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Soil depth for dahlia raised bed

jcase1664jcase1664 Posts: 29
in Problem solving
Hi - I am currently in the process of constructing a couple of raised beds for some dahlias. 

1 - How deep should the topsoil be??
2 - The raised beds were originally going to be a shed so a stone scalping base was already down. I have already scrapped away alot of it (see pic) but I presume i could leave an inch or so of the stone down to help with drainage as we are on clay here in Dorset.

Many Thanks for your help everyone  :)




 

Posts

  • amancalledgeorgeamancalledgeorge Posts: 2,736
    I would think a minimum of 25cm as they become quite big plants. But let's see what other will say. 
    To Plant a Garden is to Believe in Tomorrow
  • FairygirlFairygirl Posts: 55,117
    I'd think that would be ok, but the only thing I'd add is that, while stones/gravel is useful for extra drainage, just putting a layer down in the base isn't necessarily the answer. If the soil below is compacted, it has the opposite effect, and can become a sump, with roots sitting in water, which defeats the purpose of the raised bed and all your efforts. If the bed was 2 or 3 times that depth, it wouldn't be a problem.
    If you can, it would be worth breaking all that soil up with a fork or spade. Clay is a great medium for most plants, but not if it's solid.  :)
    It's a place where beautiful isn't enough of a word....



    I live in west central Scotland - not where that photo is...
  • purplerallimpurplerallim Posts: 5,287
    I have some in a raised bed @jcase1664 that is four bricks high, open bottomed, and on clay soil which do well.
  • LoxleyLoxley Posts: 5,698
    Yes you can leave the stone in the base - the soil will just drain into this. I think about a foot of soil, c.30cm, would be fine. A bit deeper will probably be slightly better in terms of the amount of water the soil can hold between watering in hot weather, but it's probably fairly marginal (the dahlias will probably root down through the stone scalpings anyway). 
    "What is hateful to you, do not do to your neighbour". 
  • PosyPosy Posts: 3,601
    I think it depends on the dahlias. Little bedding ones will be ok in 12 inches, but the tall ones in my garden make tubers a lot deeper than that. They can fall over, too, if you can't stake them properly. I would want about 2ft depth, minimum.
  • FairygirlFairygirl Posts: 55,117
    I'd still say that ground needs broken up, and lots of organic matter added. If it was being prepped for a shed base, it won't be in suitable condition for giving plants adequate drainage. There could be a very solid barrier of clay below those scalpings.  :/
    That's certainly the case here, and it isn't great for any plant [except bog plants] especially if there's a lot of regular rain. 
    It's why I have raised beds in most of the garden. 
    It's a place where beautiful isn't enough of a word....



    I live in west central Scotland - not where that photo is...
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