Root pruning some natives

[shibui]

I usually repot my natives a little earlier than this - late October and November but December is still not too late for me here.
It has been several years since some of these have been root pruned and the soil is starting to feel quite solid. I've also been having some trouble keeping water up to them which seems to be another sign that the pots are getting too full of roots.

Today I did some banksias from the sales benches.

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You can see the note on the pot. This one was potted in standard potting mix with standard osmocote which obviously hasn't killed it.

Out of the pot.

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Look at the funny patterns on the root ball. Those are proteoid roots.

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Those tiny, much divided roots are able to scrounge up even the tiniest traces of Phosphorous for the plant and enable them to survive in nutrient deficient soils.
Unfortunately for the plants, when phosphorous becomes more available the roots continue to do their job and can collect so much P that the plant dies of an overdose. This seems to be the origins of the half truth that Australian Natives must have low P fertiliser. In actual fact, most native plants grow better with normal levels of P in the soil. Banksias like this one are most at risk but it is possible to gradually introduce fertiliser in low doses which leads the tree to let the proteoid roots die off. It is even quicker to prune off the proteoid roots as I'm about to do. Then it is safe to give normal fertiliser.

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Most of the proteoid roots have been removed as they tend to be around the edges of the root ball. Banksias are quite tolerant of root pruning so I can reduce the roots fearlessly during the warmer months. Root reduction like this in winter could kill a banksia.
This tree will now go into a 15 cm squat pot for the next year or 2 to grow a little more. Mix is my standard bonsai mix including standard controlled release fertiliser.

This one is Banksia serrata.

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note that this one doesn't have the proteoid roots. It must have had a bit more fertiliser and has not needed to grow the special roots.

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I've reduced the roots even more this time.

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It is now also in a 15 cm pot in my standard mix, including standard controlled release fert.

[melbrackstone]

I can see I need to look at root pruning some of my natives too! Thanks for sharing Neil.

[Rory]

That's a great post Neil. I love it how you write so that everyone can easily understand why and what you are doing.
Plus it is really great for everyone to see an idea of how much root you can safely remove on Banksia and the process.

I reckon if I ever get to visit Victoria in the near future I should never visit your nursery Neil. I'd empty my wallet there

[Jarad]

Unfortunately for the plants, when phosphorous becomes more available the roots continue to do their job and can collect so much P that the plant dies of an overdose. This seems to be the origins of the half truth that Australian Natives must have low P fertiliser. In actual fact, most native plants grow better with normal levels of P in the soil. Banksias like this one are most at risk but it is possible to gradually introduce fertiliser in low doses which leads the tree to let the proteoid roots die off. It is even quicker to prune off the proteoid roots as I'm about to do. Then it is safe to give normal fertiliser.

Thank you in particular for that note. Got my first banksia a couple of months back, every extra drop of info helps!