Question on Raised Veggie Beds

memphistigerlily(7)April 15, 2008

I have been wanting to build a raised veggie bed in my backyard since last year, but haven't been able to talk my husband into it. We have a small starter home with a even smaller yard, so my husband was worried about taking up what little space we had back there.

Well thanks all you lovely ladies and gents at the swap and all the little babies you gave me of tomato, strawberry and pepper plants I have convienced him!!! He loves his jap peppers and is very excited for me to try grow some.

In the past I would always say something to him about the raised bed and he would blow it off or tell me no, and to keep using containers. Well last night I half jokinly said to him last night, we need to get starting building that raised bed and he said, Yeah I know we can try to do that this weekend!!!!!! :o)

So I had a few questions since this is going to be kind of new to me. I am building it against my fence line, so should I put a back on it, or you think I could just use the fence as the back of the bed? Also there is really not much grass back their now, they fence builders killed it and b/c of that and rain its stayed muddy the first 1-2 foot out. However I am sure in a couple of years grass would slowly start to build back. What is best to cover the ground with? I saw someone here say old newspapers. Then I remember from this weekend, John using rocks before the soil? Should I do that? Any suggestions for a first timer?

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brandon7 TN_zone(7)

I would not use the fence as the backing. Doing so will cause the wood (if it's wood) to rot much faster than the rest of the fence. Also, you might (for whatever reason) decide to move the bed or even take it out all together someday. Finally, the fence is not likely to be designed to be strong enough to hold the load. The dirt might cause the fence to lean or even fall over. If the fence has a warranty, using it as a backing would probably void the warranty.

How deep are you going to build the bed? You might consider making it about 18" tall so it would be at sitting level for ease of pulling weeds, etc. If you do make it 12" or more deep, there would definitely be no reason to put paper down. I can't think of ANY reason to put rock down. (Sometimes rock are put around the edges of block retaining walls for drainage.) There are a number of reasons NOT to put a layer of rock down. A layer of rock will make for poor drainage (it increases saturation drainage, but impedes normal drainage) and acts as a partial barrier for root growth for the plants in the bed. One idea I saw on Gardening By The Yard that sounded reasonable was to install galvanized mesh wire before adding soil if you were worried about borrowing animals.

There are three other things you might give some thought to.
1. If you decide to make the bed at sitting height, built-in seats might be helpful.
2. Be sure to secure the corners of your beds well. Nails will not be enough. There are a number of ways to accomplish this. Screws help some, but corner brackets (available at any hardware or building store) are probably better.
3. Consider adding braces for any sides over about 3 or 4 foot. The corner brackets (sometimes called L brackets) might work for this as well as for the corners.

I'm sure I have left something out, but this is all I can think of right at the moment.

    Bookmark   April 15, 2008 at 1:15PM
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I would put the newspaper in the bottom.But that is just me. Then the layer of large rocks like John did for extra drainage.
No I would not add it to the fence, the wetness would hurt the fence.
If you noticed, John had a brace in the middle of the bed, that helps support the weight of the soil in the wood.
I would make sure to use a good treated wood.That way you have a jump on the wood eaters.

I am so excited for you!

    Bookmark   April 15, 2008 at 1:53PM
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brandon7 TN_zone(7)

I just thought of something else....

Breaking up the surface of the ground (just a little, not necessarily a lot) before adding soil will help to break down the layer between the soil in the bed and the original soil surface to aid drainage. This is not a necessity, but takes very little effort and is probably worth it.


I was looking at the braces on John's beds in the pictures. It appeared that maybe it was only along the top. Was there a brace underneath that I couldn't see? The pressure against the sides will build up considerably from what it is now as the soil settles and especially when temperatures cause the soil to expand and contract. The sides of the bed will have to be braced well to hold up to that pressure.

As deep as John's beds are, hopefully the rocks won't cause a problem, but what they actually do is cause excess water to stay in the bed after it rains (the normal capillary action that is responsible for soil drainage is blocked by the rock layer) and encourage the beds to dry out worse during dry times (water can't be wicked up from below as would be the case without the rock). Rock can aid in saturation drainage (like during a flash flood), once the excess water makes its way down through the soil, but tends to keep the soil waterlogged after saturation level as been achieved.

