Banksia integrifolia - consolidation/summary of all AusBonsai posts

[Phil Rabl]

Preamble

The following information on Banksia integrifolia is a consolidation/summary of the many posts on the AusBonsai site on this species.
I developed the summary (and, yes, it took hours) to use for an article in the Canberra Bonsai Society’s newsletter later this year. But I have decided to post it to AusBonsai so it can be accessed by a wider audience p- and to get feedback, especially from those whose posts I have used. So, if you see anything you think is wrong, misleading or missing, please weigh in with a comment. I will revise my summary after I see what you all have to say.

Banksia integrifolia as bonsai

Introduction
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.
B. integrifolia grows in many locations in the wild, from the most exposed, windswept coastal headland where it may grow flat against a cliff or it may be a 25-metre high tree growing in snowy conditions in the mountains.
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 more coarse 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 don’t have a feedback mechanism to stop them taking up too much food, 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.)

Watering
In pots, B. integrifolia are both hungry and thirsty but if you give them what they need they will repay you with many years of pleasure.
Appropriate watering regimes for B. integrifolia vary from season to season and from climate to climate. Unsurprisingly, the advice on watering is not wholly consistent.
In the heat of summer, some experienced growers say that all B. integrifolia need to stand in water and that this will lead to a much-increased rate of growth and a much better resistance to the heat. Others point out that B. integrifolia are susceptible to root and even trunk rot and advise against keeping them in water. Rather, they advise that it is better to be careful and vigilant and keep your B. integrifolia trimmed to shape to reduce the amount of foliage in summer and to cut out all the old leaves to reduce the canopy as well. B. integrifolia is very quick growing and can be used as your ‘waterweed’: i.e. it is usually the first tree to droop if it is in fresh growth. Full sun is desirable, but you are in any doubt about being able to water enough to beat the heat, some afternoon shade may be appropriate. There is a general consensus that B.integrifolia can tolerate becoming a little dry but should not be allowed to completely dry out before watering.

Note that some, but not all, banksias are adapted to growing in swamp/boggy conditions.
During the period of winter dormancy, especially in colder climates, it is important to avoid having your B. integrifolia sit in wet potting mix for extended periods. Make sure you keep plenty of foliage on your tree during this time and avoid potting Banksia in winter. If you repot in winter or don’t leave enough foliage on the tree over winter you may encourage root rot, the tree effectively drowns in the excessively wet soil. In warmer climates, this advice is less relevant.

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. 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.

Sun/shade
B. integrifilia need to be grown in full sun, so except for immediately after re-potting, keep nursery stock and bonsai in as full sun and wind as they can take. Growing in full sun gives you a healthier, more disease and pest free tree. It also keeps the foliage more compact.

Heat/cold
Contributors to AusBonsai report that B. integrifilia can survive 43 degrees in full sun in summer near Sydney and minus 8 degrees or so in shade in Canberra; they really are tough if allowed to be.

Pests and diseases
If your tree is wobbly you should investigate the soil for curl grubs. If you find the roots have been severely eaten by the grubs, give it a hard root prune and put it back in a similar size pot and lightly trim the foliage. If you find Leaf Miner, which distorts the foliage, you may need to spray. Kill borers with a piece of wire if you spot any holes in the trunk.

Fertilising
As mentioned, B. integrifilia 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. integrifilia 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 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 so and then build up to full strength over the next four months or so.

At least two very experienced bonsai artists 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. integrifilia 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 proteid 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. integrifilia rapidly fill a pot with very fine roots. Eventually watering becomes very difficult and it is possible that a lot of potted B. integrifilia deaths in spring and summer could be attributed to dehydration due to excessive root growth.

Do not pot-on a B. integrifilia 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. integrifilia 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 are able to go several years before re-potting. On the other hand, re-potting too frequently can lead to root rot.

As one very experienced grower 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.”

If you live in Sydney or further north, you 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 it was, 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. integrifilia 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. There are two key opportunities for re-potting B. integrifolia: after the weather warms up (early spring to early summer, depending on your local climate); and before the weather cools down (late summer, provided there is 6-8 weeks of warm weather remaining).

In the cooler climates it is best to re-pot B. integrifolia in late spring or early summer. The main reason for avoiding potting in winter or early spring is that the tree just sits and sulks 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. One Sydney bonsai artist reports heavily cutting back roots and re-potting B. integrifoila in the middle of winter with no ill-effects. Others suggest it would be better to wait until September, even in Sydney, to do your re-potting of B. integrifoila. Some people report B. integrifoila dying after winter root pruning.

