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Hi all, I was wondering how deep a layer of compost I could add to established perennial beds without killing them. I normally just add a little in spring but can I add quite a lot and raise the soil level maybe five or six inches? My thinking is it might help with drainage if we get more wet winters. If so should I add it now before the plants come through?
Adding compost is not going to help drainage. Fine gravel and perlite will. Just adding soil may actually make things worse as it may mask what is happening under the new soil level.
To answer your question you can add a few inches of soil to established perennials.
Thanks blairs, I dug holes 1 1/2 -2 feet deep and filled the bottoms with stones under some new plants that like good drainage (Armeria, Erodium, Gypsophila and Alstroemeria) I'm so glad I did, I don't think they would have survived all this wet otherwise.
It does also depend on your subsoil as well. A compacted subsoil will raise water levels. It's a major job though to tackle bead subsoil. sharp sand and grit are your best bets to improve drainage.
The top 18" or so is lovely loam, the sub soil is a mixture of clay and sandstone, very weird.
Blairs, I am afraid I dont really agree with you. Compost applied to a heavy soil will definetely improve drainage, this is a key recommendation for its use. In light soils compost improves moisture retention.