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Raised bed with clay soil


i have a large raised bed (10'x4') which is full of nasty cloying clay soil.  Should I top it up with good soil or mulch?  I want to grow small roses, lavender and other small scented perennials.



  • Dave MorganDave Morgan Posts: 3,123

    I'd add sharp sand/grit/well rotted manure all at the same time. Lavender will struggle in clay even with the above added. If your'e going to put some in make sure it's more grit and sand around the roots than anything else, they'll need the drainage. Roses will love it, clay is great for them, with the perennials, choose carefully as some won't like the clay.

  • Sand and clay = cement in summer and root rot in winter. Never add sharp sand to clay soil!

    Fork in compost, bark, twigs and cardboard (ripped up) to loosen up the soil then add some top soil and manure. I would put Lavender at the front of the raised bed: you are supposed to add rocks at the front next to the wood to add in drainage.

  • Thanks guys.  I wish I could get rid of it all.  I shall have to just add loads of compost and fork it in and see how that goes.  Thanks.

  • PosyPosy Posts: 3,601

    It really is worth adding grit. Test your pH before you plant roses: everyone says roses love clay but they will struggle if the soil is very alkaline and die if the roots are in water all winter. Lavender won't appreciate wet roots, either. However, there are lots of plants that will enjoy your raised bed. 

  • Dave MorganDave Morgan Posts: 3,123

    Sorry blairs, your'e obviously using the wrong sand, decent sharp sand is full of grit. with very little sand.

  • i am a little out of my depth here.  This raised bed was built and filled by landscapers who assured me that they would fill it with good quality soil.  When I came to plant it I found it heavy and cloying.  When you dig it most of it stays on the fork.  The landscapers say that it is not clay but When I complained decided to add a very sharp grey gritty substance and a lot of compost.  Now I have almost a forks depth of malleable medium but below that is heavy 'clay'.  If I keep adding more compost will it eventually become 'not clay' and why should I not just add a decent topsoil?

  • If you keep adding more compost you will eventually get a deeper layer of 'not clay'.  As it is a raised bed we are talking about, not your entire garden, double digging might be feasible, if you feel strong enough. This is a way to dig down to 2 spade depths, rather than one. As you say, normal digging leaves a clay layer, which is effectively just a puddled clay pond lining!

    Double digging involves digging a trench across your plot. Put the soil you lift into a barrow and move it to the far end of the bed. Fork over the bottom of the trench to loosen all the soil, maybe adding some grit too. Then dig a second trench next to the first, tipping the soil you lift into the first trench to fill it. Fork over the bottom of trench 2 and fill from trench 3. Repeat till you get to the other end, then use the barrowed soil from trench 1 to fill in. You now have deeply dug soil, which you can top with a good layer of compost or well rotted manure. The good news is that the worms will dig this in for you. Award yourself a prize of your choice!

  • dig the top quarter of the bed out and get rid then dig plenty of organic matter (compost/rotted manure/leaf mold) into the rest and top with a decent layer of pure organic matter (rotted manure is best as it usually has plenty of worms) the worms will help turn over the bed for you in the future, just keep adding organic matter every year and after a couple of years it'll be good soil as clay soil is one of the richest nutrient wise you can get

  • Should I put any compost in the trenches?  Can you give me an idea of what sort of grit I should use?  Size of grit?

    the annoying thing is that underneath all this clay is some very good soil but if I can dig down deep enough maybe I'll get down to it?  I'll give your idea a good go.  Thanks for your advice.

  • I have a bed similar to yours. Mine is in semi shade and the clods of clay I was lifting out of it were so dense, I could have almost made a pot out of it!

    I got really fed up sifting in good stuff, turning it over, breaking it up etc.

    In the end, I took the decision to empty pretty much the entire raised bed completely and start again.

    I only left perhaps 20cm or so of clay at the very bottom, which I had turned, raked, broken up etc.

    Back filled the rest of the bed as other s have suggested - grit, good quality compost, some manure.

    My roses, by contrast, are in in the open ground (not the raised bed) in clay soil - I have turned it over so many times and add manure every year, but it is still heavy. They love it, no complaints!

    My rosemary and lavenders and any other mediterranean plants were suffering in the ground, so I stuck them all in a big planter, half backfilled (as Monty advises) with broken crocks, The rest gritty soil. They are now thriving.

    You can put mediterranean plants in a raised bed, provided drainage is excellent and as someone on here has said, add loads of big pieces of material around the plants and keep them near the front. Personally, unless I am doing a 100% mediterranean plant raised bed, where I can get the conditions perfect for them, I don't bother and always stick them in pots instead.

    If you soil, in spite of you efforts, remains fairly heavy, go for perennials that don't mind those conditions (as someone else has said). In my wetter, heavier and shadier parts of the garden, I have real success growing ophiopogon (black grass), as well as Japanese anemones, heucheras, hakkonechloa grass, cyclamen hederifoliums, snowdrops, ferns of various types, aucuba and astilbes. Worth pointing out though that the garden drains well now, even though it didn't used to. There are few plants I know of, except certain irises and ophiopogon grass, that will relish sitting in water.

    Good luck!

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