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. 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. integrifoila 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 freely drain. There are many recipes. See ‘Soil Preference’ above for a common example.

Overpotting:
Do not over-pot a B. integrifoila, even if you have exceptional drainage. If you are subjected to continual rain and very little water uptake it can still be dangerous.

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.

Trunk and Branch development
There are basically 2 stages in bonsai: the development stage; and the dwarfing stage.

It is suggested that the dwarfing stage should start when the tree has reached its 3rd or 4th ramification. If you jump the development stage too early, say when tree is just starting it's 2nd ramification, you get an imbalance in the transition from trunk to primary branch to secondary branch and so on. It is better to select the best placed young branches on this 2nd ramification and allow them to shoot away completely disregarding their length. Concentrate on their vigour and thickness, especially at the base, keep a close eye on the balance of transition from their parent branch.

If you want to develop any plant quickly, restricting the roots will set you back (but note that too much root room in a pot is not a good idea – some say it is as bad as too little room). B. integrifolia grow strongly in pots and you may very well be able to achieve good lengthening and some thickening in the small pot but in a larger pot you will be able to get the tree to do this more quickly thereby speeding the progress toward your ultimate goal. Having more vigour, the tree will also bud in more places over a longer period which further increases your options.

B. integrifolia is one of the few trees that seem to grow almost as fast in pots as in the ground. Based on a small sample-size experiment, one experienced grower kept one seedling in a 30 cm orchid pot while all the others from that batch went into grow beds. At the end of 3 years there was little difference in trunk diameter between the pot-grown tree and those from the grow beds, but by that stage the trunk and branches on the pot-grown one was far superior, with better taper, better trunk movement and better branching.

You can allow the roots of B. integrifoila growing in pots to penetrate the ground and thicken that way. However, trials show that similar to Eucalyptus, this diverts energy towards the ground roots and the tree can lose vigour.

The quickest way to grow your trunks is to allow branching to develop all over the tree. If your B. integrifoila is still in the development phase, it will heal the scar sooner in the ground than in a bonsai pot. But be mindful it can take many years to heal scars of over an inch or more in diameter.

Trees you want to grow larger or thicken can be allowed to grow without trimming to get them to grow quicker. Regular trimming will probably slow the growth but trees must be trimmed regularly if you want to get ramification and to look more bushy.

Trimming and pruning
When you come to style your B. integrifoila you must decide on a shaping method. You can just clip and grow or apply some wire.

Clip and row with B. integrifoila is very similar to deciduous trees; just cut back either to a small branch growing in the direction you want and then let it grow and thicken and then repeat the process; or cut right back very hard to a point on the bare trunk and wait for new growth to emerge. As in the case of root pruning it is best to cut back hard in the warmer months.

Once the shape of the tree is established you can maintain the foliage in its general outline by frequent tip pruning. If you fertilise well and occasionally cut out stronger growth at the top of the tree the lower branches will stay in good health. When you cut back all over the tree you may get buds popping and new growth emerging anywhere on the trunk or branches. The growth is strongly upright so you will probably need to trim the vertical growth about three times as frequently as the sideways growth. It is a mistake to constantly trim out the vertical growth completely as it doesn't allow the tree to bulk up.

When B. integrifoila are cut back in the early stages of new growth the tree will often shoot up to 4 new shoots from a past node. Simply rub off the unwanted shoots as they first protrude or a big bulky bulge of 4 shoots will shortly progress – unless you don’t mind slight bulging.

Keep plenty of foliage on the tree over winter and do hard cutting-back and trimming during the hotter months. Some say it is possible to do hard cut-backs at any time, but the tree will bud and grow back quicker if this is done in spring and summer. When making a hard cut-back, leave a big stub, seal the cut and rub off any new buds for about 6 months then reduce or remove the stub. Otherwise you may get some die back into the trunk if you remove a really heavy branch.

Prune new shoots whenever they grow too long. Cut back far enough to maintain the shape you want for the tree.

B integrifolia puts out a couple of ear-like small buds before they elongate and start to leaf out. They are bud covers that have opened as the new growth pushes out. You can cut back to these bud covers and almost without fail they will start budding and grow.
By constant fertilising, trimming and regrowing, B integrifolia bulk up and the leaves become much smaller.

