so that a plant in flower is a memorable sight.
Each flower head has somewhere between 150 and 250 small flowers, surrounded by a ring of small bracts (modified leaves). The bracts are imbricated (overlapping like fish scales), tinged brownish, hairy, and have fringed margins. The uppermost ones taper to an acute point, but the lowest ones have leaves at their tips, and some in the middle have partially developed leaves. The flower heads may be golden yellow or red-tinged, with the pistils being the most prominent part. In an immature head, the pistils curve inward, so that their ends are clustered together at the center and enveloped by the pale feathery ends of the perianth segments. In this position, they abut the anthers, which upon ripening produce pollen. Once the pollen is present, the pistil lengthens and pulls away from the rest of the flower, with a dusting of pollen on the stigma at its tip.* When they have pulled away and straightened, the radiating pistils of the flower cluster impart the pincushion look so often seen in the genus. A fully open cluster has a diameter of a little over 2 inches (5.5 cm). After serving as a means to transfer pollen to the various creatures that visit the flowers (marsupials, rodents, birds and insects – all attracted by the abundant nectar), the stigmas change their function and become receptive, waiting to be brushed against by pollinators bearing pollen from another plant. Despite the large number of flowers in a head, only a few go on to develop the woody follicles that hold the seeds.
Plants in cultivation
Like many of the Banksia species in the Dryandra group, B. nobilis is adapted to winter rainfall and dry summers, and it does not like humid conditions in summer. It is therefore best suited to growing in areas with a Mediterranean climate. In common with many other species in its family, it occurs in relatively nutrient-poor soils with an acidic pH, so fertilizer should be applied sparingly, and alkaline soils should be avoided or amended. Because of the low levels of phosphorus present in its native habitats, it is especially sensitive to high levels of this fertilizer component. Plants are reported to endure overnight winter lows down to 20° F (–7° C).
*The pistil at this stage of flowering is referred to as the “pollen presenter”. In a typical flower, it is the stamens (the male part of the flower) that make pollen available to floral visitors, with their long wiry filaments holding the pollen-bearing anthers aloft. Here we have a role reversal, with the female part of the flower (the pistil) temporarily performing this function. As unusual as this arrangement may be, plants in the Proteaceae have used it very effectively.
