A pretty thick layer of gravel in the bottom should help matters tremendously. That would be alot of digging, though!

EG

You have made a bowl. I would send a french drain downhill into a rain garden or just anywhere out of the way. Your expense would be the time, the fabric to line the channel, and gravel. A PVC pipe is nice but not necessary.

Dan

The gravel won't help because water will just move laterally to fill the pores between the stones. Unless you have somewhere for the water to 'go', you're stuck with the 'bath-tub' effect. If you can insert a tile at the bottom of the depression that will drain downhill, you can remove the water. You could also install a collection crock and pump it out. Filling the disturbed depression with anything but native or finer soil will guarantee this bathtub effect, and even native soil will create the effect because it was disturbed. It would disappear once the soil has resettled & compacted to the same degree as surrounding soil though. (See note about soil biota and the bathtub effect later in the post)

For practical application, you have to consider that the supply of water that can run into the depression is unlimited because of the fact that water will move laterally, even through clay, from surrounding soil, so the volume of the depression, or the depth, in all but a minute number of cases is not a factor. You should consider that 'when it rains. it's going to fill.

You'd have been better off had you simply built the RB directly on top of the clay and allowed the downward movement of the water in the RB soil to be arrested by the (near) impermeable clay where it would flow laterally around the RB sides and over the surface of the clay where it would quickly evaporate or flow to a lower point.

Eventually though, soil biota will incorporate the OM from the RB soil into the clay substrata & (passively?) create the same bath-tub effect as the double-digging.

Al

I started typing about the same time Dan did, but I got called away for a few. Obviously, I agree with his view.

Al

I tried to make the briefest comment possible to frame the issue as being very clear and simple. Al's better, more thorough comment explicitly illustrates the issue, and highlighted a solution I should have included above.

I failed to do so for brevity's sake but here goes now:

would the material cost be cheaper to just start over someplace else, as opposed to the 2 yd gravel, 50 ft landscape fabric, fuel and delivery charge (if you don't have a pickup)? I think it is clear the time element is a significant cost, esp if you hurt your back [sez the guy who gave up his landscaping business because of the toll on his body].

Maybe you can keep that bed for plants that don't mind wet feet, disassemble the sides, move two feet over and have your Mulligan. I imagine the old bed, with the nice soil, would do very well for nice ornamentals that require full sun and moisture, and eventually the blooms will be a wonderful addition to the garden once the consternation goes away; maybe that's where the bench goes too, right next to that bed.

Dan

I removed alot of the soil and used it in a different beds last year. The "tub" went unused all last year because of problem and has settled almost to ground level.

Could I leave the existing soil undisturbed and build my Mulligan over the top of it?

Ive already moved the smaller bed but the larger bed is untreated cedar 4x4s lagged together.Then drilled, and rebarred into the ground the corners and twice on the long sides. Overkill I know but I wanted it to be permanant. It is 6'x 18'x 16", alot of work to put together and a pain to take apart.

Since the existing soil is at ground level wouldn't the majority of the water run out at that point or would wicking effect be a problem?

Thanks for the input :)

If I must move it to solve the problem I will :( Lesson learned!

Hi, Al and Dan:I would like to know how to build a French Drain. I have dug a 'bathtub' into my backyard clay, which is also in a low area that collects water. I had planned to put some blackberry plants here after filling it with compost and making a raised bed on top of the excavated clay.After reading this discussion, I think this may not work as the organic matter will decompose resulting in the ground settling and the plants being waterlogged.So I would like to build a French drain if is not too difficult. How can I get instructions for this? How deep does the drain have to be? Thanks

You'll need to have a spot to drain it TO, so it can flow away or be pumped away.

Dig a trench under your bed(s). It's helpful if the clay bottom of the bed(s) slopes toward the drain. Use a perforated PVC pipe under the beds set on top of a piece of landscape weed barrier or other durable material as wide as the trench. It's ok (better) if the cloth extends up the sides. Cover the pipe with several inches of gravel large enough to not pass into the pipe, or(if the gravel is smaller than the perforations, place a weed barrier or other durable material that allows water to freely pass through into the perforated pipe, on top of the pipe, THEN cover the pipe with gravel, then another layer of landscape cloth and backfill with your RB soil.

The pipe should have fall (so the water runs freely downhill). It should be canted toward an area where it will continue to flow away from the sight, or a collection sump where it can be pumped away. You can change from perforated to a continuous-walled pipe as soon as the bed ends.

Maybe Dan has something additional to add or suggest if he's still following the thread.

Al

Thanks, Al. Is this a special type of PVC pipe with perforations? I have not come across it(not that I have looked yet). Where can I get it? Thanks for the help. Paul

Any big box store should have it.

Hope that helps.

