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how deep to go with the compost in a big container

Aurum66Aurum66 Posts: 65
in The potting shed
Hello wise gardeners
I have 6 large containers 90L capacity and about 60cm tall x 45cm wide. I'm planting dahlias and filling between with marigold and classic bedding. What I want to know is how deep to go with the compost. I was thinking of using broken up polystyrene at the bottoms for drainage as I've done this before with great success, but just wanted to make sure I'm not wasting a lot of compost as it's going to take a lot to fill them and they will weigh a tonne.....best to do them in situ I guess :)

Posts

  • DovefromaboveDovefromabove Posts: 88,147
    What size dahlias are you planting?  Little bedding ones will need much less root space than the bigger varieties. 

    Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.





  • ObelixxObelixx Posts: 30,090
    As long as you keep them regularly watered and fed you probably can fill almost half that depth with polystyrene chunks and the rest with compost and then a mulch on top.  I suggest you fill and plant up in situ rather than trying to move them afterwards.
    Vendée - 20kms from Atlantic coast.
    "The price good men (and women) pay for indifference to public affairs is to be ruled by evil men (and women)."
    Plato
  • SussexsunSussexsun Posts: 1,444
    My large pots are always half and half.
    i add an old cut up jumper or t shirt to the bottom( helps stop the soil from coming through the drainage holes but still allows drainage) then a layer of broken pots then my packing peanuts. Compost mixed with vermiculite to fill. I buy Poundland vermiculite and find it just as good as the much more expensive GC ones.
    To see a world in a grain of sand and heaven in a wild flower Hold infinity in the palm of your hand and eternity in an hour.

  • [Deleted User][Deleted User] Posts: 0
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  • Aurum66Aurum66 Posts: 65
    Thank you everybody.marvellous. I will fill out up to halfway and put the good stuff in the top half. These are good sized plants...last year they were growing in small buckets in a conservatory and they reached my shoulders. This year they get all the love and food they could wish for to put on a good show for the big day and then I’ll give them all away because they are far too needy and hungry for my usual gardening practice! 
  • hogweedhogweed Posts: 4,053
    If your dahlias are as big as that I would put more compost than 'fill' in the pots.
    'Optimism is the faith that leads to achievement' - Helen Keller
  • Aurum66Aurum66 Posts: 65
    Hi Hogweed
    thank you. I'll be more generous then. I don't want to hold them back
  • KT53KT53 Posts: 9,016
    Even with larger plants I'd be inclined to go along with the 50/50 suggestion.  Easier to move the pots if the need arises and you can always add fertiliser through the season.
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