Hello, I've been pondering over an idea I've had for some time now, but don't know if I SHOULD do this or not.
The idea is that because I garden on heavy clay soild and I'd like some plants that need fast(er) drainage such as Eremurus and Agapanthus, I'd dig out new beds two feet deep or so. But instead of relying and trying on amending the native clay with compost (or perhaps maybe the bottom), I'd just get rid of it altogether, leaving me with a wide bed (hole) like when you're digging a pond. Then backfill with topsoil (not completely sand) and compost.

I know people talk of adding grit/sand to a planting hole is bad because it acts like a sump, but done on a larger scale (several m²) will or won't this work?

Sketch:

Good luck with your project! Some work now will give you a GREAT garden bed for the rest of your life.

I think there's only one issue when you dig a hole or a "raised bed" below grade.
Once water gets INTO the hole you just dug, where can it drain down TO?

I forgot that water won't drain uphill, and created this mud wallow:


One solution is to keep the bed entirely "raised" and not rely on any part of the root zone being sunken below grade.



I assume your clay is so heavy that rain would fill the bed faster than it can "percolate" out the bottom of the bed. If that's true, it WILL turn into mud-soup with zero air every time it rains faster than the bottom of your bed can drain. Roots will start drowning as soon as that happens, and keep drowning until most of the water drains out of the amended soil (and you have to hope that turning into mud doesn't destroy all the structure of the soil).

What can you do? Provide drainage. You don't have to go totally bonkers as I did here:



Probably your amended soil WILL drain fast enough to keep up with most rainstorms, IF the water has some path by which it can exit the amended soil.

How?

Hopefully you have some slope or grade to work with. Ideally locate the bed on top of a high spot in your yard. If you put it in a low spot, it will become a big mud puddle and grow nothing. You can put it on a slope. Even a slight slope is probably enough.

If you can't locate it in a spot where it's easy to provide drainage, you do have a problem.

In that case, it might have to be a "very" raised bed. In other words, don't excavate or discard any clay.

Build taller walls and put your new soil on TOP of the old clay. You still want the lowest part of your new raised bed to be HIGHER than surrounding clay. Water can drain down from a "very" raised bed because even after it drains "down", it is still higher than the surrounding clay (it is "above grade").

Now it is happy to become just more run-off water, running over the clay surface down to whatever the low spot is, but AWAY from your bed's root zone.

- - - - -
If you have enough slope to provide drainage to a partly sunken "raised" bed, you might still need a trench. Remember that water will only drain DOWN, never UP.

You need something like a trench or drainage pipe that connects the LOWEST point of your water-logged mass of new soil with a spot LOWER than the lowest part of your bed.

What I do, after removing the awful clay, is to scrape the exposed "floor" of the bed into a fairly smooth slope down to one edge or one corner of the bed. Obviously that has to be the lowest edge or corner.

Then I dig a slit trench from the low point of the bed to the low point in my yard. (I'm lucky to have a steep slope in one part of my yard, so I just lead water to that area and then it runs downhill all on its own.)

The trench can be just a few inches wide (I make it the width of my mattock's blade). If we have clay so nasty that we need to create drainage, the clay is probably hard enough to hold the shape of that slit trench for years. I don't even back-fill my slit trenches with gravel, and they have held up for years.

The only tricky part is to keep the floor of the trench to a fairly even slope. It can never dip down and then back up, because the low spot will hold water and kill roots nearby, and freeze int he winter and mess up your trench.

There are fancy ways to keep a trench's slope uniform using stretched string, rulers, lasers or whatever, but I just scrape it level by eye and then wait for a heavy rain.

The low spots fill right up with water. I stand a little down-slope and dig the trench below the big puddle to be even lower than where I guess the bottom of the puddle is. Then I use the mattock to remove the last plug of clay between the puddle and the rest of the downhill run. The water rushes out and erodes the rest of the trench's slope evenly, and drops some mud and silt in remaining low spots. Repeat once or twice, and you've leveled the whole trench.

This is easiest if you start at the bottom and work uphill, but you can also jump around but accomplish the same result eventually. Since "water finds its own level", it will tell you if any part of the trench is too high and blocks the lowest point in the bed from draining down to some low spot in your yard.