Al

If it were my tub and I had other beds to plant in, I would grow a cover crop of weeds and legumes in the tub. A year or more of fibrous roots going down and filling the clay w/ OM would help alot. You would come back to a much improved soil in that bed.

It won't help, RJ. It's not about improving the soil - that's the easy part. It's about getting rid of the water in the tub. You can't drain a bathtub by filling it with towels - it (the soil) would still be too anaerobic to support proper root function/metabolism. Air in the rhizosphere is extremely important to healthy roots and o/a plant vitality.

Al

Is building an above ground raised wooden box over that area at this point a viable solution? Would it be cheaper and less labor intensive than trying to install a french drain or pump, or more practical than relocate the planting area?

Hey Al! (My citrus in pots are still alive, thanks to your watering tips.)

Al and Dan, on a similar note, can I ask you about my new raised beds, also being built above clay soil? We don't have the "red clay", but here in Northern California/Bay Area we have a pretty dense grayish clay that squishes like wet pottery clay when you rub it between your fingers. I didn't double dig or anything that intensive, but I did amend the top 6-10" of the soil before we planted with a wood/chicken manure based amendment. I did it to break up the surface, improve the texture and drainage of that surface soil so that water wouldn't basically stand underneath the raised beds we're about to make. There was some construction on that site in the last couple years (heave bobcats, etc) compacting the soil and bringing in some less than desirable topsoil, concrete bits, etc.

So we sort of raked away the ugly bits, and amended the top 8-ish inches with a ratio of about 30-50% organic matter. Then we mounded the finished amended soil/clay into little hills (kind of looked like a cemetery after a burial) which means we didn't really remove much of the original soil. Maybe only a few cubic feet in total from each 4'x8' area. Even after only a day or two, the amended mounds are settling back down to grade.

On top of these mounds, I'm planning to build 18" high raised boxes. I didn't think that the digging might create a "basin" until I read this post. Should we be worried about this?

Al's response in particular, about how just simply the act of disturbing the clay soi creating the basin, now has me worried.

Is it a good thing that we didn't remove much of the native soil?

Thank, guys.
-kmama

Let's say you're starting with a raised bed (RB) with a highly organic RB soil on top of any type of undisturbed soil. Eventually, the texture of the soil below the raised bed will be changed. When the native soil is one that allows percolation or adequate drainage, there is no problem, but in clay soils, there is. Eventually, there will be greater porosity in the clay immediately under the beds to fill with water when it rains or when you irrigate. Even if the argument is made that this will take place over an extended period, we can still make the statement that disturbing the soil under the beds, and especially disturbing AND amending with organic matter IS going to create the bathtub effect.

You are better off, on clay soils, to build the bed on top of undisturbed earth. As water accumulates in the RB soil, it pushes down on the water in the soil at the bottom of the bed. If this water is impeded by the extremely slow percolation rate through the clay, it will flow laterally across the top of the clay as long as rain or irrigation continues; or it will move laterally by diffusion through the very top layer of soil. In both cases, because the surface area whetted by the water from the raised bed is very large, evaporation will occur and the capillary pull of the clay surface will continue to pull water from the beds.

To visualize this: Saturate a sponge (the sponge is your RB soil) - hold it so the largest surfaces are horizontal until it stops draining. Set it on a counter top (the near-impermeable clay layer). Note that no water flows away from the sponge. Now pour a Dixie cup of water on the sponge (this is rain or irrigation). See how water flows laterally across the counter top?

Do it again - hold the sponge horizontally until it no longer drains. This time, set it in the center of a dry paper towel (the top of the native clay soil). Note how the water flows laterally across the paper towel? If the paper towel was larger (like the huge area of soil surrounding the beds) it would pull all perched water from the sponge. Since the towel greatly increases the surface area that is wet, evaporation can occur at many, many times the rate it would w/o the towel (the surface of the native soil).

Turn your thoughts to amended soil under the bed. The water from the bed and surrounding undisturbed soil fills the amended bowl and the water has nowhere to go. It is not exposed to the air, so it cannot even evaporate. See where the problems lie?

Al

Hey Al, Thanks! I can always count on you for a very thorough answer.

So if you have a raised bed well above the "bathtub" below, is it still a problem? Does that water under the raised bed just never drain away... like does it get stagnant and start causing problems in your landcape? Or does it just perhaps make your veggie garden not perform as well. I do guess I'm asking, what's the fuss... if you have a raised bed. If you have your raised bed tall enough, won't the roots avoid the basin?

Also, if you're saying a bathtub effect is inevitable just by gardening, then how can anyone grow a raised bed above clay soil? That part isn't making sense to me.

Also, is it still a "bathtub effect" if the entire planting area (about 20' x 40') is rototilled to about the same depth? As I mentioned, we actually rototilled the entire area a couple years ago (at the time we we laid an adjoining sod lawn.) Then again last year before the rainy season, we rototilled with more organic material then laid a thick layer of redwood mulch. Last weekend, when we hand mixed these amendments into the area where the beds would be, we worked it in only into the top 6-8", probably no deeper than the rototilling.

