Thick clay under top layer of soil in new garden

Sagehead · February 2023

Hello, I just moved into a new house and am starting to do the garden, and when I dug it I found that under about 10 inches of top soil is thick yellow clay. It's not even clay soil -- it's pure clay; I think I could make pots with it! I'm worried about the impact on drainage, so I dug a hole 12 in by 12 in, filled it and waited for it to drain, and it took nearly 2 hours. Is this a long time? I'm now sure how long it should take. I've brought loads of salvias from my old garden but I don't think they'll like it here -- does anyone have any advice or experience tackling this problem? Many thanks 😊

Slow-worm · February 2023

That happens with building unfortunately. I found a load of clay mixed in the soil of a border I cleared last year - we're on limestone here! It was about 50/50, and my salvias have been fine in it. You could scrape the top back and add some sand and/or grit, or add extra soil and mix it around where the salvias are going.
I've only ever dealt with new house heavy clay once, and it wasn't my house, but we just added kitchen scrap compost to certain (small) areas - that worked wonders but unless you have tons of compost for the whole lot, you might have to get plants which are happy in any soil, or shallow rooted. It'll work out though.
There's plenty of knowledgeable folk here to help more than me! 😁

Nollie · February 2023

Hi and welcome.

Digging in tonnes of organic matter, as much as you can, into the entire bed - compost, leaf mould, pine bark fines, composted manure - lots and lots of manure - is the only real answer to improve hideous clay. If you add sand to the mix make sure it’s horticultural sand or grit and not builders sand, because clay + the latter = concrete! You do need a lot of grit to make a difference to drainage though.

If you just imrove the planting hole you create a sump for water to collect, which is death to plants that like freer draining conditions.

I had a similar clay problem but also a shallow layer of it on rock so the only way I could make a garden and plant anything, including salvias, was by making raised beds.

Fairygirl · February 2023

I've never gardened on anything else but clay - mostly uncultivated, and 2 hours isn't very long in my experience

As @Nollie says - loads of organic matter is the answer, and there's no quick fix if you want to plant directly into the ground.
If you want salvias, you may need to have raised beds [as mentioned] to counteract the clay well enough, especially if you're in a colder, wetter area. Sharp drainage is necessary for them as I'm sure you know, and that will just make life easier until you can amend what you have.
The only salvia I can grow reliably here is the mega hardy one - S. caradonna, and it's also in a raised bed with very light soil and good drainage. Our climate isn't suitable for them unless they're overwintered, or cuttings taken. Even so, that one struggles a bit in the colder winters

Busy-Lizzie · February 2023

I moved to a new house in SW France 2 years ago, same problem. Every time I made a new bed I dug in bags and bags of compost, but only about a foot deep. Everything I have planted survived, except for a couple of lavenders. Not sure why they died as several others survived. I have several salvias which have all done well.

I dug this bed in October 2021 then planted it and this is mid August 2022. I worried that it wouldn't work because of the clay.

B3 · February 2023

A positive is that when clay is improved, it is a very fertile medium. You could have worse things under your topsoil.

Fairygirl · February 2023

Absolutely @B3.
Of course, the soil is only one aspect of being able to grow salvias easily. I still can't change the other factors, and that's what @Sagehead will also have to bear in mind. Unless the new house is still in the same general area as the previous one, and climate conditions will therefore be much the same as before.

Allotment Boy · February 2023

I agree with previous advise. The only thing I would add is, if you can't dig compost in then just put a thick layer on top 5-10 cm every year. It will take longer to improve but it will work.

B3 · February 2023

Also empty spent soil from containers into the soil.

bédé · February 2023

What you have got is sub-soil from the foundations. If you dig down you will probably find topsoil underneath. The counsel of perfection is to move the present top soil to one side, dig out the clay, dig out a lot of the old topoil. Bury the subsoil with the topsoils.

I have done just this in the past. ... I was younger and fitter.

Alternative: find plants that like this situation.

Wjs57 · February 2023

although it will take longe, I’d add my voice to those advising the use of a thick compost/ manure etc but I would say unless you’re very fit or using someone else, lay it thickly on top of beds ( assuming they are empty now) and leave it . Look up Charles Dowding and “no dig”. I am gradually converting to no dig in my front and back gardens . We garden on London clay , which I appreciate isn’t the same as “real”clay but my first ever no dig bed had a decent layer of compost and farmyard manure ( maybe four inches) put on it in early winter 21 /22 and was ready to use by spring 22. It has been a success with no digging at all and all the new plants survived the drought and the freeze. Given that you have ten inches of top soil I think you will be fine if you do that despite what’s down below.

Thick clay under top layer of soil in new garden

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