If you worry about guests breaking their ankles, you can back-fill the trench with drainage gravel. But that is expensive and HEAVY! I learned that after digging a honking big trench, and then realized that the back-breaking part had only BEGUN. Hauling enough gravel to fill that trench would have broken my back and my car's springs if it didn't break my bank account first. IF you dig a trench big enough to drop a perforated drainage pipe into, do that! Then only backfill a little gravel around the pipe.

Here are some photos of the time I turned into an insane mole-man and dug up the entire front yard.
http://garden.org/thread/view_...
That was so unnecessary!

The problem with that is, the soil you add will be fast draining, but then it will hit the clay underneath and stop. You would basically be creating a bog. Once the new soil is saturated it will drain very slowly through the clay drowning, you plants. It would be better to either amend the clay or go with a raised bed.

Horntoad said:The problem with that is, the soil you add will be fast draining, but then it will hit the clay underneath and stop. You would basically be creating a bog. Once the new soil is saturated it will drain very slowly through the clay drowning, you plants. It would be better to either amend the clay or go with a raised bed.

But isn't that exactly like a raised bed? I mean, you're just piling better draining soil on top of lesser draining soil except the walls in my proposal are the original soil (which is porous indeed).
I mean, in a raised bed the water eventually also hits the bottom and stops or slows draining away. It's not like during rain the water that has already penetrated the clay one foot from either side (in my proposal) suddenly decides "Oh that soil over there is faster draining, I think I'll head that way."...Same goes for amending it. I don't plan on amending/digging out my whole garden.

Place on planning this is behind the hammock

Except that with a raised bed you don't have water potentially also draining into the bed from the sides, which you may have with a sunken bed. Some people call the excessive wetness in a bed dug into clay the "bath tub effect". I know of someone who did this and had problems with plants because of the poor drainage.

Why not dig a test hole, pour water into it, or do it before it rains a lot, and see how it drains? If it doesn't drain well then maybe dig the bed less deep and raise part of it. Arborists no longer recommend backfilling tree planting holes with amended soil because of problems with water movement (either way) through the soil interfaces between different textured soil, which is pretty much the same effect. The compost or other organic material in the mixture will also continue to decompose and create a depression.

With a raised bed the excess water can drain underneath the wall unless you bury the edges. You can also provide extra drain holes in a raised bed wall if necessary.

Here is an area I dug in my clay soil. This was taken after a short shower. With a long or hard rain it would be full all the way to the landscape timbers. In the driest part of the summer, it may take a week or two for the water to dry up. If I were to fill this with good soil it would be a bog garden (which I have considered doing).

Another raised bed is not an option I'm afraid. And my clay is not THAT bad that water sits on top of it for hours on end. Sure it doesn't drain like sand, but I wouldn't consider it a boggy area either.

I did something like you are planning, however, I only dug down one foot in a 15m x 7m garden, removed the clay-like soil and replaced with a topsoil/sand/peat/wood chip/whatever I could find mixed with some of the clay. It is on a very gentle slope, almost unnoticeable. I also did it all the way around the house for the flower beds. I did that five years ago and have never had a problem with roots, rot, drowning, sloppiness or ponding. Unless you expect several inches of rain over a short period, or your clay is absolutely not permeable, I'd go for it. if its the raised area behind the hammock (with cinder/concrete blocks), you'll be fine. If things get ugly (ponding), put a 1/2 inch drain pipe in the block.

removing the soil is backbreaking work and you will have to have somewhere to put the removed soil.

The short form of my long post would be: "You can get drainage by either raising the bed above grade (a truly RAISED bed) or by lowering the temporary water table by providing rapid drainage through a trench, so that excess rainfall escapes before it water-logs the amended soil.

I don't think it takes hours for root hairs to drown when underwater (depending on the plants). Remember that after every drowning event (rainfall) the plant has to get rid of rotted dead root tissue and then rebuild it before the plant can take up as much water and minerals as it used to. All its energy goes into re-building the root system after every rain.


