How top stop concrete plant pot cracking?

[vanilla00]

When I purchase my apartment unit from the developer, the developer has built a few magnolia trees inside the concrete plant pot in my balcony. Recently, I noticed that one of the plant pot is cracking (4 vertical cracks and 2-3 horizontal cracks), and with new roots of magnolia trees growing out of one the cracks. I am a bit concerned about the cracks, given the size of the cracks, the weight of the trees and the general aesthetics. As a quick solution to strengthen the plant pot, we have wrapped luggage belts around it for now. Is it possible for us to fix the cracks? As in, to stop the cracking to develop further, and to seal the cracks? One of our next steps will be to trim the tree to stop it from growing further. Many thanks!

Measurements for your consideration:

Circumference of the plant cot: Approx. 238cm 

Length of vertical cracks: Range from 42cm - 70cm

Length of horizontal cracks: Range from 11cm - 20cm

[Jason]

Welcome to the Bunnings Workshop community @vanilla00. We're pleased to have you join us.

Let me tag one of our resident gardening experts @Noelle to see if she can share her thoughts on whether these pots are of sufficient size to accommodate the magnolias.

Jason

[Noelle]

Hi @vanilla00 

The magnolia trees are far too large for the concrete pots so it is little wonder they are cracking under the strain! These trees are quite tall and will have developed vigorous root systems by now that will be exerting pressure on the concrete. The pots are ideal for plants that grow no more than about 60cm tall - and these magnolias are going to be quite a lot bigger than that!

Their roots will already be completely filling the pots and may also be circling around the inside of the pots because they have no room to grow.

The magnolias need to be repotted as soon as possible into large tubs that are at least 1.5m in diameter and about the same depth. If this is not practical, then remove the magnolias, and replace them with smaller growing shrubs. If you can, find someone with a large garden who has the space to grow these magnolias as they should be grown - in the ground.

Pruning the trees back will not reduce the size of the root ball of each one. Eventually the trees will start to suffer as there is not enough space or soil in the concrete pots to sustain them or keep them healthy.

In essence - these trees are not suitable for the concrete plants. They may have looked great when they were small but they are not at all suitable long-term.

[robchin]

Hi @vanilla00 

Ouch. Sorry to tell you that those pots are way too small for those trees. Those trees will potentially get a lot bigger - even in a pot. The cracks are from the roots expanding in the pot. You have a dilemma here for sure. Even if you repair the cracks - they will just form new ones elsewhere. I can't tell from the picture but I also suspect the roots from the plants might be growing out of the bottom of the pots as well which could create additional problems. Pruning the tops of the trees probably won't stop the roots growing either. 

So you could try repotting them into a larger planter - which might help in the shorter term. 

If you want to keep those plants and those pots - you might need to practise a bit of bonsai - and it will take some doing. You'll need to prune the shoots and the roots. It'll be a big job and is not for the feint hearted. Do it in late winter for best results and when you replant with a good quality potting mix - use some seaweed plant tonic when watering to help them re-establish. 

It may be easier to start again with some plant that will give you the size you want without getting too big. 

Good luck.