And if I'm understanding you, no matter what we've done, we'll eventually create a "basin" effect by virtue of having clay soil? What happens then?

Thanks,
kristi

I am interested in this top because we just moved to NC and I have plenty of clay in the yard.
The place I want to put my raised bed is on a small slope. It drops about 8 - 10 inches over a 14 foot distance. I plan on making a level raised bed on this site. It seems to me that this would help with any excess water. But after reading through this thread I am very confused.

Kristi - Any time you pile organic matter (OM) on top of native soil, the structure of the native soil below the OM will be altered. How fast it is altered depends on the native soil. The loams will change fastest because they have the most life, then sand, and slowest is clay, but the structure WILL change to something different. Sands will become heavier and clays lighter. With this change to a lighter soil comes more porosity. The ultimate porosity is a depression - one big pore. Think then of a depression filled with rock or gravel - it will still fill with water in a clay soil. Sand? It will fill with water. Clay mixed with OM - it will fill. Even though you make no active effort to incorporate the OM, soil biota will eventually do it for you. It may very well happen so slowly that it's not going to be a problem. In clay loam with reasonable drainage, and all other soils more open than clay loam, it's probably a non-issue.

I guess my point is that on heavy clay soils, it's better to build on undisturbed soil than to incorporate OM into the soil below, UNLESS ... you have something in place that ensures water can be removed from the beds.

If you start with 1,000 sq ft of hard clay and dig a 4 x 8 depression and amend the soil, you have a 4 x 8 bowl. If you do it to a 20 x 40 area, you have a 20 x 40 bowl. Water still falls from the sky (don't take that as my being a s/a) and it still moves laterally through the soil to fill the bowl. How large an issue this is depends on the percolation rates of the native soil or your ability to drain the excess water away.

Snowman - Review the sponge illustration above. It's a very good illustration of how a raised bed behaves above hard clay. If you do have hard clay, and for long term applications, you only need to dig a shallow trench under the RB and fill with gravel next to the RB or install a short drain. Either should be directed so collected water can run downhill away from the RBs.

Al

This has been a very useful discussion. Thanks, Al. I had filled my clay 'tub' with compost and then it rained, turning it into a kind of watery jello.The berry plants would never tolerate that. So I will move my berry bed to another area of the yard that is near the boundary where we have a open storm drain.I plan to put a (hopefully short) french drain to the storm drain. I feel more optimistic now that I have a plan.
Paul

Ahhh - an actual illustration of what I've been conveying. I hope your plan works, Paul.

Al

OK, I have thought about the sponge analogy. It makes sense.

Now why would I need a french drain if I have a raised bed over the "bathtub".

Is it because the bathtub will retain the water below my raised bed and cause it to keep the "good soil" of the raised bed too wet?

If this is the case, short term a raised bed over undisturbed clay will not need a french drain because there is no bathtub affect yet.

Snowman

"Is it because the bathtub will retain the water below my raised bed and cause it to keep the "good soil" of the raised bed too wet?"

Yes. Like Paul noted above, a depression in clay will cause all the (lighter) soil in the depression to be soggy for as long as it takes for the water to percolate through the clay soil. Your next question might be, "Why can't I just build the beds deeper (over the depression)?" The answer is in the fact that how deep roots go isn't determined by the plant's natural habit. Plants that usually grow with their roots very close to the surface will send roots very deep, as long as there is sufficient aeration in the soil. It's almost always aeration that limits root depth. When the bowl is empty, roots will easily grow deep into the bowl and even into the clay below. The problem arises when you irrigate or when it rains. The bowl fills and after only a few hours of being deprived of O2, fine roots begin to die. As the length of time roots are in anaerobic conditions increases, incrementally larger roots die. Roots will regenerate after ample O2 finally returns to the soil, but the cyclic death/regeneration of these roots is very taxing to the plant and could kill it outright. It uses valuable energy that the plant might have spent on an increase in biomass (more foliage, fruits, .... better production).

"... over the short term a raised bed over undisturbed clay will not need a french drain because there is no bathtub affect yet."

True.

Al

Maybe Dan has something additional to add or suggest if he's still following the thread.

I'm traveling quite a bit this month and missed this interesting discussion.

The key to the issues in this thread, in my view, is that you want to be able to utilize the soil underneath your RBs.

Why? The soil underneath the RBs should be a water reservoir and a mineral source for plants. You don't want to shut it off from plant roots. If your RB is, say, 6" high and you have clay bathtubbing underneath, your effective rooting depth is 6" unless you get frequent saturating rains, then all that bathtubbing may wick up into your bed.

