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What are the best soil amendments for blueberries grown in containers? What's the best mulch? Last year I used chopped leaves from a friend's landscaping job that is no longer around. I also remember hearing about adding coffee grounds to the soil to help with acidity. I haven't tested the soil yet, but have a bunch of coffee grounds from a local coffee shop. Do they raise the ph so much that I shouldn't add them if I haven't tested the soil?

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A good soil mix for pot grown blueberries is a porous, acidic mix high in organic matter. A good mix could be made by mixing these materials by weight:

  • 5/8 topsoil
  • 2/8 peat moss
  • 1/16 vermiculite
  • 1/16 coarse sand

Plus some slow release plant food. To help with the acidity, mix 1/4 pound of aluminum sulfate into the top 6" of soil. There are several mulches that fit the needs of the blueberry bushes. Bark mulch is acidic, long lasting and attractive. Also, shredded wood, aged sawdust, and pine straw are good mulches. Coffee grounds are only very slightly acidic, so they will not be useful in maintaining ph. But they do make good compost, and if composted with an acidic material high in carbon, such as the mulching materials mentioned above, they would make an excellent mulch that would feed the plants and help maintain soil ph.

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How would your recommendations change if planting blueberries outside? –  Om Patange Mar 15 '12 at 4:15
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I would say to use less sand and no vermiculite, and not use slow-release plant food. –  J. Musser Mar 15 '12 at 13:04
    
Thanks @jmusser. If these containers are already established would you recommend top dressing them with this mix or replacing a couple inches? –  Ann Addicks Mar 16 '12 at 0:09
    
I would recommend replacing the soil down to the top layer of horizontal roots, but not further, because blueberries have shallow root systems and do not tolerate deep cultivation. –  J. Musser Mar 16 '12 at 1:08
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@EmmyS, There are dwarf varieties meant for pots. My favorite is Tophat, a dwarf container variety that grows 2'x2' and is self fertile. –  J. Musser Mar 18 '12 at 1:26

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