Banksia grossa

Banksia grossa grows as a bushy shrub, generally 0.7 to 1 m (28 to 39 in) high,[2] or occasionally up to 1.5 m (4.9 ft) high.[3] Its many stems rise from a woody lignotuber.[2] 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 and 12 cm (1.5 and 4.5 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.[4] New growth occurs in the spring and early summer.[3]

The first known specimen collection of B. grossa was made by botanist Fred W. Humphreys in 1965. Alex George has expressed surprise that it was not collected by earlier visitors to the area, speculating that "perhaps they were diverted by its similarity to B. leptophylla and B. sphaerocarpa." George formally described the species in his 1981 monograph "The genus Banksia L.f. (Proteaceae)", based on a specimen collected by him on the Brand Highway, 76 km north of Regans Ford, on 14 May 1969. He placed it in subgenus Banksia because of its flower spike; section Oncostylis because its styles are hooked; and the resurrected series Abietinae, which he constrained to contain only round-fruited species. It was placed in phyletic order between B. micrantha and B. leptophylla. The specific epithet is from the Latin grossus ("coarse") and refers to the leaves, flowers, and fruit, all of which George observed were more coarse than other members of B. ser. Abietinae.[2]

George's arrangement remained current until 1996, when Kevin Thiele and Pauline Ladiges published an arrangement informed by a cladistic analysis of morphological characteristics. They retained George's subgenera and many of his series, but discarded his sections. Banksia ser. Abietinae was found to be very nearly monophyletic, and so was retained. It further resolved into four subclades, so Thiele and Ladiges split it into four subseries. Banksia grossa appeared as the most basal member of the second of these:

Typical shrubby habit, in cultivation, Kings Park, Perth, Western Australia

This clade became the basis for the new subseries Sphaerocarpae, which Thiele defined as containing those species with lignotubers, styles loosely curling around the infructescence (although this trait was reversed in B. micrantha), and "transversely aligned cells of the seed wing inner face". The species other than B. grossa also have shouldered follicles.[7]

Questioning the emphasis on cladistics in Thiele and Ladiges' arrangement, George published a slightly modified version of his 1981 arrangement in his 1999 treatment of Banksia for the Flora of Australia series of monographs. The placement of B. grossa was the same as in 1981, except that B. telmatiaea was moved to set between it and B. leptophylla. The placement of B. grossa in George's 1999 arrangement may be summarised as follows:[4]

Foliage, highlighting new growth

Inflorescence in early bud in front, old cone with follicles behind

Banksia

B. subg. Banksia

B. sect. Banksia (9 series, 50 species, 9 subspecies, 3 varieties)

B. sect. Coccinea (1 species)

B. sect. Oncostylis

B. ser. Spicigerae (7 species, 2 subspecies, 4 varieties)

B. ser. Tricuspidae (1 species)

B. ser. Dryandroideae (1 species)

B. ser. Abietinae

B. sphaerocarpa (5 varieties)

B. micrantha

B. grossa

B. telmatiaea

B. leptophylla (2 varieties)

B. lanata

B. scabrella

B. violacea

B. incana

B. laricina

B. pulchella

B. meisneri (2 subspecies)

B. nutans (2 varieties)

B. subg. Isostylis (3 species)

Since 1998, Austin Mast has been publishing results of ongoing cladistic analyses of DNA sequence data for the subtribe Banksiinae. His analyses suggest a phylogeny that is very different from George's arrangement. Banksia grossa was inferred to be sister to a polytomous clade consisting of B. leptophylla, B. telmatiaea, B. scabrella and B. lanata:[8][9][10]

Early in 2007, Mast and Thiele initiated a rearrangement of Banksia by merging Dryandra into it, and publishing B. subg. Spathulatae for the taxa having spoon-shaped cotyledons. They foreshadowed publishing a full arrangement once DNA sampling of Dryandra was complete; in the meantime, if Mast and Thiele's nomenclatural changes are taken as an interim arrangement, then B. grossa is placed in B. subg.. Spathulatae.[11]