So double-digging is a great idea as long as you can take the moisture away. I have done hundreds of feet of French drains as a matter of course in my old landscaping business. All you need is 1" of fall over 15-20' and you can make it work. Of course, more fall is better. Briefly, your trench is the key. Take the energy and time and make a good trench. Borrow a trenching shovel, as it is built just for this task. Line the trench with landscape fabric, hopefully repurposed and it doesn't have to be perfect. Put an inch or so of pea gravel on the bottom of the lined, angled trench; this makes it easier to adjust any spots that don't drain as well. I prefer the perforated black corrugated pipe with a sleeve, as it makes it easier to bend. Test your drainage, fix, then cover with pea gravel and fold over extra fabric. Bury. You want to have enough soil and gravel on top so you know where the drain is when you dig, so you don't cut the line accidentally with a pick-shovel. You now have extra soil for either a raised landscaping berm or other purpose.

Making drains is easy, put the work takes time. But take the time, because this is one task you never want to do again, as fixing it is horrible. If you have more than, oh, 40 feet of trench to dig, spend the money and rent a power trencher. If you do this, start a thread and I'll walk you thru how to use one.

I suggest the terminus of the drain become a rain garden. This may solve some of the issues around what to do with the water.

Dan

My entire backyard fills with water after a rainfall. To fix the issue, I started by building a raised bed, to ensure that the roots were not sitting in water. The raised beds I have now are just six inches high, but the beds have very good drainage. Conversely, the area around the beds still gets standing water, but its a little better.

My plants do very well in the beds.

Eagles - if the percolation rate of the soil surrounding the bed is such that the standing water disappears within a couple of hrs, it probably didn't need fixing. If it doesn't disappear within a couple of hours, it's not fixed - it's just another raised bed, built in a depression, and the roots of the plants in that bed are suffering energy-sapping, cyclic root death and then regeneration. You may not see it, but it is definitely happening.

Al

If drainage is fixed, then couldn't landscape fabric keep the roots from growing too deep?

Landscape fabric keeps seedling plants from growing upward, but not roots from growing downward. Root hairs and the finest roots are microscopic. When they grow through the fabric, what happens to them? They either grow into the saturated soil zone below or are girdled by the fabric as they increase in diameter. In either case, the result is the same. The plant pays good energy to make roots that are killed and have chance to offer no return, or very little return on the plant's investment.

If percolation rates of native soils are fast enough to get water out of the saturated zone quickly, then incorporate OM into the native soil under the bed. If it is not, then use a drain system and incorporate OM into the native soil under the bed. If you will not be using any type of drain system, and the native soil's percolation rate is very slow (heavy clay)- then build your beds atop undisturbed native soil.

Al

wrt to my comment above, for clarification: landscape fabric is to keep soil from falling into pipe and mucking up the works.

Dan

I personally like the pipes with the fabric already around it like a sock. Saves a little bit of effort

I personally like the pipes with the fabric already around it like a sock. Saves a little bit of effort

Yup. And the movement and frost heave etc makes gravel rub against the sock, which is why I like to add the landscape fabric as well. After following behind substandard landscaping for years, I don't like to leave messes for the next person.

Dan

Good to know. I've only put in that type of drainage a couple times and it was just the tube with the sock and rock, but the fabric makes a lot of sense. Especially if you live in an area that has a deep frost line. I think maybe the difference is the few that I've done have been below the frost line which is much easier to do in zone 6 than zone 2. Also nice to see someone who's through with their work and doesn't cut corners.

Dan or Al:
What do I do with the ends of the perforated pipe? Should i cover them with fabric or use a pvc cap, or leave open?
Where can get rock/gravel in bulk. The gardening centers have 2 cubic ft. bags. I will need more than that I think.
Thanks, Paul

Since there should be a 'fall' built into the pipe, the high end can either remain open and covered with water-permeable fabrique to keep the soil out of the pipe, or you can cap it. The low end, of course, must remain open so water can flow away from the bed via the natural surrounding gradient or fall into a collection sump where it is removed by a pump.

Any landscape/excavation company will be able to provide you with crushed stone in a suitable size (by the yard).

Al

Thanks everyone, specially Al and Dan for the advice. I have completed my project by using a 25 foot long 4 inch perforated pipe with a fabric sheath. It came as a kit at my local big box garden store. I recommend the kit as it is really useful. I got gravel(coarse gravel, 1/2-1 inch pieces) at a landscaping company. I think it is going to work. The depth of the trench was 8 inches because that was all I could manage to dig by hand. I hope it is sufficient.It helped that the ground was softened by recent rains. I can't imagine how long this would take if the ground was dry. I had the local utilities come and mark the location of underground lines. I had to work near some lines but did not damage anything. Hope this is useful to anyone with the same problem. It took me about 2 afternoons.

Here is a link that might be useful: