Soil advice - large planters for trees

[pinny73]

I am seeking some advice with regard to the best soil/planting mix to use in very large cylindrical bottomless planters for medium sized trees.

The trees are mature Lilly Pillies (Waterhousia and Green Screen), that have been grown in large pots at the nursery. The trees are currently about 1m wide and 3m tall. I will be maintaining them at this height width with rugular pruning to form an informal hedge.

I plan to grow the trees long term in very large corten steel tree rings. These are open at the bottom. They are 85cm in diameter and 1.22m tall.

I won't be re-potting them so I will require a mix that I can be used long term with surface feeding with liquid fertiliser.

I have attached photos of the tree ring planters and trees.

Any advice will be much appreciated.

 

[Mr_Yan]

That will be impressive and could also be used as a traffic barrier (I used to live on a very busy street and still think about protection from cars).

My initial thought / reaction is to keep the soil in the open container as close to the native soil where these will be planted as possible. You want the roots from the tree to extend out of the container in to the ground. There is a school of thought that if the soil is significantly better in a container (amended hole, garden bed) the roots will tend to stay in the container and not spread out.

Have you looked at getting a load of top soil delivered to fill these?

As they are so tall and contained you can "refresh" the soil in a few years by removing the top 150 mm or so and putting in a compost. Then as you top water it the compost and nutrients will leach down to the roots.

 

[Marck]

Large narrow planters like that will still basically act as containers, even with the open bottoms. Use a well-draining potting mix, especially as yo will need to water regularly.
Roots might extend beyond the cylinders, if the soil is appropriate, but the root-crown of the tree will still be confined which might still lead to to a rootbound situation.

 

[pinny73]

Thank you both for your comments.

Will there be any issues with a soiless potting mix breaking down after a couple of years given they won't be repotted? I will only be able to replace nutrients by replacing topsoil and liquid feeding.

Any suggestions on a potting mix recipe are welcome! e.g. soil-less or loam based mix. Should it have perlite or similar that doesn't break down to ensure it stays free draining. They are native Australian plants so a low phosphorus mix is ideal.

 

[pinny73]

I have also considered an inner bottomless "airpot" which may prevent the rootbound issue.

AIR-POT Root Pruning Tree Pot Kit, 400L | Arborgreen

AIR-POT Root Pruning Tree Pot Kit 400L, 925mm diam (w/o base), 595mm high

www.arborgreen.com.au

 

[Oliver Buckle]

I would avoid creating a barrier by putting the ring in place, putting some of the soil you are going to fill with in it and then digging to mix it with the subsoil a bit, then do the same sort of thing when you plant the trees, mixing the soil they came in with what you are putting them in. The idea is to make a gradation rather than one type of soil, hard line, another type of soil. If there is a certain amount of organic material in the soil you fill your cylinders with it will decompose and shrink; it will probably bed down a bit over time as well. That should leave you a space at the top of the cylinder which you can use to top dress with compost. I find with big pots for things like lilies, a pittisporum and my choisia I can do this every year and I don't see why it shouldn't work for you. I sometimes add a bit of bonemeal or blood fish and bone to the compost if I don't look like getting much top dressing in.

 

[Marck]

Choose a mix of sand and some organic matter for drainage, perhaps 80%/20%. Their will be settling with time, root growth and top mulching will offset this partly.
This will be a curious planting experiment, so by all means, do give it a go. Besides subjective aesthetics, I suppose the appeal is that there is some immediate added height, though ultimately you can grow a hedge taller in the ground. However, the tradeoff may come later when you may find you have an ailing tree oddly jammed into a rusted steel pipe.

 

[pinny73]

Thanks for the feedback. In a few months the corten steel will patina like these. The insides will be painted with bitumin paint to stop rusting from the inside.

I did decide on the planters for immediate height because they will be in a lower area with the surrounding retaining wall, path and house that is about 700mm higher. The area they will be planted is graded aeay from the boundary with compacted clay to a spoon drain to avoid water flowing to the neighbouring property which is cut 1m lower. The planters will sit on top of this compacted clay. We didn't have a lot if luck planting some capital pears in the ground there.

I really hope they will be ok in the planters longer term. Thanks for the suggestions. I will post progress pics once they are in.

I was thinking of some ground cover to spill over the sides of the planters although this may make it difficult to replace top soil!

 

[pinny73]

This is the area they will be installed. The pebbles will be moved and just surround the pots. The drain runs all along the left hand side under the pebbles next to the raised garden bed to the open grate.

You can see the first two pear trees are dead. The next two seem to be recovering since we added the spoon drain, but are very stunted compared to the others planted in a free draining location.

 

[cpp gardener]

I would use something that has a minimum of organic matter, 20% max. As it decomposes it will gradually disappear and the soil will settle. Also, highly organic soils can be extremely tempting to sowbugs and other consumers of decaying organic matter. We have had some real problems with sowbugs eating roots along with the compost, thereby killing the plants.

 

[pinny73]

Finished result..

 

[pinny73]

Managed to get a custom speciality mix of bulk potting mix delivered which included sand and humate. I mixed in scoria, zeolite myself.

 

[pinny73]

I lined the pots with polystyrene for insulation because the corten gets really hot. The planters are staked into the ground with 1.6m star pickets bolted to planters.

 

[cpp gardener]

Really cool look!