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Compost for raised concrete borders

Hello everyone. I have a number of raised borders in my garden, made of concrete block. The block has been waterproofed. The taller borders are about 4-5" deep and have a lot of concrete at the base, so not much contact with open soild. Others are shallower but with good contact with the underlying soild.

My question is this: what is the best compost/soil mix to go for given that I will be planting them quite full and with greedy plants? I would imagine lots of organic material? I want to avoid having to dig them out each year to re-energise them, almost as you would with potted plants...

Thanks lots!


  • hogweedhogweed Posts: 4,053
    Did you mean 4-5 inches deep or was that a typo?? Do they all have drainage facilities?
    'Optimism is the faith that leads to achievement' - Helen Keller
  • Sorry typo! 4-5 FEET!

    They all open to the soil below to some degree.
    I can add gravel into the bottom before anything goes on top?
  • ...4-5".... what was I thinking... rockery anyone?
  • Lizzie27Lizzie27 Posts: 11,999
    I would fill them with a mixture of bagged horse manure and John Innes No.3 which is a gutsy mix, suitable for a wide range of plants.  You may well find that you have to top this up each year as the soil settles/you dig out some plants etc. Next year add in some Blood, fish and bone as per the packet instruction.You could also add in some horticultural grit to provide more drainage.
    North East Somerset - Clay soil over limestone
  • FireFire Posts: 18,143
    I would add a load of grit if you are going to plant anything with bulbs, or plants whose root easily / need good drainage. I lost a lot of bulbs last winter in a new raised bed. The medium was very water retentive (which is what I wanted) but too heavy for anything prone to root rot.
  • FairygirlFairygirl Posts: 54,358
    Hi Windsor - when you say 4 - 5 feet [ ;) ] are you talking about the height of the beds or the depth front to back? In any case - you'll need a big volume of material. Perhaps a bulk bag of top soil to start with would be beneficial, then you can add manure, grit and compost according to your planting needs   :)
    Can you drill drainage holes through the block along the bottom of the beds? That will help enormously with drainage, and give you more scope with planting. 
    It's a place where beautiful isn't enough of a word....

    I live in west central Scotland - not where that photo is...
  • Hey Fairygirl! Sorry the height is 4-5'. The front to back is about 3-4 feet on most of them. I can dril holes in the back of some of them but was kinda hoping them being open to the soil below and being quite deep (tall) and adding some grit would be enough to keep roots from sitting in water? I was thinking of adding worms so that, if I add lots of organic stuff all the way down, they will, over time, churn things up. The planting will be quite lush with greedy big leaf stuff so I am trying to go over the top a litt on adding nutrients / organic matter.

  • (PS - chap helping me thinks we need 10 tonnes...)
  • LynLyn Posts: 23,088
    4 - 5 ‘ I would be able to see in them! 
    Gardening on the wild, windy west side of Dartmoor. 

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