Of the 77 species of banksia, the majority grow in southwest Western Australia - and (as many a gardener will be able to painfully recall) the majority of these cannot cope with Sydney's weather. The reason for this is that summer humidity provides ideal conditions for the spread of the rootrot fungus Phytophthora cinnamomi, to which western banksias are exquisitely sensitive. Some species, such as Banksia speciosa may appear to grow quickly and thrive for a couple of years before succumbing rapidly one summer.
Banksias are used to well-drained sandy soils or sandy loams and sunny positions. Heavy or clay soils pose some problems. The only species which copes well with heavy soils is Banksia integrifolia, though Banksia robur is fine with poor drainage. Some forms of Banksia spinulosa are found naturally on clay soils in the Sydney basin but growth can be slow. Alternately, soils may be 'improved' by adding gypsum or other soil conditioner or by raising the growing level by at least 30-60 cm. Banksias appreciate extra water during dry periods, especially during summer. Special care shold be taken not to let them dry out until established (i.e. they get roots deep into the gorund and find the water table, which may take anything up to 2 years). Fertilizing with phosphorus should be minimal. A slow release low phosphorous treatment is best. If new leaves turn yellow use iron chelate or iron suphate according to the instructions. Species which are lignotuberous, such as Banksia robur, B. spinulosa and B. serrata, may be pruned -even back to ground level! Others, such as B. ericifolia and B. "Giant Candles" are nonlignotuberous and should be pruned lightly (not below green foliage) and often.
In the past, banksias in nurseries were usually grown from seed or from cuttings from plants raised from seed from the wild. These days more and more named and registered varieties from recognised companies such as Austraflora or Merricks are seen in nurseries - these are all cutting-propagated. This means that the plant grown will be predictable in habit and flower colour. Some seed from dwarf plants may not remain so. Local volunteer or community nurseries are the best source if you are looking for banksia populations that come from your local area (known as 'local provenance' or indigenous plants)- many eastern species are widespread, so you or the nurseryman may have no idea whether the nice Banksia spinulosa you're looking at comes from North Queensland, NSW or Victoria!
Some of the most showy Banksias are those from WA which are very difficult to grow in the Sydney region, generally succumbing within a couple of years of planting (and well before they've had time to flower!). Some species may be available, grafted on to fungal-resistant eastern species. Combinations to look out for include Banksia brownii on B. integrifolia and B. speciosa on B. aemula.
If you want to grow Banksias, and every garden should have them, start with EASTERN SPECIES. As your knowledge and experience grows, you can-then experiment with other more difficult types.
Banksia integrifolia is the easiest banksia to grow and the most forgiving in regards to soil (it will grow in most soils and tolerates alkalinity too). It is a fast growing plant of open, rangy habit and can grow into a large tree. Some unnamed dwarf forms are in nurseries, though some reports indicate they do grow larger than predicted! The prostrate form, "Austraflora Roller Coaster" does remain prostrate. Leaves are serated on young plants, entire on older. The pale yellow to yellow flowers occur in autumn, some forms are greenish in bud, others yellow. In Sydney, the local form is variety integrifolia, which is also the form most commonly seen in nurseries. There is a mountain subspecies monticola with narrow leaves, though this is not seen in nurseries yet.
Banksia serrata occurs in near coastal regions from Queensland to Tasmania. 10 to 12 m high. Large flower spikes summer-autumn, grey-green in bud, turning yellow. Large woody cones outstanding. slow growing but long lived. It has attractive foliage and develops a gnarled warty grey trunk over time, however may take several years to flower. Especially good for exposed coastal sites and as a sand binder. A prostrate form known as B. "Austraflora Pygmy Possum" is available. Banksia aemula is similar but leaves are smaller and flowers brighter. Its fruit is the "Big Bad Banksia Man" of May Gibbs' stories. Both are fairly finicky in their soil requirements, preferring a sandy well drained sunny site.
The Swamp Banksia (B. robur) grows in sand or peaty sand in Qld and NSW. It is a spreading shrub to 2.5 metres. It has very large, leathery leaves with serrated margins up to 30cm long. New growth is colourful, wih shades red, maroon or brown with a dense felt-like covering of brown hairs. Stunning large flower spikes, metallic green with pinkish styles in bud, becoming cream-yellow and fading to brown. This one likes plenty of moisture.
Banksia oblongifolia used to be known as B. asplenifolia and grows from Qld to Ulladulla, NSW, mostly near the coast on moist sites. A 1 to 3m spreading shrub. Very attractive (but slow growing) garden shrub with velvety red or brown new growth on foliage. In bud, flowers can have mauve or aqua highlights before opening to a dull yellow. Best in moist soil in sun or partial shade. B. paludosa is related. From central NSW in coast and mountains. This is a compact shrub to 1m x 1.5m. It does best in sunny position. Both can be pruned to shape.
The Hinchinbrook or Blue Banksia (Banksia plagiocarpa) from North Queensland grows readily in Sydney and is frost tolerant. It is related to B. oblongifolia and has more vivid furry red new growth. The other outstanding feature are the spikes, which are generally blue-grey in bud. It grows to 4m or so and prefers sandy soils. Hinchinbrook gets a lot of rain so this plant would appreciate extra moisture.
B. ericifolia is found in the Sydney Basin and Blue Mountains on sandstone soils. A strong growing bushy shrub with long red to orange spikes with red styles. Some forms, such as 'Kanangra Gold' have paler orange spikes with gold styles. It flowers in late autumn and winter and is very attractive to birds. Plant in sandy soils and don't prune hard.
The Hairpin Banksia (B. spinulosa) is a showy shrub 1-3 m by 1-2m, with long, narrow leaves. Flowers over a long period through autumn and early winter with spikes from yellow to orange, and styles of yellow, orange, red, pink, maroon or black colour. The subspecies most commonly seen in nurseries is variety collina (Hill Banksia), which naturally occurs north of the Hawkesbury whereas variety spinulosa occurs to the south. Given that the varieties hybridise, try and get a local provenance form from your local community nursery, Bushcare or Australian Plants Society group. There are, however, some marvellous dwarf forms available for the city gardener - 'Stumpy Gold' is a form of variety collina originally from the Central Coast, while "Birthday Candles", 'Coastal Cushion' and 'Golden Cascade' are forms of variety spinulosa from the south coast of NSW. In general, Hairpin Banksias prefer sandy well-drained soils, though some local forms hailing from Wiannamatta Shales may do OK on heavier soils.
Banksia "Giant Candles" is an excellent garden plant - believed to be a hybrid between B. ericifolia and B. spinulosa. It grows to 5m high and wide with tall (30cm) all-orange spikes in autumn. Best in well-drained soils and sunny aspects.
Finally, of the Western species, the most successful are the two more vigorous prostrate species - Banksia blechnifolia and B. petiolaris. Both have been grown by many gardeners in well-drained sandy beds. Banksia blechnifolia in particular appreciates some extra moisture and will reward you with odd orangey pinkish spikes in late spring. Banksia media is the other species which has been grown with some success.
- Cas Liber