building a raised flower bed in response to poor drainage

gee_ess(z6)August 24, 2007

I am new here and have spent the last half hour looking for help with this issue. I have found some help but not specific to my problem.

I have a 170' long flower bed that runs along the back edge of our backyard. This flower bed is sitting directly in front of a 7' tall retaining wall(concrete covered in stucco) that is holding back the hillside that slopes downward into our yard. There is a drainpipe along the back of the concrete retaining wall, but nothing in front of it.

My plan three years ago when we built this house was to plant several shrubs and flowering plants to disguise this wall but everything we planted has died (actually, it has drowned). The soil is solid clay and the yard is totally flat with no place for any water to drain. (picture a courtyard type backyard) Currently, the view from the back of my house looks like a prison yard.

So, I think my only answer is to build raised beds. My plan is to dig down about 8" in the current bed and lay gravel back to ground level, then build the beds 24" high. Do you think this will work? Would 24" be high enough for the root systems of azaleas and other shrubs like japonica?

Also, should I put a drainpipe in the bed on top of the gravel? And if so, shouldn't it be at a slight slope? At 170' I don't know if I can achieve that.

I would LOVE to get any ideas from you on ways to fix this. I live in a small town and landscape designers are nonexistent.

Then, there is the small problem of what to build the beds out of...

Thank you SO much

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Professionally speaking, drain pipes should have a minimum fall of 2 in. (50mm) in 10 ft. (3 m.) to avoid sediment clogs.
Our area is like yours on a smaller scale, I am following the ARS-endorsed guidelines for rose beds.

Here is a link that might be useful: Have more time to search for ideas?

    Bookmark   August 25, 2007 at 2:40PM
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Thank you for your response, Simplex!

I just don't see how I can put a drain in this bed -especially if it needs to have 2 inches per 10 feet. It might work if I could start at the center of the bed and drop the pipe from both sides. But, one side dead ends into our garage and I don't think that will work.

I am hoping that by digging down about 8" below existing soil level and adding gravel, that might help drain the bed.

Another thought is I could take the current 4 - 6" of mulch that is on the bed right now and till it into that soil in hopes that will improve drainage below the raised bed material. Everything I read on amending clay soil says to do something like this. I am just afraid that if I limit my "fix" by only amending the soil and don't go ahead and raise the bed, I will have only prolonged the inevitable.

    Bookmark   August 25, 2007 at 3:41PM
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mad_gallica(zone 5 - eastern New York)

Where are you?

That is the short answer to things like how much annual rainfall do you get, how is it distributed throughout the year, what type of soils are you likely to have, what types of solutions have worked in your area, etc.

In my area, what that solution would mostly get you is wet gravel, and many people asking why you just didn't plant swamp azaleas if you wanted to grow azaleas in a swamp.

    Bookmark   August 26, 2007 at 12:11AM
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karinl(BC Z8)

Unless you can drain your gravel layer to somewhere, there is no point in having such a layer.

What I suspect you need is drain holes out the sides of your raised beds, at the bottom of the walls. If you also have a gravel layer AT ground level in the raised bed, the water will be able to go somewhere - out of the bed at least.

What to build with - depends on taste, style of house and yard, budget, what work you can perform or hire, and availability. Any material can be made so it drains.


    Bookmark   August 26, 2007 at 1:57AM
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mad gallica - I am in southeast Missouri and the entire bed is clay. Very poor in nutirents and holds water for weeks. My DH is asking the same questions, "why aren't you just planting things that like to be wet?" But this 100+' wall is the THE focal point as you look out of our house, and I would really like it to be a good old fashioned bed filled with things that are interesting throughout the year.

karinl - I don't have unusual amounts of rainfall in this area so that is why I am thinking that I will build the bed up 24". That amount of depth would be able to handle a normal year of rainfall. (sorry, I don't know exactly what a normal year amount is, but we are not considered a rainy climate) The gravel base is probably overkill.
As far as materials go. Of course, I would love stacked stone, but that is going to cost an arm and a leg! I was wondering if the lnadscape experts knew if there is any new, cheaper material out there that I could substitute for that look - and not interlock blocks, too commercial looking for this project.