    Bookmark   April 15, 2008 at 2:34PM
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great advice so far, but I thought I would toss in a reference to square-foot gardening since you're building raised garden beds. A simple 4'x4' garden bed, divided up in a SFG plan can give you the ability to plant orders of magnitudes more crops/flowers than you'd have normally been able to lay out in your standard, unorganized bed.

*image curtesy of the website...

    Bookmark   April 15, 2008 at 3:08PM
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Brandon, As this is a question for opionions.I will agree to disagree with you and stop posting on this thread.

Kim, I am still excited for you! Have fun!

    Bookmark   April 15, 2008 at 3:11PM
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brandon7 TN_zone(7)

Don't stop posting; tell us (or at least me) what you disagree about and why!

If it's about the drainage with the rock layer, that's soil science 101 in every basic college level soil-science textbook (unless there's something you see in my explaination that's not well written or wrong). If it's something else, I'd love to know so I can re-evaluate. We might or might not agree, but I always like to hear different opinions! That's why Gardenweb is so valuable, there are always people that know something that I don't or think of something that I didn't think of, and I try to learn from that.

    Bookmark   April 15, 2008 at 3:31PM
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I wouldn't recommend using the gravel as a base for the raised bed either, unless you were going for beds that would be over an existing slab of concrete on your patio or if you planned on making them deeper than 18". I've come to learn that the soil here, although very high in clay, drains very well and the gravel just isn't needed.

Also, should you ever design to scrap the raised bed you'll have a gravel pit where the raised bed once existed and will be fighting those stones for the rest of your natural life - or the new owner of the home will be after you've moved on...

One additional tip I'd like to offer, unrelated to the gravel debate, is to get the bed up and filled with your soil mixture as soon as possible and to let it sit & settle for a couple weeks before you start planting. Lightly watering it down every other day or so will help it settle down more quickly but you'll find that over the course of a couple weeks what filled the raised bed up to the brim only occupies between 50-66% of the same amount of space.

Tamping it down by hand, while providing instant gratification, only serves to compact the fresh soil and squeezes out the valuable oxygen within the mixture making it less fertile for your plants.

It's for this very reason why I'm trying to get my raised beds constructed at my new place by this weekend. I'm hoping to have everything "settled in" by this time next month so I can begin transplanting my seedlings out of my garage greenhouse...

    Bookmark   April 15, 2008 at 4:03PM
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Thanks for all the advice. I hadn't thought about using the back fence wearing it down faster. Good thing I have you guys to help me get started. :o)

That square foot gardening is rather interesting.
Thanks for all the advice. Didn't mean to start a debate but you know what they say about opinions.... we all got one :o)

Oh and I am totally excited!

    Bookmark   April 15, 2008 at 5:31PM
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brandon7 TN_zone(7)

Arjo brought up some good points. I hadn't thought about putting the bed on an impermeable or semi-impermeable surface like concrete. That would be a good reason to use a gravel layer.

    Bookmark   April 15, 2008 at 6:52PM
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I have raised vegetable beds and LOVE them! We have bermuda or some other tenacious grass that I cannot get rid of easily. And organic has been my choice for gardening overall, so raised beds help with weed control.

My husband and I first tried 6 inch beds, but that really wasn't enough for good root depth. So now we have built beds 12 inches deep for this year. Even so, we had a much better garden with the shallow raised beds than ones in the ground because we brought in organic soil, which was rock-free, weed-free, and was not compacted and was obviously not red clay, all of which helped a great deal.

The book I would recommend to you is called "Lasagna Gardening". It talks about the layers the author used for fabulous results with her raised beds, and all are very inexpensive or free. Just one of the suggestions is to use cardboard and/or newspaper (I'm going to use both this year) since it all breaks down into the soil and still helps with weed blocking. Then she talks about layers of other organic sources with which to layer. The book is a great resource and it's an enjoyable read as well.

I perused the SFG book and it seemed helpful. I did use a couple of tips insofar as how space is used and how much seed to plant based upon how many people are in your household. However, other than that, I'm not sure I got much more out of it.

Personally, I would skip the gravel as I don't think it adds significantly to the project. Positive results without backbreaking work are always a top goal in our gardens. Although, I could see how gravel might make the soil more friable if it was solid red clay and you were adding other items into the mix, such as sand, bone meal/blood meal, compost, and bagged soil. That's really too much work for my consideration, however!

Everyone, including this writer, seems to agree that the fence should not be used as support for the raised beds.