Do not cut-back a large B integrifolia branch if it not in good health.

Wiring
You can wire B. integrifoila at any time, but wiring a B. integrifoila has a few interesting foibles. If you wire a branch down below horizontal it tends to lose vigour, it only thickens at about one third of the rate as a branch wired up then out. If you must for artistic reasons wire a branch down, leave it long and untrimmed and wire the very tip upwards. This allows the branch to stay vigorous as it thickens and when it has set you can unwire and trim it back the growing tip. Once the branch is set in the downward position it then allows the side growth to grow and thicken as normal.

If a bud bursts on the trunk and you wish to keep it and turn it into a branch you will probably need to wire it (clip and grow won’t work in this situation). Wire the branch very loosely so that it can expand without the wire cutting in. You will be surprised at how quickly the branch thickens and you can take the wire off and rewire in the reverse direction if you have any marks from the wire.

Carving
B integrifolia can be carved, if that will add to the tree artistically.

One experienced grower reports cleaning up a carved section of a B integrifolia that had died back a few years earlier. It had died down to ground level but had not rotted into the ground as the roots were strong and entire, that is wrapped in healthy bark above ground which then strengthened as it grew over time. After new bark had regrown over carved cambium, he painted the scar with lime sulphur coloured black with Indian ink to imitate a burned-out trunk. Very soft wood is easy to carve.

Propagation
B integrifolia can be easily grown from seed or cuttings but must be potted up and on quickly; always pot up to the next size and do not over pot.

[Phil Rabl]

Although my original post has not attracted any comments, I have a few to add myself. As signalled, I have converted the summary of AusBonsai posts into an article for the Canberra Bonsai Society’s newsletter. The first part is in the December newsletter. Part 2 will be in the February 2020 edition. Based on input from CBS members including Grant, Roger and Neil (all well known and highly respected contributors to AusBonsai) as part of our CBS consultation/vetting processes, I submit the following clarifications/comments:

Roots
This isn’t exactly true that Banksia do not have a feedback mechanism to stop them taking up too much food. It is more correct to say Banksia can’t quickly stop the working of their proteoid roots when the supply of nutrients suddenly increases. They will 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.

Watering
It is true that some, but not all, banksias are adapted to growing in swamp/boggy conditions, but B. integrifolia is not so adapted.

How often to repot
The comment that re-potting too frequently can lead to root rot may be true, but re-potting is only indirectly implicated. It is likely to be the aftercare – e.g. too much watering – that has caused the root rot.

Overpotting:
The suggestion that over-potting a B. integrifoila can be dangerous is a controversial one. It is not clear what it means to have ‘too much room’. Plants in the ground don’t appear to suffer from ‘too much room’. If your tree is not growing well, there may be something wrong with some other aspect of the horticulture, rather than the generous root-run of a larger than necessary pot. Accurate diagnosis of your tree’s problem is essential if you are to address it.

One issue is that you should avoid the tree sitting in wet soil for extended periods. When there is much soil without any roots, as immediately after re-potting, that new soil will tend to stay wet after watering (or rain) for too long for the health of the tree, unless you are very careful in regulating the amount you water.

Trunk and Branch development
The comment you can allow the roots of B. integrifoila to penetrate the ground and thicken needs clarification. It would be better to say: trials show that similar to Eucalyptus, this diverts energy towards the in-ground roots and away from the in-pot roots. If left too long, the tree will suffer when the in-ground roots are severed and the tree is forced to rely on the in-pot roots.

[melbrackstone]

Thanks for all your work on this!

[Rory]

That's a nice summation of notes Phil.

Overpotting:
The suggestion that over-potting a B. integrifoila can be dangerous is a controversial one. It is not clear what it means to have ‘too much room’. Plants in the ground don’t appear to suffer from ‘too much room’. If your tree is not growing well, there may be something wrong with some other aspect of the horticulture, rather than the generous root-run of a larger than necessary pot. Accurate diagnosis of your tree’s problem is essential if you are to address it.

One issue is that you should avoid the tree sitting in wet soil for extended periods. When there is much soil without any roots, as immediately after re-potting, that new soil will tend to stay wet after watering (or rain) for too long for the health of the tree, unless you are very careful in regulating the amount you water.