I think my main worry is that I keep reading that raised beds are the answer to everything and I just want to be forewarned if that is not true and I need to do something differently. All the raised bed experts seem to be growing veggies and not shrubs!

    Bookmark   August 26, 2007 at 10:36AM
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What is the soil like above the prison wall? Would it be possible to plant stuff up there to hang down?

    Bookmark   August 26, 2007 at 11:29AM
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low quality fill dirt was used to backfill against the wall...I have considered plantings that hang down as part of the overall plan but have not pursued it too far because I am obsessed with fixing the bed... But it is definitely part of the overall solutions. Any suggestions? The wall is in full sun all day so whatever I plant must be able to tolerate that situation.
One plant I want to try is a carolina jasmine that is supposed to be hardy in my zone called Margarita. But I would plant it in the bed and train it up to and across the wall.

    Bookmark   August 26, 2007 at 12:08PM
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saypoint(6b CT)

I'm not an expert on drainage, but if you laid a perforated pipe in the gravel and covered it with landscape fabric to keep soil from filling it in, before adding more gravel, even if it had very little slope it should carry some water away. You still need somewhere for it to go, however.

How deep an area do you have to work with? If it's deep enough, you may be able to build up an area with improved soil and lots of compost to create a berm in front of the wall, leaving the old bed behind it, and plant in the berm. If this is an option, read up on berms, and don't make it so high or so abruptly rising from the ground that it looks like a burial mound.

    Bookmark   August 26, 2007 at 9:27PM
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Thanks for the suggestion, saypoint, but I don't have a lot of depth to work with. Because we literally cut into a hillside to build this house, our backyard is only about 30 or 35 feet deep. I purposely made the beds at our house foundation and along the retaining wall at least 5' deep so the plantings would be full and lush (oh if only!)

I would still like to hear from someone who has built raised beds for shrubs/flowers instead of vegetables.

    Bookmark   August 26, 2007 at 10:14PM
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A raised bed will automatically improve drainage - that's just the nature of a raised planting area filled with imported soil. As long as there is a point of egress for accumulated moisture to drain through the beds, drainage will be far superior than a in-ground situation in the same location. There is no need for a layer of gravel at the base of the bed - introducing layers of different texture/structure actually slows water penetration and impedes good drainage.

A 24" height is sufficient for most shrubs and even a few small trees, but I'd make sure depth was sufficiently generous enough to accomodate the spread of more than just a single line of plants.

What you use to construct the bed is up to you, the look you intend and your budget - there's a whole bunch of things that can work. Just keep in mind that moisture will need to move through the material via joints, weep holes, etc. Use a good quality planting mix to fill. And if drainage in this area is so compromised, a border of several inches of gravel or drain rock 6-8" deep at the outer edge will help to keep the lawn or whatever from turning into a wading pool during the rainy season.

    Bookmark   August 27, 2007 at 11:02AM
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Thank you Gardengal!

Okay, let me understand this a little better. I shouldn't put an entire layer of gravel down but should have a gravel border at the front of my bed? Since the area in front of that bed already has the wading pool effect during rains, I love that suggestion as a potential fix. At the risk of sounding totally stupid, would that gravel be at the base of the planter?
I am hoping to do this raised bed out of stacked stone so I will have natural ways to move the moisture vs weep holes.
The deepest spot of the bed is about 8' and the narrowest is about 5'. Yes, I am one of those people who made the bed curvy. I just learned on this forum that makes me a novice! If the shoe fits...
Also, after reading tons on this forum and others, I think it can only help the soil if I till in the 5" or so of mulch on top of the current beds before building the walls and adding the new improved soil. Surely that will improve the clay base, and the drainage problem, somewhat at that level.

Thank you again for your help. I am now waiting for dollar numbers from the yard guy who will be doing this project.

    Bookmark   August 27, 2007 at 11:56AM
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saypoint(6b CT)

The short version of the drainage issue with gravel underneath the soil IN the planter is that the water will want to cling to the soil, but gravity makes it drain down. When the moisture level falls to the point where the forces of gravity that want to pull it down and the attraction of the water to the soil particles are equal, some quantity of moisture will remain in the bottom-most portion of soil. Adding gravel raises the level of the bottom of the soil, raising the wet part at the same time. No gravel means the wet part is lower in the planter.

