How Banksia serrata Works With Mycorrhizal Fungi

Banksia serrata, commonly called old man banksia, is one of the most recognisable plants of eastern Australia. Its gnarled trunk, serrated leaves and large woody flower spikes are familiar across coastal heath, sandy woodland and dry sclerophyll forest, including parts of the Tomaree Peninsula around Nelson Bay, Fingal Bay and Anna Bay. Beneath the surface, however, its survival depends on a less visible partnership with soil fungi.

The symbiotic relationship between Banksia serrata and mycorrhizal fungi helps the tree obtain nutrients from soils that are often sandy, acidic and naturally low in phosphorus. In return, the fungi receive sugars produced by the banksia through photosynthesis. This exchange connects root biology with local geology, rainfall, fire history and the health of the wider plant community.

What grows beneath old man banksia

Banksia roots can form specialised structures known as cluster roots, sometimes called proteoid roots. These dense groups of short rootlets greatly increase the area available for nutrient uptake. They are particularly useful where phosphorus is present in forms that ordinary roots cannot easily access. The roots release organic acids and other compounds that help mobilise nutrients around them.

Mycorrhizal fungi extend this absorbing network into the surrounding soil. Their fine filaments, or hyphae, can reach tiny spaces beyond the immediate root zone and transport water, phosphorus and other minerals back to the plant. The fungus gains carbohydrates from the banksia, while the tree gains access to resources that would otherwise be difficult to obtain. The arrangement is cooperative, although its success depends on the species of fungus, soil conditions and the health of the host plant.

Banksia serrata may associate with several kinds of root fungi rather than relying on one permanent fungal partner. Ectomycorrhizal fungi are especially important in many banksias, forming a sheath around fine roots and penetrating the spaces between root cells. Other root-associated fungi can occur as endophytes, living within tissues without producing the classic external sheath. These communities can change with age, season, fire and neighbouring vegetation.

A partnership shaped by Tomaree soils

On the Tomaree Peninsula, old man banksia often grows in deep coastal sand, dune swales and heathland edges. Areas near Birubi Beach, One Mile and Shoal Bay can experience salt-laden winds, intense summer sun and rapid drainage after rain. Sand holds little water and few nutrients compared with richer clay soils, so a broad fungal network can be valuable during dry periods.

The banksia’s low-phosphorus strategy also explains why fertiliser can be harmful in native gardens. A heavy application of soluble phosphorus may upset the balance between roots and fungi, encourage fast-growing weeds and damage plants adapted to nutrient-poor country. Locally sourced native plants, free-draining soil and restrained feeding are generally more compatible with the conditions in which old man banksia evolved.

Field identification also matters because several woodland trees can appear together. A plant labelled simply as a “gum tree” may belong to Eucalyptus or Corymbia, and their root associations and habitat preferences are not identical. The photographic guide to distinguishing eucalyptus from corymbia provides useful context when recording banksia habitat during a walk.

Nutrients, drought and the role of fire

Mycorrhizal fungi are particularly valuable when water and nutrients are patchy. Their hyphae can improve contact with damp soil after light rainfall, although they cannot prevent a banksia from suffering through a prolonged drought. The relationship may also improve the plant’s tolerance of salt exposure, heat and certain soil pathogens, but the outcome varies according to the fungal community and local conditions.

Fire is another major influence. Banksia serrata can regenerate after bushfire from seed, and mature individuals may survive some fires because of their thick bark and protected buds. A fire can temporarily reduce living fungal networks near the surface, alter soil chemistry and create a flush of available nutrients. Fungal communities then rebuild from surviving roots, deeper soil and spores carried by wind, animals or water.

The timing and severity of burning are important. Frequent, intense fires can remove mature seed-bearing banksias before they have replenished the soil seed bank. They may also simplify the plant community, reducing the range of fungi and host plants available for recolonisation. In Tomaree National Park and nearby bushland, fire management therefore has implications that extend below ground as well as across the visible vegetation.

Reading the relationship in the field

The partnership cannot usually be identified by looking at a leaf or flower alone. A healthy old man banksia may have firm new growth, well-formed leaves and regular flowering, but those features reflect many influences, including rainfall and browsing. The fungi themselves are often hidden, while the mushrooms or other fruiting bodies they produce may appear only briefly after suitable weather.

Careful observation can still reveal useful ecological clues. Note whether the banksia is growing in pale sand, heavier soil, a damp depression or a wind-exposed ridge. Record nearby species such as tea-trees, she-oaks, eucalypts, grass trees and hakeas, because different hosts help support a varied below-ground fungal community. Avoid disturbing roots or collecting fungi in protected bushland; a photograph and location record are usually more useful than a specimen.

Seed movement adds another layer to the story. Banksia follicles can retain seeds for years, releasing them after heat, drying or physical disturbance. In wetter forest systems, water can also move plant material and nutrients through drainage lines and swamp margins. The account of seasonal seed movement helps place banksia regeneration within the wider movement of seeds across coastal landscapes.

Why the fungal partnership matters

For gardeners in Port Stephens, the relationship is a reminder that a native plant is part of a living soil system rather than an isolated ornamental. A newly planted Banksia serrata may struggle if its roots are waterlogged, compacted or surrounded by rich imported soil. Good drainage, careful watering during establishment and minimal disturbance give existing soil organisms a better chance of remaining active.

Commercial mycorrhizal inoculants are often marketed as a quick solution, but adding a packaged fungus does not guarantee a useful partnership. The introduced organism may not suit the local soil, climate or host, and many native plants already encounter compatible fungi naturally. Protecting remnant bushland, retaining leaf litter and avoiding unnecessary soil movement are more reliable ways to conserve fungal diversity.

This connection also has value for students and bushwalkers learning to read Australian ecology. The flowers above ground attract insects, birds and mammals, while the roots below ground negotiate access to scarce nutrients with microscopic partners. Old man banksia therefore represents more than a distinctive coastal plant: it is a visible expression of the hidden relationships that allow Australian vegetation to persist on difficult soils.