This species has narrow green leaves, and brownish, orange or yellow round flower spikes which may be seen from January to July.

B. sphaerocarpa is classified as Not Threatened under the Wildlife Conservation Act of Western Australia, although two varieties have been placed on the Declared Rare and Priority Flora List—var.

All varieties bear a lignotuber, a swollen starchy root crown from which the plant resprouts after bushfire.

The leaves are stiff, narrow and linear, and measure 2.5–10 cm (1.0–3.9 in) in length, on a petiole 2–3 mm (0.08–0.12 in) long.

Anthesis takes place over two weeks, as the individual flowers open in a wave across the inflorescence.

[2] Northern specimens can be quite small shrubs and may be hard to distinguish from B. micrantha, which has smaller inflorescences and large flattened follicles.

[2] The earliest known botanical collection of B. sphaerocarpa occurred in December 1801, during the visit of HMS Investigator to King George Sound.

The specimen was collected from "A single plant observed between Princess Royal Harbour & Oyster Harbour on a heath",[6] and is credited to English botanist Robert Brown, though it is not possible to be certain on this point, as Brown incorporated the collections of junior expedition members into his herbarium without attribution.

[10] He also did not state the etymology of the specific epithet, but it is accepted that the name derives from the Ancient Greek sphaera- ("round"), and carpos ("fruit"), in reference to the shape of its infructescences.

[9] Swiss botanist Carl Meissner published a more detailed arrangement in 1856, placing B. sphaerocarpa in section Eubanksia because its inflorescence is a spike rather than a domed head, and in series Abietinae, whose members have inrolled leaf margins with no, or only very fine, serrations.

[12] George Bentham's revision of Banksia for his 1870 Flora Australiensis overturned Meissner's series; instead, B. sphaerocarpa was placed in a new section, Oncostylis, because of its hooked styles.

[10][16] George placed B. sphaerocarpa 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.

Banksia sphaerocarpa appeared in the second of these, initially called the ' grossa clade' for its most basal member.

The placement of B. sphaerocarpa in George's 1999 arrangement may be summarised as follows:[2] A 2002 study by American botanists Austin Mast and Tom Givnish yielded a surprise when molecular analysis mapped out dolichostyla as a sister taxon to its geographical neighbour B. violacea, in a clade with B. laricina and B. incana.

[18] Early in 2007, Mast and Thiele initiated a rearrangement of Banksia by merging Dryandra into it, and publishing B. subg.

[3] Banksia sphaerocarpa is a widely distributed Western Australian species with four (or five if one includes var.

dolichostyla) varieties: Banksia sphaerocarpa is distributed widely across southwestern Western Australia—from Eneabba in the north, south to the Whicher Range, Nannup, Albany and Jerramungup, and eastwards to the vicinity of Hyden.

It is mainly found on sandy soils in flat or gently sloped areas in shrubland, mallee or open woodland.

[20] As a species, B. sphaerocarpa is not considered to be under threat,[21] but two of the five varieties have been placed on the Declared Rare and Priority Flora List.

[24] Botanist Stephen Hopper found pollen of B. sphaerocarpa on New Holland honeyeaters (Phylidonyris novaehollandiae) and honey possums (Tarsipes rostratus) at Cheyne Beach in a field study published in 1980.

[b] 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".

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.

[29] 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".

[30] Like most Western Australian Banksia species, B. sphaerocarpa is susceptible to dieback from the soil-borne water mould Phytophthora cinnamomi.

[32] In another experiment, 37% of plants were dead within three months, and 85% within a year; yet this study rated the species' susceptibility as "high".

[34] Otherwise, they adapt well to gardens with good drainage, sandy soils and sunny aspects in drier and Mediterranean (winter moisture) climates, and are also frost tolerant.