Banksia and fertiliser

For many years there has been a myth that ‘normal’ fertiliser will kill Australian plants.

The truth is that most Aussie natives don’t care. Many actually grow way better when they have access to reasonable levels of P so they grow better when fertilised with standard fertilisers. I have actually come to the conclusion that many of my early failures with Australian native plants can be attributed to starvation because I was too frightened to fertilise.

Some of the proteacea plants are sensitive to P but astute plant persons will note that this group also includes South African proteas so P sensitivity is not just confined to Australian species.

Recent work done with plants from the proteacea family is showing that P is not always fatal to these plants. If they are underfed, banksias and proteas will develop special ‘proteoid’ roots that absorb all available P. This is how they thrive in the poor soils of their natural range but it is also the aspect that causes problems when phosphorus suddenly becomes available because the proteoid roots are so efficient that the plant can die of a P overdose. In the picture below you can see the distinctive ‘starburst’ pattern of proteoid roots on a potted banksia along with normal root structure.

Proteoid roots on Banksia marginata

Plants that grow up with higher P levels don’t develop proteoid roots and can tolerate normal fertiliser like any other plant.

Here are the results of an experiment carried out at Shibui Bonsai over the last 12 months. A number of banksia seedlings were pricked out of a seed tray as normal when they were a few months old. I potted half in mix with low P ‘native’ osmocote. the remainder were potted in mix with standard osmocote and placed alongside the others. Since then they have all received the same supplementary liquid fertiliser, water and light.

The results are very clear. All the plants in normal mix have grown strongly and are healthy green. All those in the mix with low P osmocote are yellow. Many are stunted and a number have died. While this may not be totally the fault of the fertiliser I think it is pretty clear that banksias can grow happily with standard fertiliser.

Alternating left to right, B. integrifolia native and standard fertiliser

I think the results speak for themselves. I’ll be giving all my banksias the same mix as my exotics from now on.

B. integrifolia in native mix (left) and standard fertiliser (right)

If you have banksias that have been underfed you do need to be careful about applying too much P too quickly. Studies have shown that you can increase the amounts and frequency slowly and the plants will gradually adjust, shedding many of the proteoid roots that they no longer need. There is also growing evidence that you can manually remove most of the proteoid roots when repotting and the tree will be able to tolerate higher P levels immediately.

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