Distribution in Western Australia

Banksia grossa mainly occurs from west of Yandanooka in the north, south to Badgingarra National Park, and east almost to Coorow. It grows in shallow sand over laterite, in deep sand, and occasionally atop lateritic rises, on flat or gently undulating land, among kwongan shrubland.[3][4] Around 40% of plants occur on road verges.[3]

Banksia grossa resprouts from its woody lignotuber after bushfire. Shrubs carry relatively low numbers of seeds compared with plant species that regenerate by seed after bushfire. A field study in Eneabba showed shrubs bore anywhere from 30 to 90 seeds per plant. Seeds borne on plants, however, are less likely to be eaten by animals than seed lying dormant in the soil, and hence survive to produce future seedlings. Populations of resprouting plants, like B. grossa, benefit more from longer intervals between fires as their stored seedbank takes longer to accumulate than reseeding species.[12] The follicles generally open after bushfire, releasing seeds which germinate after disturbance.[6] Species with persistent flowers on old spikes often require higher temperatures for follicles to open after burning; laboratory testing resulted in 50% opening after exposure to 270 °C and 90% after 400 °C.[6]

The leaves of B. grossa are adapted to a dry climate. The lower surfaces are protected by the strongly revolute shape of the leaf, the leaf margins curling around underneath almost to the mid-vein. The trichomes (leaf hairs) run along the mid-vein and the margins, further protecting the surface where the stomates are located, thus minimising water loss.[13] Cladistic analysis suggests this species and its relatives in the series Abietinae developed long narrow leaves with inrolled margins as they invaded drier climates in Australia's southwest, having evolved from ancestors with broad leaves.[9]

The inflorescences are highly attractive to insects.[5] The rare bee species Hylaeus globuliferus is a recorded visitor.[14] Large numbers of ladybirds have been reported drinking the nectar of this species, while ignoring nearby Banksia attenuata inflorescences.[5] The white-tailed dunnart (Sminthopsis granulipes) has been observed visiting flower spikes. Pollen has been found on the snouts and in the scat of several species of dasyurid marsupials, which suggests these animals are likely pollinators of Banksia species in general.[15] No pollinators of this species were recorded in The Banksia Atlas survey.[3]

Banksia grossa is one of five closely related Banksia species that have highly unusual flower nectar.[lower-alpha 1] Whereas other Banksia species produce nectar that is clear and watery, the nectar of these species is pale yellow initially, but gradually becomes darker and thicker, changing to a thick, olive-green mucilage within one to two days of secretion, and eventually becoming "an almost black, gelatinous lump adhering to the base of the flowers".[17] It was first noted by Byron Lamont in 1980; he attributed it to cyanobacteria that feed off the nectar sugars. Noting that many of these cyanobacteria had heterocysts, he speculated that they aid the plant by fixing atmospheric nitrogen, which is then washed off the flower heads by rain, and absorbed by the proteoid root mat. This purported symbiosis was investigated by Barrett and Lamont in 1985, but no evidence of nitrogen fixing was found.[18] Further investigation by Markey and Lamont in 1996 suggested that the discolouration is not caused by cyanobacteria or other microorganisms in the nectar, but is rather "a chemical phenomenon of plant origin". Their analyses indicated that the nectar had unusually high levels of sugar and free amino acids,[5] but three of these species, including B. sphaerocarpa, have since been shown to have normal nectar sugar compositions.[19] The purpose of coloured nectar is unclear, especially as pollinators such as nocturnal mammals are not thought to forage by sight. However, nectar that becomes more obvious by appearance or smell as it ages might encourage pollinators to prioritise it over newer nectar. It is possible the colour change is unrelated to pollination.[16]

Banksia grossa is cultivated for its attractive needle-like leaves and rusty-brown flower spikes, both of which can be quite variable in colour. This species favours well-drained sandy or loamy acidic soils with a pH of 5.5 to 7, and a sunny aspect. Once established, it tolerates dry spells. It can be pruned heavily as it resprouts from its lignotuber.[20]

Propagation is by seed.[20] The seeds do not require any treatment before sowing, and take around 14 days to germinate.[21] This is a slow-growing plant, taking 5 to 7 years to reach maturity and begin flowering.[20]

This banksia was featured on an Australian postage stamp in 2018.[22]