Growing banksias: phosphorus, drainage and picking the right species

Banksias that fail in home gardens usually fail in the first two summers, and three mistakes cover nearly all of it. Someone fed it. Someone planted it into soil that holds water. Or someone bought a Western Australian species for an east coast garden that gets its rain in summer. Avoid those three and banksias are long lived and ask almost nothing of you.

Why ordinary fertiliser kills proteaceae

Banksias, grevilleas, hakeas and the rest of the Proteaceae evolved on old, weathered Australian soils where phosphorus is scarce. To cope, they produce dense mats of fine cluster roots, often called proteoid roots, that release acids and strip out whatever phosphorus is there. It is an efficient system with one flaw: many of these plants do not throttle that uptake back when phosphorus is suddenly plentiful. Put a banksia into soil carrying ordinary garden levels of phosphorus and it keeps mining, keeps absorbing, and eventually poisons itself.

Older leaves yellow between the veins, then blacken at the tips and margins, then brown off. New growth stalls. Damage appears weeks or months after feeding, which is why gardeners rarely connect the two.

Avoid general purpose garden fertiliser, lawn food, poultry manure, blood and bone, mushroom compost, and any controlled release pellet not labelled for native plants. Watch indirect exposure too: a banksia below a fertilised lawn collects what washes downhill, and off-the-shelf potting mix is fed for hungry plants, so pot into a low phosphorus native mix. Use a fertiliser labelled low phosphorus or formulated for phosphorus sensitive natives, applied lightly in early spring and not at all in the first season. Composted leaf litter and coarse bark mulch are safe, and seaweed extract is a root tonic rather than a fertiliser, usable at label rates. In reasonable soil, most banksias never need feeding at all.

Drainage is the other half of the problem

Phytophthora cinnamomi, a soil borne water mould, thrives in warm wet ground and accounts for a large share of banksia deaths. Proteaceae are among its preferred hosts, so a banksia in soil that stays damp for days after summer rain is on a countdown.

On clay, do not dig a deep hole and backfill with good mix: that builds a sump which fills every time it rains. Mound instead. Raise the planting area 300mm or more with a sandy, gritty, low fertility mix, set the root ball slightly proud of the surrounding soil, and let water shed away sideways. On sand you can plant flat, though you still water through the first summer. Keep irrigation off established plants; the usual mistake is leaving a banksia on the same daily dripper line as a lawn.

When not to buy: Western Australian species in humid gardens

This is where most of the disappointment comes from. Western Australia has a Mediterranean climate: rain in winter, a long dry summer and low humidity. Acorn Banksia, Banksia prionotes, Candlestick Banksia, Banksia attenuata, Banksia sessilis and Banksia nivea all come from deep, sharply drained sand in that world. Banksia telmatiaea sits in seasonally wet WA swamps rather than dry sand, but it runs on the same winter wet, summer dry cycle and reacts the same way to a wet January.

Bring them to Brisbane, the Sydney basin, coastal NSW or subtropical Queensland and you are asking them to sit in warm, wet soil through the months they are built to spend bone dry. They commonly look fine for a season, flower once, then collapse over a wet summer. That is a climate mismatch, not a care failure, and no fertiliser or watering schedule fixes it. Do not buy these species for a humid summer rainfall garden and expect a long lived shrub.

Where they do work: Perth, Adelaide and the drier inland districts of Victoria and South Australia, in sand or a sharply drained raised bed. A large pot of coarse, sandy, low phosphorus mix also works where you control every drop of water, though not for as long as an eastern species would live. In a humid climate, plant Banksia ericifolia or Banksia spinulosa instead. Two other times to skip the genus altogether: when the bed shares a dripper line and a spring feed with the rest of the garden, and when the soil is heavy clay and you are not willing to mound.

Choosing a species for the space you actually have

Coast Banksia, Banksia integrifolia is the workhorse of the genus. It handles salt wind, poor sand, humidity and drought, grows quickly, and reaches roughly 8 to 15 metres, so it needs genuine room. This is a tree, not a shrub: think windbreak, large front garden or exposed coastal block. Where space is narrow, the Sentinel form is upright and much tighter, suiting a driveway edge or side boundary.

Old Man Banksia, Banksia serrata is slower and smaller, usually 5 to 8 metres, with thick corky bark, saw edged leaves and heavy grey green cones. Plant it as a feature for character rather than screening; the gnarled trunk takes years.

Hairpin Banksia, Banksia spinulosa is the safe pick for a normal suburban garden. It stays a shrub of one to three metres, tolerates part shade and heavier soils better than most of the genus, flowers through autumn and winter, and suits a mixed native bed or a large pot. Heath Banksia, Banksia ericifolia is denser and taller at three to five metres with fine heath like foliage, and it is the one for an informal screen rather than a single specimen. Silver Banksia, Banksia marginata, suits cool temperate gardens in Victoria, South Australia and the tablelands, and is less reliable in subtropical humidity.

Pruning, and whether to cut below the flower

The thing to know about pruning banksias is that most of them do not break new growth from leafless stems. Cut into a length of stem carrying no leaves and it dies back to the next live growing point, or simply dies. Hence the rule: prune lightly, prune often, and always leave foliage below every cut.

So should you cut below the flower? Yes, but only as far as the leafy wood beneath it. Take the spent flower with a short length of stem, finishing just above healthy leaves, and the plant stays compact. Reaching into bare wood for a longer vase stem is how people end up with dead stubs.

The exceptions are the lignotuberous species. Banksia spinulosa, Banksia serrata and Banksia marginata form a woody swelling at the base and can be cut back hard, in some cases close to ground level, and will regrow from it. Do it as growth starts in early spring, once flowering has finished, rather than in mid summer heat, and only on an established plant. A young shrubby plant takes it far better than an old Banksia serrata carrying a real trunk, which is better reduced over two or three seasons. Whatever the species, start tip pruning in the first year: keeping a banksia dense is far easier than making a leggy one dense again. Leave the old cones on the plant; removing them does not improve flowering.

Bird value, and how to get more of it

Banksias earn their place largely because of when they flower. Spinulosa, ericifolia and integrifolia carry nectar through autumn and winter, the months when most other garden plants have shut down and honeyeaters, wattlebirds and lorikeets have little else to feed on.

The cones matter too. Yellow tailed black cockatoos work over serrata and integrifolia seed, and the prickly framework of ericifolia and spinulosa gives small birds cover, often the missing piece in a garden with plenty of nectar and nowhere to hide.

To spread feeding across the year, plant banksias alongside grevilleas and bottlebrush, which cover the spring and summer gap. Our guide to grevilleas and bird attracting plants sets out that sequence.

Buying and planting notes

Proteaceae resent root disturbance, so smaller stock often establishes faster than advanced stock. A 140mm to 250mm shrub planted in autumn will usually overtake a much larger plant put in the same day. Autumn suits most of the country, though in frost prone tablelands and inland districts spring is the safer window. For immediate height from Coast Banksia, larger sizes do work, but handle the root ball gently, do not tease the roots, and plant slightly high.

Water in with plain water, mulch with coarse bark clear of the trunk, and water deeply weekly through the first summer before tapering off. After that these plants sit in the low maintenance and native part of the garden. For exposed blocks, read our guide to coastal planting for salt and wind, and for size from day one the advanced trees and feature trees ranges carry Coast Banksia in larger pots.

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