Yeah that's a golden point. Very fast drainage and ensuring you don't over-water after a root cutback is the key. From my experience the only times I've had issues with root rot - specifically with Banksia integrifolia - was when they had been inadvertently over-watered too often or the drainage wasn't fast enough and thus the roots were staying wet too long. I blamed myself for not checking each pot individually and instead just watering the entire collection. If the drainage is fast, its usually less of a problem being over-potted.
I've discovered root-rot more often from being in bonsai pots, where the drainage was not as good as in orchid pots. Its a pain in the bum if the pot is not level, and the water has collected or pooled in the pot without you realizing it. This is just not an issue with orchid pots.

When people say, "Plants in the ground don’t appear to suffer from ‘too much room’", I feel that its too generalized a statement to make. If the roots have penetrated deep into the ground then they are much more likely to be able to regulate their water intake better, because the entire root system isn't getting 'flushed' with water daily like we often do, and also the entire root system isn't sitting in damp soil for longer periods like it might be in a bonsai pot. Underground, the root structure will be exposed to more diverse conditions. Parts of the root structure will have more water than others etc, so it naturally has a better resistance as there is more diversity available in the ground. The roots also don't tend to spread straight away like they have to in a bonsai pot, so the roots are usually longer and have a greater area to utilize.

Trunk and Branch development
The comment you can allow the roots of B. integrifoila to penetrate the ground and thicken needs clarification. It would be better to say: trials show that similar to Eucalyptus, this diverts energy towards the in-ground roots and away from the in-pot roots. If left too long, the tree will suffer when the in-ground roots are severed and the tree is forced to rely on the in-pot roots.

Exactly. I have had this happen many times with Eucalyptus. With B. integrifolia I have experienced this too, but they are so hardy it only caused die-back and long sulks, but not death with B. integrifolia. I no longer ground grow B. integrifolia and many others for this exact reason. I feel its more beneficial to maintain roots inside the pot rather than sever the ground grown roots and then try to nurse the roots inside the pot back to optimum health. To me it is safer to develop the trunk of most species slower, with the roots kept inside the pot and not ground grown (in my opinion).

[jehsiboi]

This is amazing. Thankyou.

Jesse

[TimS]

I don’t grow Banksia but excellent work on compiling all that information into one place.

[rodm]

Great work Phil. Am reading with most interest, reason being my son-in-law is disguarding some over grown banksia intergrifolia at his nursery. Now I am looking ahead with much excitement
Cheers RodM

[1trackmind]

Thank you very much for this post. A treasure find for this newbie

[Phil Rabl]

On 14 April 2023, I posted a summary of advice on Ausbonsai about black spots on the underside banksia leaves. Here is a link to that post. viewtopic.php?f=69&t=30827&p=299762#p299762

It is clear that Banksia integrifolia is especially prone to getting the black spots under their leaves, so I am making this post to give the matter some visibility on the advice about B. integrifolia.

IMG_3875.jpeg

The top leaf in this photo is from an unknown species of banksia (let me know if you can identify it). The bottom leaf is from my integrifolia. Looking at these black spots with a strong magnifying device, they certainly are a kind of scale.

Here are some extracts from that post, repeating the parts particularly relevant to B. integrifolia.

If the spots are all on the underside of the leaves, it is more likely they are a kind of scale. They can be killed by rubbing off with a fingernail or something else sharp if you are patient enough. You can also get at them with pest oil/white oil.

In a 2021 post, Grant Bowie said you should definitely get rid of them. If they get in large numbers the leaf will be sucked dry and the branch can die back badly. You can cut back hard to get rid of the bulk of it and grow new growth. Keep the tree in full sun to lessen the possibility of infestation. You could spray with a safe insecticide, and this should reduce populations.

Grant said he has had this problem on 3 out of 4 of his Banksia integrifolia, but not on the marginata or serrata. Others, me included, have also had bad infestations on integrifolia. But I have recently had a minor infestation on a marginata. The unknown banksia in the above photo is also very susceptible. Like Grant, my serrata have never been affected.

Is white oil / pest oil effective? In my experience, yes. In January 2023, I largely defoliated my badly affected B. integrifolia, and took the opportunity to do a major redesign. As far as I could tell, it had no black spots remaining, but by April some had returned (see above photo). They are a bit hard to scratch off – they seem to slightly bury themselves into the leaf. But within days of spraying with pest oil they come away quite easily (mostly). From now on, I will both spray and remove leaves that develop the spots.