Extreme Clay Soil - Plants for extreme clay

Sometimes it feels like the word "soil" in the phrase "clay soil" feels like is a misnomer, what to do if what you have is clay with a hint of soil.


Questions received and answers.

Q. I have inherited a clay hillock for a back garden, with about an inch at best of clayish top soil, very clayish sub soil, and total clay beneath that. Is there an easy answer to growing a lawn with out resorting to the obvious. (importing tons of worms/topsoil)?

A. The good news is that grass will grow on the most unpromising of soils, though could always do with a little help. If you're going to lay turf, then a ton or two of topsoil will help tremendously, it won't add much more % to the total cost especially if you are going to pay some-one to do the job for you.

You could just lay the turf directly on what you have (it comes rooted in about an inch of soil anyway) and it will more likely than not be ok (how is the grass in neighbouring gardens doing?). If I were starting from scratch though as it sounds that you are, then your "obvious" answer is the one to go for. Even if you're going to do it yourself, a ton or so of topsoil is not so bad to shift with a wheelbarrow in reality as it is in prospect.

Q. I have a Garden 10 meters by 3 meters. The topsoil seems very thin, then there is clay for at least another 60 cm or so. Underneath this is a chalk/flint layer (so my next door neighbour tells me). When it rains, and for some time after the ground is waterlogged and I can't even step into the area without loosing my shoes! It is fairly depressing. Unfortunately the only access to the garden is through the house and I have no outside drains in my garden (It is a terraced house). How can solve this problem?

One of my neighbours dug out all of the clay and replaced it with topsoil and put in drainage to the main drains (but they are at the end of the terrace with easy access and a back gate. The other dug a big hole (2' x 3' x 2') and filled it with pea shingle. I can't help thinking that the reason mine is so bad is because the sump hole is next to my fence! What can I do?

I live in Burgess Hill, this is on the south downs and used to be well known for its pottery works! Hence the heavy clay soil.

A. Not an easy problem to solve. If you want to deal with it properly then you aught to call in a drainage expert who could assess the site at first hand and suggest a solution, probably involving pipes set beneath the soil leading to a soak away at the end of your garden. If you want a cheaper answer however it's going to entail a lot of work. It's difficult to advise not seeing the garden directly. My first approach would be in penetrating the clay layer to the chalk and flints underneath, a soak away like your neighbours may help or a series of mini-soak aways. Dig a couple of holes about 12" in diameter (about 6 ft apart) down to the chalk layer and fill the hole with shingle apart from the top few inches where you can replace topsoil. See if this has an effect and then extend if it appears to work.

You'll end up with a lot of clay to dispose of down the tip, but there's no real alternative other than call in the professionals, do it a bit at a time and you'll get there.

It will also help to dig organic matter, and pea shingle into the soil in as large quantities as possible, ultimately however it is the removal of the clay that will help unless you want your garden to be elevated thanks to the added material.

A. from George Shiels, agronomist, McMillan-Shiels Associates.

With soils and drainage it is always better to see the site and look at the profile to see what is really there and what is causing the problem It is not always a case of the obvious cause being the actual cause. Topsoil can be a clay topsoil and chalk might be a hard or a soft type. the soft types do not always drain well and may not be good for a soakaway. The problem may be more due to compaction of the upper layer than to soil type problems, in which case compaction relief would be the best advice.

Generally the advice is reasonable but I would not advise adding sharp sand or pea gravel to a clay soil because the clay, if very wet, will simply absorb the sand and you then have gritty clay which in the long term will be no better drained. The idea of linking through to the chalk is the most obvious one for a gardener.

On a larger scale we use a post hole borer but a pit excavated down to the chalk may be all that is required. Several pits across the area of the garden would be effort well invested if the chalk is suitable. In Burgess Hill it should be suitable. The pits can be spade width in such a small site. As you say, backfill any pit with pea shingle and blind with a grit, cover with a good 150-200mm depth of sandy loam topsoil or the existing topsoil mixed with a lot of medium/coarse sand. The mix must remain permeable and will be around 9:1 sand:soil if it is to work long term. A larger pit with drains running into it would be another option. The drains should be 45cm deep minimum and backfilled with pea gravel and then covered with a sandy loam topsoil to 200-300mm depth to ensure adequate growth as well as drainage.

Q. Last year we moved into a property with 2/3rds of an acre of paddock surrounded by fields. Last summer was very dry and the land was rock hard.  In July, we hired a digger and driver to excavate a pond and discovered that there was a few inches of top soil after which it turned to clay and was still clay four foot down.

We also had the digger man dig some large deep pits which we filled with top soil and planted 12 decent sized trees  (cherries, rowans, silver birch, etc) which cost several hundred pounds. Now it is winter and we find that the tree pits are totally waterlogged.  The land itself is very wet although there is no surface water accumulating  except in the tree pits themselves. 

