Making a raised bed on clay

[rodofgod76]

I want to build a raised bed this year at the bottom of my greenhouse. The trouble is, the lawn in that part of the garden has a bit of a depression in it and fills with water when it rains. It seems that it's all clay in the part of the garden. Now I'm thinking of levelling the lawn off where the depression is, it's maybe 8-10 inches deep at it's deepest part, so I'm wondering whether I can just fill the depression with soil and then make the bed on top of it, or will I need to break the clay up to improve drainage in the area first, then build the bed on top of it? I don't want the bed being surrounded by a moat everytime it rains.

[Eyren]

Hmm, tricky one! I seem to recall something I read about digging drainage ditches for that sort of situation, but I can't remember the details - anyway it sounds like you need to solve the problem before you set up your bed, otherwise your veggies' roots will suffocate

Perhaps someone with more experience of really heavy soil can help - I'm on light clay at a comparatively high elevation by East Anglian standards, so we don't have a problem with waterlogging.

[jemma_charlie]

I had this problem, i dug the top soil up and put it to one side, then dug about a spades depth up and put that seperately to the top soil, I then layed some hard core in the bottom of the trench, only a few inches deep, then replaced the clay and then the top soil. This has levelled off my garden and drainage hasnt been much of an issue over the winter. Back breaking work though

[Norm]

Just a thought, but if you have a natural reservoir you want to put a raised bed over, the soil/compost in the bed should absorb most of the water.

[tootles]

You could dig a 'french drain' a deep hole at the lowest point filled with gravel. It doesn't have to be huge, just big enough to act as a soakaway so that your veg doesn't become waterlogged.

[snohare]

The theory I've always heard is that you need a soakaway two feet deep and eighteen inches on a side, filled with larger stones at the bottom and gradually smaller ones at the top, in order to drain ground like that properly.
I have never yet met anyone who doesn't lose the will to live before getting two feet down ! Like Jemma says, it is back breaking work.
The short cut I and friends have used many a time is to dig a hole a spit's depth, then simply to violently slam a large pinchbar as far down into the underlying hard pan as possible, if you can break that up to a depth of six inches it seems to make a heck of a difference. (If you don't wear gloves, you will very rapidly get blisters doing this.) Backfill with stones, and if you are worried about the compost soaking up lying water, put a layer of hardcore down below it to mitigate the wicking effect.
This is why posh folk employ gardeners...

[rodofgod76]

Thanks folks.