The photo looks like you don't have much slope to work with. Maybe forget trenching. Maybe make the new bed partly raised like the area behind and to the right of the hammock? Like an extension of that bed? Maybe only 12-18 inches tall?


One thing that I do when I want more drainage but less digging is to put a partially sunken walkway around a partially raised bed. Loosen the soil or double-dig the bed before digging the sunken walkway. Throw soil from the walkway up onto the just-turned bed, making it deeper at the same time you dig the footing lower.

I do trench to drain the walkway, but if you have no "low spot" to trench TO, your sunken walkway could form a moat after each rain. The bed would drain rapidly into the moat while the roots in the raised bed stayed alive. Then the moat can take an hour to drain if it wants to.


If your clay soil drains slowly, digging a hole below grade (lower than the surface of the clay) will cause that hole in the ground to fill with water in a heavy rain. Clay "perks" very slowly, and rain can fall pretty fast.

If you have a sunken bed in a below-grade hole (amended soil in a hole below grade), it will turn to mud in the first heavy rain, and that will probably collapse the amended soil structure as well as drowning roots.

If the clay drains faster, and you don't have heavy rainfalls, you might be OK with a bed that goes below grade. You'll soon find out. It all depends on "how heavy" your clay is, how fast your rain is, and how tolerant your plants are of having drowned roots.

If the seed packet says "needs well-drained soil", don't plant it in an un-drained sunken bed, unless your soil is at least as open as loam or sandy clay loam. Clay, clay loam and sandy clay are not "well-drained soil".

Unfortunately, not even sandy gravel is "well-drained" if it is the back fill for an un-drained sunken bed surrounded by poorly-draining soil. You would just have a puddle with a nice sandy gravel floor.


>> And my clay is not THAT bad that water sits on top of it for hours on end.

The grass is clearly not drowning. Maybe your soil does drain fast enough for a sunken bed to have a chance.

However, the rain water MIGHT be running off in a thin surface sheet you can't see, under the grass, down a very slight slope. If that is going on, squint hard and figure out what the slope is. BLOCK any uphill runoff from entering any sunken bed.

Digging a hole would tell you as soon as you had a heavy rain. Sue has the right idea. Real estate agents call that a "perk test". I guess in areas other than the coastal PNW you might have to fill the hole from a garden hose. Where I live, just wait a day or two.

I'm a big fan of sufficient drainage. It doesn't help to put fast-draining new soil into a puddle.
The BED has to drain well, not just the soil IN the bed.

Well you don't want to see my lawn right now, it's a mess. It's been raining almost every day now for the past few weeks and it's really suffered because of it. Even more so when we or the dogs walk over it. Bare patches here and there and a mud trail where we all walk. Nonetheless, my windmill palm that I've had for second year in full ground now is doing well so far and all I did to that planting hole was mix in a lot of gravel, sand and compost and also backfill with it. on top of that and around is just the original clay. I think I might try it with less valuable plants first such as grasses or my calla lilies which like wetness.

>> It's been raining almost every day now for the past few weeks and it's really suffered because of it. Even more so when we or the dogs walk over it. Bare patches here and there and a mud trail where we all walk.

That makes me think: "If you dug a hole, it would fill right up with water."

Maybe here is a way to decide how long is safe for a hole to remain full of water. Imagine that you have a healthy plant in a pot. Suppose you dropped that pot into the hole full of water as soon as it had a few inches of water in it. Would the plant still be healthy if the water took 30 minutes to drain out? 40 minutes?

>> Nonetheless, my windmill palm that I've had for second year in full ground now is doing well so far

THAT makes me think: "His soil can't be THAT bad if below-grade palm roots are still alive."

II guess it is another case where you would have to try it to find out.

Unless your new bed on clay is the lowest spot in your yard, part of a long term fix will be a raised layer on your bed.

Short-term it will provide air space for growing plants, and residential space for worms. It will be these that tunnel drainage for the whole bed in time.

If you say another raised bed is not an option, but don't forget about vertical gardening in case you needed more space. Many herbs and small veggie plants can grow just fine in a vertical garden type of setup.