If you were to put in a drainage pipe, you need to set it in gravel to keep the soil from clogging it. Gravel in a trench will also direct the flow of water because the spaces between the stones are larger (unless they fill up with soil and debris) and are the "path of least resistance" for the water.

There is nothing wrong with curvy beds if there is a reason to make it curvy, such as to make the style informal, to repeat curves elsewhere in the landscape, etc.
A common mistake is squiggly lines instead of broad sweeping curves.

    Bookmark   August 27, 2007 at 6:29PM
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saypoint - your gravel explanation is very interesting to me since my "landscaper" who originally put in this bed, dug holes for the shrubs and put gravel in the bottom of each so the shrub would drain better in the clay soil! If I understand your post, that only made it worse! I am SO frustrated!

I am going to take some pictures of this prison wall/flower bed and try to post them this evening...Hopefully you will think my curvy bed is not too bad! :)

    Bookmark   August 27, 2007 at 7:01PM
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    Bookmark   August 27, 2007 at 10:57PM
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saypoint(6b CT)

I would try to find out why the drainage that was put in behind the wall is not doing its job. Can you call the builder/contractor that installed it? The town inspector?

The problem with clay soil is that even if you amend the soil in the planting area, the hole in the clay will form a "bathtub" and hold water, if indeed there is that much water collecting there.

A raised bed that allows the water to escape will work, but it seems like a very expensive fix for this project. Stone or brick are the obvious choices because of the curves, but I'm not sure even those will look right against the wall.

Can you put in some drain pipes and have a couple of dry wells put in to drain the water into?

You bed is pretty squiggly. Sorry. You have a fair amount of space on either side of the paved area to widen the beds. Maybe you can use large free-standing containers in the narrow areas and widen and berm up the other areas?

Your wall is more attractive than I expected, the brick piers look good, and it really doesn't look like a prison yard to me. Have you considered growing a vine up the wall? It could look very nice.

    Bookmark   August 28, 2007 at 8:44AM
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Gravel or coarse sand added to an entire planting area can help to improve drainage because it introduces spaces in between the soil particles for water to penetrate, but it takes quite a lot to have much of an effect. And it needs to be fully incorporated into the soil to a significant depth, not applied as a layer. The accepted horticultural convention is that organic matter (i.e. compost) is preferred for this purpose, as its decomposition over time improves soil fertility and structure in addition to increasing the pore space. The incorporation of the gravel or compost into the entire planting area is the same theory behind not amending individual planting holes - amending creates situations of different soil structure adjacent to each other - this impedes root development as well as water penetration. It's called soil interface issues and if your "landscaper" is self-taught or long out of school, he may not be familiar with the both the theory and the current practices.

Establishing a border of drain rock at the front base of the wall where it meets the lawn (or whatever) should help to reduce the wading pool effect. It is a pretty common application here with heavy, poorly draining NW soils and if deep and wide enough, relatively effective. It can even have the appearance of a dry stream bed (or mostly dry) and become part of the garden aesthetic as well as being a highly functional application.

    Bookmark   August 28, 2007 at 9:03AM
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Gardengal - my landscaper is definitely both self taught and long out of school(local high school) so any technical know-how is learned from experience! He is coming by tomorrow to talk more on the subject and I look forward to updating the forum on my situation.

Thank you, all for the useful comments!

    Bookmark   August 29, 2007 at 10:07PM
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18 is good. 24 inches should be plenty but I would forgo the gravel. This can actually worsen your problem. If you don't mind putting in a little extra sweat equity you may be able to save some cash by tilling in some coarse sand and compost to amend the soil. It's A LOT cheaper when you get it by the truck load vs. the bags you buy at the store. The pieces are bigger too which really helps the drainage. It seems like a lot of work but it goes a lot faster than you would think. Another potential concern is the wall itself. In the photos it appears as if the sun is going down on that wall. One reason your plants may be having issues in that area is that hot afternoon sun reflecting off that wall onto your plants. Maybe an arborvitae backdrop would help shade that hot wall, take focus off of it, and make the area feel a little less linear. They will get tall enough, with no trees directly above to pretty much hide that red dirt hill above the wall too. Then you can plan the rest of your garden in front of that.

    Bookmark   April 25, 2008 at 2:09AM
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