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We have recently needed to build a retaining wall to prevent the garden sloping off into our neighbor's garden at the rear. We had to back-fill the gap created in building the wall with rubble and stone that was unearthed whilst digging the footings. The stone and rubble has been flattened and compacted for drainage and we plan to plant bushes or high shrubs to add privacy around the perimeter of the wall but obviously we need to replace soil to the area on top of the stone.

To what kind of depth would we need to replenish in order to plant the shrubs? At this moment, the types of shrubs and bushes are undetermined, but to create a bed suitable, I am unsure how much soil would there need to be to grow successfully?

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I imagine there's some limitation to how much soil you can add on top of the rock because of the height of the wall, but three feet or a metre really, certainly not less than two feet of topsoil, if all that's there is stone currently, because you have no subsoil and you want a 'high' or tall plant barrier. Usually, 'topsoil' is a spade's depth, sometimes more, with sub soil beneath. However, I'm somewhat concerned at your description regarding the stone and rubble. It's the word 'compacted' that's worrying me, because if its been crushed and compacted down by machine, that means drainage won't be fantastic, and it will act like a solid barrier or pan to plant roots, even assuming there's subsoil beneath the rock.

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    I share your concerns. There are some shrubs that grow naturally in places where there is bedrock under a shallow layer of soil, like witch hazel, hazelnut, and juniper that might do well, but only if the drainage is good. – michelle Jul 22 '15 at 15:43
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Based on what I have seen in virtually any situation where I've dug a hole in the ground, 6-8 inches of topsoil is pretty normal in a natural state. Of course, I haven't dug any holes out in the great plains states where they supposedly have 6-8 feet, but that's an abnormal rather than normal condition for most other parts of the world.

My entire garden (USA-ian usage - "veg plot" to UK) struggles to maintain a foot of decent topsoil with me adding horse manure every year, and it gets along fine. Shrubs are rather less demanding, so they'll manage fine with less. Blueberries are a particularly handy shrub where you have limited soil depth, as they are naturally shallow-rooted (they happily grow in soil pockets largely built of their own leaves on top of solid rock, in the wild.)

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  • But presumably, your veg plot has sub soil, not rock, beneath? Your answer is useful to me, its meant I've clarified mine somewhat! – Bamboo Jul 22 '15 at 12:44
  • Correct. Abysmal clay subsoil. But calling for 2 feet of topsoil is rather much, other than for a very pampered bed. 6" over solid rock would be fine for blueberries (and probably lignonberries, though they don't make much of a bush.) – Ecnerwal Jul 22 '15 at 12:47
  • Maybe, but it won't be fine for very large shrubs or trees... which the owner says he wants, or at least 'high' shrubs. You can't buy subsoil, so he has to use topsoil instead. – Bamboo Jul 22 '15 at 12:48
  • Sure you can buy subsoil. Just order a load of sand or gravel. As for high shrubs, 4-5 feet is well within what you can do with blueberries. – Ecnerwal Jul 22 '15 at 12:52
  • No substitute for proper subsoil, particularly if the owner wants to grow large shrubs, they'll be falling over... – Bamboo Jul 22 '15 at 12:54

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