On a smaller scale, many people do what you are describing when they dig that 12-18" hole to transplant some new plant in. They make that hole with such nice growing material. Occasionally, the plant struggles because if the root grows beyond that 18" radius, it meets a new kind of soil that might make the roots struggle. I have forgotten the term for it but that is a cause of some plants struggling during their first one or two years in that spot.

However, your hole will be much bigger, and for smaller plants that might even die off each year, so it may not face the same problems. I just bring it up as something to be aware of when the plants might be facing a kind of border between two drastically different kinds of soil types.

Well--I did this last year.
Had an area between the house and the sidewalk around my porch filled with nasty, clay/silt back-fill--gummy muck when wet, like concrete when dry.
Dug out to about 2 and 1/2' by the foundation and sloped the 'floor' away from the porch to the sidewalk to about 4'--filled the whole thing back with amended 'soil' including a lot of compost and grit.
So, effectively, I have created a giant bathtub on each side of the porch with well drained soil with no-where to drain to...
So far, everything I planted in there did fine last summer and seems to have made it thru the winter as well...time will tell.
When I planted some bulbs in there last fall, I noticed the amended area was in fact more moist than other areas, but it certainly wasn't soggy.
This may or may not work for me in the long run--but the one saving grace is that we don't get much annual precipitation around here. I figure as long as I don't over irrigate it, it will probably work out better for me than that muck/concrete crap I was dealing with before.

Do you have any before and after pictures of that?

>> Dug out to about 2 and 1/2' by the foundation and sloped the 'floor' away from the porch to the sidewalk to about 4'--filled the whole thing back with amended 'soil' including a lot of compost and grit.
So, effectively, I have created a giant bathtub on each side of the porch with well drained soil with no-where to drain to...

That sounds like a great bed. Deep enough for anything!

>> sloped the 'floor' away from the porch to the sidewalk to about 4'

Can't it drain to the sidewalk? If it does, the "runoff" over the sidewalk might be too shallow to notice, even during a heavy rain.

Or maybe there IS a slight amount of percolation down though the muck below 3 feet, and THAT is keeping the bathtubs from filling with water and staying full.

Or being 2.5 feet deep means that top 18 inches drain down and flood the bottom 12", and the roots don’t try to go deeper than 18".

>> When I planted some bulbs in there last fall,

Many bulbs need fairly well-drained soil. If they thrive and come back for a few years, you must have drainage somehow!

>> including a lot of compost and grit.

That is exactly what I would do. Given the budget and carrying capacity.

>> we don't get much annual precipitation around here.

AHHH HAH! Whatever your hardest rainstorm is, say one inch per hour times a few hours, check out how high the water table rose inside that bed, within 10-20 minutes of the rain stopping. Well, really, check how close the water table gets to the soil SURFACE. Then check again a few hours later, and you'll know how fast the whole bed perks (no matter WHERE the water is going TO.)
 

Arico said:Do you have any before and after pictures of that?

maybe...
not really that I took on purpose that will show a good before and after though
I'll see if I can find some...

Rick
yes--somehow, the bathtub must drain. I have never seen 'the water table'

>> 'the water table'

Where the soil is wet enough that there's effectively no air. That might not be the true definition of "water table", but that's what I was trying to say. The depth where slow drainage kills roots during hard rains.

Sorry--
This is not a great before pic--late summer 2014

but, you can see the junked out porch and steps there, and kinda see the area my dear, sweet BF dug out.
On each side of the steps are some large yuccas--I never really intended to go that deep but I did intend for the yuccas and the milkweed not to grow back in this spot so I wanted to get their extensive roots, and well, then, once you're already down there, what the heck


This was last spring--start of the hole

start of the dirt pile

start of the rock and other buried stuff pile

the hole

me in the hole


and I don't have any good 'after' pics but I'll get some this summer
here's some freshly planted babes

There were dahlias on one side last year, but I am not going to put them there this year

There's this rose now in place of the yuccas

and clematises and alstroemeria in there now

I love the "me in the hole" photo!

For a while I had a sunken-walkway-plus-dirt-pile going, and all the neighbors could see was my head,
sitting like a pumpkin on top of the dirt pile.