What steel posts for long & high garden bed?

[TTN]

Hi everyone,

I’m planning a raised garden bed and would love some advice about post options for long-term durability.

Details of the project:

• Size: ~8.3m long x 0.9m width x 0.9m high

• Purpose: planting 3 fruit trees (citrus, persimmon, and fig)

• Fill: approx. 7.5m³ of soil

• Location: bed will sit directly on the ground (existing soil is 20cm garden soil over compacted yellow builder’s fill)

Question:
I was advised by a tradesman to use steel pickets coated with bitumen (star pickets, see attached image) instead of hot-dip galvanised posts to cut costs. I want the structure to last around the 20-year mark.

• Will bitumen-coated steel pickets realistically last that long when buried in soil and holding back this much weight?

• Or should I be looking at galvanised steel channel posts, like these sleeper uprights used for retaining walls: Bunnings Jack Galvanised Corner Post Upright

Other things I’m unsure about:

• What spacing between posts would be recommended for a bed this long and high?

• Would I need heavier posts for the corners?

• Any suggestions for balancing cost vs durability without compromising strength?

Any guidance, especially from those who’ve built high, long garden beds or retaining walls for fruit trees, would be much appreciated.

Thanks in advance!

Steel Picket

[EricL]

Hello @TTN 

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your high garden bed.

For a raised garden bed of this size and height, the structural design is closer to a low retaining wall than a typical timber garden box. Since you are filling it with over seven cubic metres of soil and supporting fruit trees, the lateral pressure on the walls will be significant. I therefore suggest consulting a structural engineer in regards to the construction of your raised garden bed. Your other requirements will also need to be taken into consideration such as longevity and structural integrity. 

Bitumen-coated star pickets are economical and easy to drive into the ground, but they are generally not designed for long-term soil contact under heavy loads. Even when coated, thinner steel sections are more prone to rust, especially where the coating gets scratched during installation. In moist or clay soils, the expected lifespan is closer to 5–10 years, not twenty. Their shape also makes them less effective for restraining wide, tall boards or sleepers. They can work for smaller beds under half a metre in height, but at nearly a metre tall, the stresses would be too much.

Hot-dip galvanised channel posts, like the Bunnings galvanised sleeper uprights, are designed specifically for retaining applications. They are far thicker, resist rusting far better, and provide secure channels to hold heavy sleepers in place. A properly installed galvanised steel upright can easily last twenty years or more, even in contact with soil. If your goal is longevity and safety, these are the far superior option.

For spacing, with a wall nearly a metre high, posts are generally placed every 1.2 to 1.5 metres along the run (please confirm with your structural engineer). Tighter spacing provides more rigidity, especially given the pressure from soil and tree roots over time. Corners experience higher stress and should definitely use the heavier corner uprights rather than just straight posts. That will lock the walls together firmly and prevent bowing or spreading. I recommend consulting your structural engineer in regards to how deep the posts need to be buried and how big the footings will need be.  

Let me call on our experienced members @Dave-1 and @Nailbag for their recommendations.

If you have any other questions we can help with, please let us know.

Eric

[Nailbag]

Hi @TTN 

I can't say the advice from the tradesperson was a good one in suggesting star pickets. There is no way long term that would hold back the weight of 7.5c/m of soil. plus the retaining walls, plus moisture and the ever growing weight of the trees themselves.

Your own suggestion of the Jack gal posts is 100% the way to go. Based on sleepers being 100mm high, 4 to make the bed 800mm a little shy of your 900mm. Stick with the shorter 2400mm x 75mm sleepers over 3m ones as there will be less chance of bowing. Remember to line the inside walls of the box with builders plastic to prevent debris seeping through the layers. Ands consider running irrigation tubing prior to filling with soil.

Nailbag

[Dave-1]

Good morning @TTN 

I am also wondering at the height you want the garden bed to end at, The width and length of the raised bed sounds good but the 900mm height will add a load of complications, much more along the lines of a retaining wall that @EricL has mnetioned plus once those fruit trees start stretching thier legs...

With three trees in that 8m + space you will end up with some non used/unusable soil.

The Hot dip gal posts mentioned plus concrete sleepers would be the way to go for that length of time tho a pricey solution.

