Huge Mistake?

Hello everyone.

First time poster and relatively new gardener here. I have a new build garden and seem to have made a huge mistake! I'm hoping for some expert advice from you wonderful people

Like most new build gardens, it suffers with heavily compacted clay. Coupled with an ever so slight slope I’ve experienced water logging issues, not only in the winter but even during the wet periods of the summer. The most painful part of this is that the drainage issues are isolated to the part of the garden that is the sun trap, and where most of my flowers are located – some of which were lost over the winter period.

It has been my ongoing mission to fix the issue, and this is where the problems start. I sought to improve the soil along the fence line as this is the worst affected area. After many evenings googling and advice from my local nursery I started by adding organic matter to improve soil drainage and structure. I thought that if I dug holes in between each tree (see image below) and improve this soil then it would stop the drainage problems by capturing any rainwater on the move towards the lower ground - mistake number 1 it seems.

I may have been a bit over enthusiastic here - I dug each hole about a metre wide and around a metre deep. Removing heavy clay and the associated builders aggregate along the way. Replacing this with bark, straw and composted manure mixed into some clay and topsoil. I may have gone a little over board with the organic material. I thought this would have done the job; however, upon inspection after some smelly soil, all it has done is:

1- Acted as an area of least resistance for all water to go, including the neighbours gardens (I hadn’t considered that soil doesn’t adhere to garden boundaries!)

2- A result of the localised water flow and heavy organic content I am now suffering with quite bad anaerobic soil as well as high water content in these areas.


So my question to all you wonderful gardeners is:
How terminal is my problem? Will the anaerobic soil fix itself once the organic material breaks down?
How can I fix it? I no longer have the clay soil to replace my over organic’d soil (is that a term?).
I have some wonderful Hydrangea Annabelle’s in between my laurel trees and I don’t want to lose them because of the anaerobic soup that is bubbling away underneath them.

 

It's shocking how badly estates are built nowadays with zero knowledge or care about landscaping.

It seems to me that you went too hard on solving the issue. Soil improvement can take years as it's a gradual process. Digging massive holes creates sumps where the water makes its way to.

From the photo it isn't clear how big your border is but if it's anything smaller that 2m deep I'd move all those plants because they won't have enough space to grow against the fence.

 

I think you have half the solution. Next action would be to 'broadfork' between the areas you have drained, linking them and when you get to the downhill end, gently channel away. You have always the right to enjoy drainage on to your neighbours land. Its common law.
However, you must not cause his house to flood, needs careful ,subtle engineering with due regard to the lie of the land....

 

If you are like me you might need a read of this....

 

Some very quick replies. Thank you.

 

It genuinely is shocking how bad new build estates are! There's some parts of my lawn that are dense with fist sized rocks no more than 6 inches below the surface. Fortunately I'm getting it all re-turfed next spring so they will correct all that then. The borders are 1.25m deep if i remember correctly - I may move them forward so they overhang the lawn a little, I don't mind that.


Broadforking is a great idea, thanks for the link also. I've been using my garden fork until now and it's been a hell of a workout!

Channeling isn't a bad idea I will see if i can link them together. I had considered linking under the lawn as an organic soak away method.

 

I have read similar, they could sell you one for £140..
A contractors fork £35 ;works well enough:
Bulldog Contractors Steel Fork

 

My current solution is to replace the deeper parts of the sumps with low organic content topsoil and leave the organic mix for the top foot or so. The Westland topsoil looks to have no organic chunks but is quite dense.

 

I wonder if it would be a good idea to add some gravel to the infill for better structure. I'm in London but thankfully don't garden on clay...so maybe anyone with hands on experience of clay can confirm.

 

I've added gravel to the lower elements of the top soil layer to trying an increase air pockets. The real issue seems to be that the clay has no draining quality at all, I may be wrong and it may just be incredibly slow but when i hit that layer and churn it, the soil appears to be bone dry - even with a wet layer above! A symptom of the heavy machinery compaction I expect

 

Perhaps a further exploration may give you some answers. If its just a localised issue; that is that other areas within your garden do drain , then why is it thus ? I suggest that you dig a hole(s) down to ,and through, the suspected compacted area. Try and find if there is a layer beneath that is dry, and hopefully, porous.
You might then cover the hole and wait 12 hours to see if water has appeared.Does it disappear over time ? If the hole remains dry, you could fill it with a bucket and see if that disappears, and in what time ?
You may well find a buried structure, from the previous use of the site. Local research of the area, examination of planning applications and drainage mitigation requirements could give you further clues as to what you are up against.
Sometimes developers choices of names for projects and road names tell a story. If you are 6 The Willows, Marsh side Avenue; then its likely going to be wet....

 

I do admire your willingness to correct the problems! I had similar issues on a former agricultural field of 2 acres...clay and years of compaction, albeit not above the depth of a plough.
After seeing how the water ran and collected through the winter, the solution was a land drain right across the slope to the borders with the surrounding fields.
Don't worry too much about your anaerobic 'pits' They'll be the best bits of the garden soil within a few years.
The old advice of double-digging, incorporating organic matter and horticultural grit if you can get hold of some gives plants an excellent start. Combined with regular mulching, you'll have soil to die for

 

I did this. And the results were interesting:

A hole 1m x 1m x 1m in the centre of the grassed area - no water.

A hole 1ft x 1ft x 1m+ on the right fence line now occupied by the Salix - Filled up to to around 10 inches from the top.



The house on the other side of the fence has a plant border against the fence aswell and is a bit of an over waterer which I think contributes to the issue somewhat. But when I dug the hole on the right you could see the water seeping from the topsoil into the pit.

 

Did it then disappear (overnight)?
It is possible to find 'ground water 'levels from the Environment agency site for your locality. These are updated in real time and give a clue as to what may be 'normal'; and trends going forward:
Find river, sea, groundwater and rainfall levels - GOV.UK

 

Thanks Infradig - no groundwater data for my town unfortunately.

The water did not disappear and this is why i found the result so interesting, that another hole no more than 2 metres away remained dry.