A couple of my smaller bonsai banksias started to develop dieback in some twigs toward the end of spring.
Many people would assume that, because these are banksias, the dead shoots would be the result of phosphorus toxicity. My experience with this genus led me to a completely different conclusion.
I have noticed that banksias have very dense fine roots that develop very quickly. Here’s what I found when this one was removed from the pot.
It definitely needs repotting.
First, trim back any long, fresh shoots.
When I cut through the root ball I found exactly what I was expecting – a dry patch in the middle.
That area is dry despite a thorough water last night and again this morning. The roots are so crowded that water has great difficulty penetrating which means the tree starts each day without a full pot of water. The dead shoots have nothing to do with P toxicity. They simply show a lack of water.
The remaining roots were trimmed quite a lot.
Note that there are no visible proteoid roots on this banksia because it gets regular fertiliser. Proteoid roots appear when banksias are short of nutrients and tend to disappear when the trees are fertilised regularly so paradoxically, regular fertiliser actually helps reduce the chance of P overdose for banksias.
Then back into the pot with fresh mix.
My experience with banksias as bonsai shows that the roots grow so fast that these need repotting every year to prevent them becoming root bound and having difficulties with water absorption. Fortunately they also seem very tolerant of the repotting process during the warmer months. This repot was carried out during a string of 40+C days in early January. New shoots continued to grow and more buds have sprouted since the repot. As usual, the freshly repotted tree went straight back to its usual position on the bench. The area is covered with light shade cloth this year but no other ‘aftercare’ was given.
I’ve found banksias to be very rewarding for bonsai. Why not have a try? Shibui bonsai normally has banksias available in a range of sizes including field grown trunks – see our banksia catalogue – hhttp://shibuibonsai.com.au/wp-content/uploads/2018/12/Banksias-2018-1.pdf