Summer is also time for repotting many of our Australian native plants. I was repotting some banksias and came across this interesting phenomenon.
We hear so much about Australian Native plants being allergic to phosphorus and needing very careful fertilising. the truth, however, is that only a very few Aussie natives are sensitive to phosphorus and there are also a few plants from other parts of te world that are phosphate intolerant as well. I give most of my Australian natives the same fertiliser that I use on the exotics and they grow very well on it.
The exception is Banksias and here is the reason
proteoid roots on banksia
That white patch that looks like fungus is actually a patch of specialised roots called proteoid roots that have evolved in some of the proteacea plants to help them extract phosphorus from the nutrient poor soils they tend to live in.
a closer look
Looking a bit closer you can actually see the hundreds of tiny white root tips that are all collecting every available scrap of phosphate from the soil.
And that is the problem. When we apply phosphate rich fertiliser these tiny, super efficient phosphate absorbers continue to grab all the available phosphate. That results in a phosphate overdose and a very quick death for your banksia so be quite wary of what sort of fertiliser you give to your banksias.
Banksias do not NEED proteoid roots and when they are in an environment that has adequate phosphate in the soil they do not develop these special roots. If we slowly incease the amount of phosphorus in the soil the tree will gradually shed any proteoid roots and can then tolerate normal amounts of phosphate fertiliser but as soon as the nutrient levels drop off again the plant will again grow more proteoid roots and will be sensitive again.
When repotting banksia bonsai most of the proteiod clusters are cut off but that will not hurt the tree any more than normal root pruning hurts any other species.
I do not want to give the idea that banksias can grow without any fertiliser. When grown in a pot nutrients are quickly leached out of the potting mix by regular watering so even banksias need regular fertiliser but its best to play it safe and use a low P formulation.
Many of our banksia species make excellent bonsai. They will shoot on old wood when pruned, They tolerate root pruning very well (in the warmer months) and continue to thicken even in the cramped confines of a bonsai pot so try some banksias for bonsai, just be careful what you feed them.