In 2009, Grant Bowie posted a ground-breaking article on Banksia integrifolia as bonsai to AusBonsai. In the decade since, Grant’s article has been viewed over 56,000 times and generated a discussion that has not ended. There are now hundreds of posts on AusBonsai on B. integrifolia, many of them by Grant in response to questions raised by others.

I have trawled through all posts on B. integrifolia, developed a summary of the advice they contain and posted it to AusBonsai. I have lightly edited the AusBonsai post, mainly to focus on advice that is most relevant to the Canberra climate, and divided it into two parts. Part 1 follows. Part 2 will be published in the next edition of Wirrabara.

About Banksia integrifolia

Banksia integrifolia, the Coast Banksia, has had a complicated taxonomic history, with numerous species and varieties ascribed to it, only to be rejected or promoted to separate species. The taxonomy is now largely settled, with three subspecies recognised: B. integrifolia subsp. integrifolia, B. integrifolia subsp. compar and B.integrifolia subsp. monticola.

According to the Australian National Botanic Gardens, this hardy Banksia grows very well in coastal areas and can tolerate salt spray. Plants may grow to 25 metres but are often smaller. The dark green leaves have a silvery underside, which adds to the beauty of the foliage. The pale yellow flowers open from late summer to winter. Plants are frost tolerant. See photograph 1.

The species can be quite variable in terms of bonsai vigour, if you get a non-vigorous one toss it out, it may struggle on for years but return nothing in the end. There are also variations with the bark of B. integrifolia: some have a whitish rough but fine bark and others have a darker and coarser bark.

Leaves

B. integrifolia leaves are large, long and smooth edged, but with constant trimming as described below, the tree will put out a juvenile form of leaf that is small, blunt and toothed, which is most suitable for bonsai. These leaves are still strong and vigorous and not deformed or delicate in any way.

Defoliation

Although some bonsai artists say they defoliate their B. integrifolia, especially when re-potting, others report say the practice is either unnecessary or detrimental to the tree’s health.

Roots

The roots of B. integrifolia are very fine and will fill the pot quickly; in summer you will need to water more often than most other plants in pots. Be aware that Banksia can grow proteoid roots. Helen Purnell, the scientist who coined the term ‘proteoid root’ in 1960, described them as a root with dense clusters of rootlets of limited growth.

They look like the mycorrhizae you may see on the root system of pines, that is a fine web-like white patch. Proteoid roots are the trees natural response to a lack of nutrients. The tree puts out a set of roots that are very fine and are able to scavenge very small amounts of nutrients from the soil or potting mix.

If there is suddenly an abundance of nutrients it can take up too much and suffer badly or even die. Banksia can’t quickly stop the working of proteoid roots when the supply of nutrients suddenly increases. They will, however, slowly replace proteoid roots with regular roots if increasing amounts of nutrients are slowly introduced. So, if you have a tree that is rootbound or very hungry, you must fertilise lightly to start with and then build up to a normal fertilising regime.

See below for more on fertilising. See Wirrabara July 2019 and the CBS Knowledgebase for more on proteoid roots.

Soil preference

There is general agreement in the literature that B. integrifolia (growing in the ground) requires an open sunny position in a soil with very good drainage.

Your choice of potting mix will clearly have a major impact on your watering regime: the higher its water-holding capacity the less water you will need to apply. See Watering re-imagined in Wirrabara June 2019 for more on the question of balancing the volume of water applied to a pot to the water-holding capacity of the potting mix.

Some people use straight akadama; others use a more complex formula such as 4 parts diatomite, 2 parts zeolite, 2 parts mini pine bark nuggets, 2 parts coconut fibre and 1 part akadama.

Fertilising

B. integrifolia can grow proteoid roots (see ‘Roots’ above) but appropriate fertilising and root-pruning/re-potting will manage this contingency.

It is a good idea to fertilise your B. integrifolia regularly. Use your fertiliser of choice: e.g. the recommended dose of slow-release fertiliser in the potting mix and liquid feed every week or two (except winter) with a mild liquid fertiliser like Maxicrop, Nitrosol, or Charlie Carp etc. Experienced growers report no bad reactions to the use of Seasol and some have been including it regularly in their fertilising regimes. It is suggested that you start with low doses of Seasol until you build up the vigour of the particular plant.

It is better to have healthy, vigorous plants (as per photograph 2) and then deal with the resultant growth in the shaping process, rather than having weak, sickly plants that have been dwarfed by starvation. If you choose to follow a higher fertilising regime than you have been using it is recommended that you build up to it gradually.

Use fertilisers at full recommended strength unless plants are sick or new to your collection or you have never fertilised your tree. In these cases, apply fertiliser at around 40-50% strength for a month or two and then build up to full strength over the next four months or so.

At least two very experienced CBS members (Grant B and Neil P, both of whom run bonsai nurseries) have conducted side by side tests with trays of Banksia, applying Osmocote and Native Osmocote to the surface of both trays. In both cases, the ones with standard Osmocote grew better than those with Native Osmocote, so both have ceased using Native Osmocote on Banksia and other Australian natives.