We have a septic tank but when we purchased the property was advised that it overflows into the septic tank of the neighbouring property which we understand then overflows into a nearby stream so I do not believe this is contributing to the wet ground problem. Basically it seems that the clay is causing the ground to be waterlogged but when I have researched field drains/soakaways, etc, it seems to be a bit of a minefield and I understand not necessarily the answer to the problem.  It appears that improving the condition of the clay by lots of digging and mulching, etc, is the answer but with 2/3rds of an acre what is the best (and most economical) way to tackle this problem.

Is there a mechanical method of dealing with this effectively? Apparently rotovators are not a good idea as they don't go deep enough and are often ineffective in the hands of the novice. I would like to have a nice lawn, raised beds, trees, shrubs, paths, etc, as quickly as is reasonably practical and which won't die because they are virtually sitting in water over winter. Incidentally, would it be possible to save the waterlogged trees - what would I need to do to manage this?  Any advice you can give me would be very welcome.

A.  "Now it is winter and we find that the tree pits are totally waterlogged" - It can happen like that on clay soil, you've dug below the winter water table and inadvertently made a series of sumps. 

With 2/3rds of an acre, you need to work with the soil and not try to change it wholesale. Drainage of such an area can be quite a complex problem, particularly if there is a stream nearby, you'd need to get an expert in really to take a look and give advice, all this and the work involved would be potentially expensive, though as you say there is no standing water probably not necessary.  

Blessings - clay is very nutrient-rich soil and when plants get established, they will be better off than in most other soils.  Lawn - shouldn't be a problem at all, grass will grow on all kinds of soils and the top-soil over clay means that it's far less likely to ever dry out and get scorched.  Raised beds - no problem, fill them with top-soil bought in.  Paths - shouldn't be a problem as there's no standing water.  

Rather than try to change lots at once, tackle smaller areas one at a time,  digging is better than rotavating, over time even surface organic matter  will be dragged into the soil by the worms.  As for the trees, are they still alive? Try breaking a few buds open to see. The rowans are more tolerant of wet conditions, but the cherries and birches less so. They might survive regardless, but the only way you'll know this is by trying and if they don't - well too late. The answer is simply to move them somewhere they will like to be instead, do you have a better area around? somewhere at the top of a slope that they could be moved to? The only other alternative is really to put them into some large containers or raised bed so the roots aren't rotting in the wet. The cherries will be the most sensitive to wet.  Take a look around the surrounding area, what kinds of trees are there? If there are similar ones to any of yours, then they are probably the ones to leave where they are.  I'd also suggest getting a landscaper in as it could well save you a lot of money in the long run and there's nothing like specific advice for a specific area.

Further Q. I have approached a couple of local landscape gardeners, both have suggested installing drains and leading them to a large sump hole at the bottom corner of the garden. It appears some of the farmer's field from which our patch was purchased by the previous owners of our house is inclined to drain in our direction, no doubt exacerbating our water problem.

Terraces have been suggested, formed by using large limestone blocks and backfilling which along with the drains, 280 tons of top soil and lawning with seed is around £7k for the cheaper quote. I don't have a clue whether that is a realistic proposition and would value your expert opinion regarding whether you think the proposal is reasonable both from a resolution and cost perspective. At least it might tidy up the land and make it reasonable to work with because at the moment it is a liability which I feel devalues the property.   

I can put tree, shrubs and beds in myself once the basics are done.   Perhaps Groundforce would come and work one of their 48 hour miracles ( as if !!!)   Incidentally, we lifted the trees last weekend and I think they are still alive. Hopefully they will survive whilst we get our act together.

A. Terraces - well they'd work, but I don't like them except on very steep slopes, cost seems about what I'd expect. If you are considering spending that amount, I'd seriously consider getting in a designer as a contractor alone isn't necessarily going to come up with the best solution. From what you've told me, I'd proceed with either working with the land, or get in a designer or two to give you some possibilities.  Before you do any of this though, sit down and write a list of what you'd like from the garden. You might despair about the soil and the clay, but looking from the property point of view, all most people want is a patio / deck area, something to look pretty from there and maybe a kids play area which at its simplest could be just a level area of lawn.

Further Q. I have not contacted the landscape gardeners awaiting your reply.  I can see the logic of getting in a garden designer.  Presumably the ground work (ie, drainage) would need to be sorted regardless but as you suggest a garden designer might have more imagination and it seems it would cost a lot less than I imagined so I think I'll try that if I can find one in my area.

A. "Presumably the ground work (ie, drainage) would need to be sorted regardless." - I was thinking the opposite maybe. As you say you don't have a problem with standing water, it sounds like something that could be worked around. If getting a large extension done on the house, you wouldn't just take the builders word for what he thought was ok, you'd have an architect in.

As you don't seem to have any particular desire for a specific garden type, then working with the garden rather than bending it to your will seems a far easier way to go. A good designer will take such things as soil type and drainage into consideration and overall I think you'll save money and have a more pleasing result.

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