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 posts, mainly to focus on advice that is most relevant to the Canberra climate, and divided it into two parts. Part 1 was published in December 2019. Part 2 follows below.

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.

Note that some, but not all, species of Banksia are adapted to growing in swamp/boggy conditions. B. integrifolia is not a swamp dwelling species.

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 if 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.

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 re-pot in the Canberra 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.

Heat/cold

Contributors to AusBonsai report that well-managed B. integrifolia are tough enough to survive 43 degrees in full sun in summer near Sydney and minus 8 degrees or so in shade in Canberra.

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.

Sun/shade

B. integrifolia 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.

Tree development, trimming and pruning

There are basically 2 stages in bonsai: the development stage and the refinement stage.

Development stage

During the development stage, the main purpose of trimming and pruning is to shape the trunk and major branches.

The quickest way to grow your trunks is to allow branching to develop all over the tree. If your B. integrifolia is still in the development stage, it will heal the scar sooner if it is growing in the ground than in a bonsai pot. But be mindful it can take many years to heal scars of 2.5 cm or more in diameter.

When B. integrifolia are cut back in the early stages of new growth the tree will often produce 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 soon develop – unless you don’t mind slight bulging.

If you want to develop any plant quickly, restricting the roots will set you back, but note some growers say that too much root room in a pot is not a good idea. This is a controversial concept, however, and 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.

Trees you want to grow larger or to thicken can be allowed to grow without trimming. Regular trimming will probably slow the growth, but trees must be trimmed regularly if you want to get good ramification and your tree to look bushier.

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.

Photograph 1: B. integrifolia on display at 10th National Exhibition of Australian Plants as Bonsai at the Australian National Botanic Gardens in March 2014. (Sourced from Wattos Bonsai Blog)

As an experiment, one experienced grower who runs a bonsai nursery kept one seedling of B. integrifolia 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 for use in bonsai, with better taper, better trunk movement and better branching. This is an interesting observation. In each case, the trees are responding to the conditions they are growing in and since the responses are different, the conditions can be expected to have been different.

If your pots are sitting on the ground, you can allow the roots of B. integrifolia to penetrate the ground and thicken that way. However, 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. Photograph 1 shows a superbly developed and maintained B. integrifolia.

Refinement stage

It is suggested that the refinement stage should start when the branches have reached their 3rd or 4th ramification. If you jump the development stage too early, say when tree is just starting its 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.

When you come to style your B. integrifolia you must decide on a shaping method. You can clip and grow, or you can apply some wire.

Clip and grow with B. integrifolia 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, in Canberra 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.

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 more quickly in Canberra 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 dieback 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 you can expect to have a strongly growing tree with small leaves.

Wiring

You can wire B. integrifolia at any time but be aware that wiring a B. integrifolia 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 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.

Photograph 2: This B. integrifolia was judged the best Australian native at the 2019 CBS show.

Carving

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

Grant Bowie 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 (see photograph 2.) The very soft wood of B. integrifolia is easy to carve.

Propagation

B integrifolia can be easily grown from seed or cuttings but must be potted up and on quickly.

Canberra Bonsai Society
Phil R

Banksia integrifolia as bonsai #2