I had 9 raised garden beds in my back yard, I used only 1 200mm high recyled sleeper edge around them. They were approximatly 800mm width and around 2.4m long. The garden beds lasted maybe 8 years before the edge lost its structure due to rotting. At this point I choose to remove it back to a level lawn. I am in the process of releveling it as I type Maybe a shorter height garden bed would work better for the type of trees you want, (some fruit trees do not like being raised that far) Also a hard edge will last around a decade (my edges were reclaimed from someone throwing them out) I would go for a lower height edge if possible with a plan to revisit it in 8 years. Having a garden bed that can be moved/added to compared to the solid gal posts ect is what I am thinking.

Dave

[TTN]

Hi @EricL , @Nailbag and @Dave-1 ,

Thank you for your advice. This high garden bed is proving more complicated and costly than I expected.

The reason I’m considering a high raised garden bed is our soil: only 20–30cm of topsoil over a compacted clay-based builder’s fill. I don’t know how thick it is. Apart from the iris, everything I’ve planted has struggled or died within a year, likely when roots hit that layer.

My other option: To dig 3 large holes for the 3 trees right through the clay-based fill into the soil beneath. My concern is that while the trees could initially establish in the holes, their roots would eventually spread out horizontally and hit the clay/fill again.

A couple of questions I’d love your thoughts on:

1. Do you know if there’s a way to tell how thick that clay/fill layer usually is in new builds?

2. If I got someone to dig and remove that layer in just a few spots (say, 3 big planting holes for my trees), would the trees be able to survive long-term?

3. The area I’m planting in is about 5m from the house and right next to a fence. If I dig deeper but avoid disturbing the soil right against the fence, would that still work?

4. What exactly is that builder’s filler soil? Is it just compacted clay or a mix of something, and is it generally hostile to tree roots?

Would love to hear your experiences or suggestions.

[Dave-1]

Good Evening @TTN 

I have heavy clay soil, mixed with soapstone (flakey chunks fo rock) Mostly Clay. I dug whole 2 and a bit time bigger then the pots the trees came in and also about the depth plus a half deep then planted them and backfilled with proper compost (not all premade compost is the same)

I have done this with all my trees

Liquid Ambers (planted in the ground level with the lawn)

Coffee trees (these are actually doing well tho the frost really hammers one, in the ground, level with the lawn. Needs windbreaks where I am)

Magnolia (heavy clay but raised bed as in a mound of soil)

Bay leaf? Not sure of that one, could be a mango lol and is on a raised garden bed.

Not sure what builders fill is but surely cant be worse then the not so great clay and builders rubish Ive unearthed in my yard  

When digging your holes, keep in mind gas lines (generally straight from the gas meter to the street) water pipes and NBN/phone lines As the trees grow they will encroach on areas (neighbour had a trees roots wrap around their gas pipe and when the pulled the stump out... Yep it was an interesting afternoon

Dave

[EricL]

Hello @TTN 

I've had a good look and can only find this discussion about trees being planted in clay soil - How to plant trees in clay soil? by @rosey55. Just as you predicted, once the roots reached the clay border the trees started to die off. It's also possible that the tree's roots got drowned in water as it got trapped in the clay base. 

I propose engaging the services of a local arborist in your area. I feel confident that they will have a good idea on how to prepare your planting area for trees. I'm sure your place is not the only one with thick clay soil. But I also suspect that a great deal of digging will be involved, and the installation of drainpipes will need to be included in the design. 

Let me call on our experienced members @Noelle and @Adam_W for their recommendations.

If you have any other questions we can help with, please let us know.

Eric

[Noelle]

Hi @TTN 

Digging into or through clay beneath the topsoil is not really recommended from a horticultural perspective when planting trees, as inevitably the root systems will either drown (because you've created a 'sump' in the clay layer into which soil moisture drains and remains) or the roots hit the clay and start growing laterally and horizontally, ending up circling around themselves.

Digging an inspection hole to try to determine the depth of the clay layer may be an option - you might find, if it's builders' fill, it may only be 600mm or so deep, under which is the natural soil. In this case, you could dig out extensive planting holes as per @Dave-1 's reply above, backfilling with a mix of good quality compost mixed with topsoil so the trees' roots can grow down deep into the soil past the clay layer. 

However, if the clay layer is deeper than about 600mm, that process is not likely to be successful as the planting holes would be exceptionally deep. Trees should not be planted any deeper than they were in pots, especially if they are grafted. The grafts need to be well clear of the soil level. 

Building up planting mounds or raised beds is the optimal solution if the clay layer is quite deep and, as @EricL  has suggested, you may need the services of an arborist or engineer to guide you on how these should be constructed.