What depth of soil for raised beds

Hello. We're planning on building raised beds this weekend, and my question is how deep should the topsoil be.

We'll have 2 lots of beds; each one 5m long X 0.4m wide and 0.3m high.

For the base, the plan was to use leftover builder's rubble to improve drainage and bulk out the bed. The beds won't be for vegetables, but for perennials and maybe small shrubs. I'm envisaging tulips, daffs, hyacinths, then allium, lilies, echinacea, lavender, phlox, black-eyed susans, maybe peonies, sedum and grasses. You get the idea.

How deep should the topsoil be? And how much compost would you include in this? I fear it's going to get expensive... We can't transplant any soil from elsewhere.

Thanks

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  • raisingirlraisingirl East Devon, on the Edge of Exmoor.Posts: 3,771

    Ideally you'd roughly fill the bed. Not saying that for the plants, particularly, but because it's quite awkward digging into soil with a high lip around the edge (having done exactly that for a few years). I just laid out the edges, dug over the soil and started because I couldn't afford to buy in imported soil and compost. Over time, because they don't get walked on and are regularly mulched, the soil level gradually builds up inside the bed. It's possible, but not ideal. Ideal is a lorry load of sieved top soil. I can dream.....

    Your bulbs will be happy straight into your soil. If you need to improve the drainage with the rubble, then the rule with bulbs is they need to have at least twice their own depth in soil above them (and they'd need some soil below for the roots), so crocuses don't need much and tulips need a good 6 to 10 inches at least.

    Peonies are greedy, they may not like sitting on rubble. Lavender will love it.

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  • KT53KT53 Posts: 3,856

    You are going to need to use topsoil for most of the depth, and be prepared for it to settle a bit over the first year too.

  • SandTSandT Posts: 66
    KT53 says:

    You are going to need to use topsoil for most of the depth, and be prepared for it to settle a bit over the first year too.

    See original post

    So you think we need 30cm of topsoil in the bed? And proper, proper topsoil, or a mix of compost/ manure too?

    Last edited: 21 February 2017 22:42:12

  • Hi SandT,

    for what you are intending to grow - mainly bulbs and perennials - I would suggest 1/3 : 1/3 : 1/3 topsoil / compost / well rotted manure and then mulch perennials every year with more manure. You wouldn't plant root veg in this but for flowering plants which are presumably going to be there for some time you need a richer mix. The compost and manure will help with moisture retention too - personally I'd mulch with 4 or 5 inches of well rotted manure as soon as you plant your perennials too as this will keep down weeds whilst they establish.

  • FairygirlFairygirl west central ScotlandPosts: 28,102

    If you're growing so many plants which like good drainage -tulips, lavender, alliums, peonies, you'll need a decent amount of soil with grit mixed through it. Some compost/well rotted manure added is beneficial.

    I'm not keen on the 'rubble at the bottom' that many people recommend. Mixing grit through the soil is always more effective, especially if you get a lot of rain. A bed that depth won't have much room for soil if there's a lot of stuff at the bottom, and the rubble itself can create a sump in heavy rain - even worse if the ground below th ebed is compacted and excess water doesn't drain quickly. Just my personal experience.

    As KT says - be prepared for it settling - you'll need to add to it each year as well - mulching with compost or similar will be ideal on a regular basis, but be careful not to bury the crown of the peony. image

    It's a place where beautiful isn't enough of a word....


  • hogweedhogweed Central ScotlandPosts: 3,966

    My maths aren't that good but I think you could almost fill the two beds with one cubic metre of topsoil.but when I think of the bags in my head and the size of your beds that doesn't seem enough! Perhaps someone else can help.

    'Optimism is the faith that leads to achievement' - Helen Keller
  • SandTSandT Posts: 66

    Oh that's helpful, thanks. We need to find another way of disposing of a lot of aggregate then!

    Whilst we're talking topsoil, can I pick your brains? The same garden that the raised beds are going in also needs preparing for turf. This small garden faces North-North-East and has compacted clay soil. We checked the drainage and it seems ok, just needs properly turning over as it was under paving stones for years.

    When it comes to lawn prep, would you add 5/ 10/ 15cm topsoil? Each garden centre tells me a different thing! And would you order a blend that has farmyard manure already in it, or would you apply a layer of compost or manure when turning it over, then apply a layer of topsoil? Not sure what order to do things in and what composition is best.

    Thanks

  • Allotment BoyAllotment Boy North London Posts: 2,259

    You have good advice on the deep bed. I suggest you look at RHS advice or Lawnsmith.co.uk they have excellent advice on all things grassy! Get any topsoil in Bulk you will need more than you think for both jobs & its much cheaper in the long run.

    AB Still learning

  • KT53KT53 Posts: 3,856
    Fairygirl says:

    I'm not keen on the 'rubble at the bottom' that many people recommend. Mixing grit through the soil is always more effective, especially if you get a lot of rain. A bed that depth won't have much room for soil if there's a lot of stuff at the bottom, and the rubble itself can create a sump in heavy rain - even worse if the ground below th ebed is compacted and excess water doesn't drain quickly. Just my personal experience.

    See original post

     I understand your concerns Fairygirl, and I suppose it depends on the type of soil the raised bed is on, and on the depth of the beds.  I've not had any of the problems you describe.  If anything the rubble actually minimises the risk of a sump being created because it's so free draining.

    My ground is also free draining which will have an effect.  The biggest issue for me has been the amount of settlement of the soil, which has dropped about 6" since the beds were constructed.  The beds are 8' x 3' x 2' deep so would have taken a massive amount of soil to fill without the rubble base.

    Last edited: 22 February 2017 11:43:38

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