Fertilising B. integrifolia immediately after root pruning is safe because the root-pruning will have removed most of the proteoid roots and they are far less susceptible to over-dosing on phosphorus at that stage. By withholding the fertiliser there is a risk the tree will grow proteoid roots so when you do fertilise the risk of overdose is higher.

In summary: Banksia rarely recover if you over fertilise or fertilise inappropriately; start low and build up the fertilising over time.

Re-potting

B. integrifolia rapidly fill a pot with very fine roots. Eventually, watering becomes very difficult and it is possible that a lot of potted B. integrifolia deaths in spring and summer could be attributed to dehydration due to excessive root growth.

Do not pot-on a B. integrifolia at any stage if it is weak, skinny or wobbly. If it is weak or scrawny looking, just give it a light trim and put it back into its pot.

How often to re-pot

Particularly during the earlier stages of development, B. integrifolia are best left to get a little pot bound before re-potting; this allows the trunk to thicken in the pot.

There is general agreement that younger B. integrifolia ought to be re-potted every 1-2 years. There is strong evidence that mature trees in bonsai pots can go several years before re-potting. On the other hand, re-potting too frequently increases the likelihood of re-potting out of season which makes after-care more difficult and increases the risk of root rot.

As Grant Bowie said: I repot about every 12-18 months, but I don’t do a heavy re-pot and trim each time. I will just do a ‘slip pot’ into a larger pot. This involves just a very light root prune or even just a teasing out of the roots and a light tip-prune of the foliage. This allows the tree to grow at a good rate in the pot and not slow it down.

If you have a tree that is hard to keep the water up to and it is mid-summer, then the ‘slip pot’ technique is the one to use. After using the ‘slip pot’ technique a few times in a row you can then do a normal re-pot. It doesn’t matter if the tree is actively growing because I trim the tree to the shape I want and then leave the tree to start forming its new buds for a week or two. I then give the roots a hard root prune and put it into a smaller pot.

People living in Sydney or further north may be able to pot-on nursery stock every six months or so to achieve quick growth. If you live in Canberra or cold areas of Victoria or N.S.W, you may need to allow more time in each size pot, say twelve months.

When the tree has used up all its store of fertiliser and is not growing as vigorously as previously, it is time to re-pot. Don’t wait until the fertiliser is absolutely exhausted, nor the tree so pot bound that water has trouble penetrating.

Letting a B. integrifolia become root bound can lead to the development of proteoid roots. If you see proteoid roots, you can just cut them off with no adverse effects for the tree. See ‘Roots’ above, for more on proteoid roots.

When to re-pot

It is very important to get the timing of your re-potting right so that your tree does not sit inactive for extended periods of time after a root-prune. In Canberra, there are two key opportunities for re-potting B. integrifolia: early summer, after the weather warms up; and late summer or early autumn, before the weather cools down (provided there is 6-8 weeks of warm weather remaining).

The main reason for avoiding potting in winter or early spring is that the tree just sits until the warmer weather arrives; there is no advantage in potting early in cooler climates. In fact, by sitting in a larger pot in wet conditions for 3-6 months you may encourage root rot. Potting in early summer will also avoid excessive new growth on the tree that can be badly damaged by winter frosts and lead to problems with die-back.

In the warmer climates, including coastal regions, re-potting can be safely done earlier.

If you miss the optimum time for re-potting, leave it for the following year if you are unsure. Otherwise a light re-pot in late summer would be another alternative but it must be warm with a good growth period to follow. Root-bound trees never die of root rot, so if you keep the tree healthy (consistent fertiliser regime) you should have no problems if you miss out on a re-pot.

How hard to root-prune

Root pruning can be quite hard at the right time of year (see photograph 3). Some people report reducing roots of young B. integrifolia drastically - up to 80% of roots removed with no obvious ill effects, but most advice is to take off around 50% of the fine roots at each re-potting.

Although some people report defoliating mature B. integrifolia at re-potting time, most advice indicates that they recover more quickly if you leave lots of foliage on the tree during and after root trimming.

Potting mix

The potting mix should be neither too fine nor too coarse but above all should drain freely. There are many recipes. See ‘Soil Preference’ above for a common example. See also, Soil and bonsai soil mixes in the CBS Knowledgebase.

After-care

The generally espoused best aftercare is a sheltered position not in full sun. This works for most people. One experienced grower says “I put most of my re-potted trees straight back on the benches in full sun but that may only work here under my conditions. I have not tested this treatment fully enough in enough different places to recommend it to everyone”.

B. integrifoila can sulk for a while (some a long time) after re-potting, so after-care is important. You will see the sulk is over it when those little pink bursts of growth start appearing.

‘Sulking’ is the term used when, for an extended time, there is no new above ground growth visible after repotting. During this time, there will either be no or slow new root growth, but once sufficient new roots have been produced, then above ground growth re-commences.

Canberra Bonsai Society
Phil R with assistance from Roger H, Grant and Neil