Good luck and keep us posted! :)

    Bookmark   April 17, 2008 at 1:16PM
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thnx for the advice and book suggestion! :o)

    Bookmark   April 18, 2008 at 2:42PM
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>As deep as John's beds are, hopefully the rocks won't cause a problem,
They should not.
>but what they actually do is cause excess water to stay in the bed after it rains
Indeed, I 100% agree, and that is exactly WHY they are there.

You will not that the beds are 28" tall. And they are filled with "Pro-Mix" which is a very sandy fast draining soil. It will most certainly dry out faster that if I had mixed in the native clay.

> (the normal capillary action that is responsible for soil drainage is blocked by the rock layer)
My theory was to move away any saturating rains quickly down to the bottom, so as not to rot the roots.
But still retain it and hopefully act as a "reservoir" that might be able to keep the sub-soil moist and hopefully the evaporation/capillary action will help us through periods of drought.

If the beds were less than 12" tall, and filled with the native clay, I would NOT utilize the gravel.

    Bookmark   April 18, 2008 at 3:42PM
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brandon7 TN_zone(7)

John, I hope you don't mind some questions. I would really like to know how you determined that the rock layer would be beneficial.

Are you saying that you think the gravel layer below the soil will increase the initial drainage rate? Don't you think that the soil's ability to drain water in excess of saturation level will be limited by the pore space of the soil in the bed rather than the rock layer's ability to divert the water once it filters through the soil in the bed?

Also, why would attempting to hold the soil (at least at the lower level) at saturation level (where air is pushed out and eliminated from the soil) be more beneficial than relying on moisture reserves present in the ground level soil (that should be available without the rock layer)? Have you determined that the soil moisture tension in a bed of this height would not be sufficient to effectively draw sufficient moisture from the ground to root level? I have a raised bed in the back of my house that's about 6' above ground level (part of a retaining wall), and I used a very well draining soil mix (looked closer to potting soil than garden dirt). I rarely (last year excluded) have to do any supplemental watering even in that bed.

Did you consider using moisture-retentive polymers in the beds?

It seems to me, at least without further information, that the rock layer would only tend to exaggerate the overly moist / drought extremes.

    Bookmark   April 18, 2008 at 7:58PM
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brandon7 TN_zone(7)

Memphistigerlily, I hope you don't mind us taking your thread so far in one direction. I guess it is related info, kinda.

    Bookmark   April 18, 2008 at 8:06PM
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I find the whole topic very intellectually stimulating. I never thought water drainage and capillary action could be so intriguing! LOL There are definitely many ways to garden and I value every single person's contribution. The free exchange of varying ideas makes us all better gardeners because we have an increased repertoire of knowledge from which to draw. Gardening ideas build on experiences and opinions and science. I dig it (pun intended)! :-)

    Bookmark   April 18, 2008 at 9:54PM
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katiedidcottage(z7 TN / Chattanooga)

IMHO I think that the gravel could be a good idea on the type of soil we had in the suburbs of Memphis in northern Mississippi (Olive Branch) for the raised beds. We did not try the gravel in our beds and they seemed to drain well, I assume because they were above the ground.

Just for information purposes, we also had dug large holes to plant trees which rotted the roots because the water would not drain any at all because the clay there is totally non-pourous. We had to dig shallow holes, put lots of gravel (about 3 inches) and then spread the roots out covering with good topsoil and covering the whole thing with mulch to have any success at all with the trees. They seemed to thrive this way. I spoke to a landscaper at a lawn and garden show in Memphis at the Agricenter one year who told us to plant them this way with the gravel in a shallow hole and sort of halfway above the ground. He said that's the way they planted everything.

    Bookmark   April 22, 2008 at 8:17AM
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brandon7 TN_zone(7)

Related to the drainage conversation:

If you dig a hole in an impermeable or semi-impermeable soil (like some clay) on level ground and fill it with well draining soil, you have a bathtub full of mud after the first rain unless you dig the hole deep enough to go completely through the layer of soil that doesn't drain well. Rock at the bottom of such a hole will do nothing to help and could (in the case of semi-impermeable soil) make it just a little worse. A lot of homeowners try this with disasterous results. The only way to really solve the problem (other than digging all the way through the poor draining soil layer) is to use a raised bed or berm.

You can replace poorly draining soil with well draining soil on a slope (in other words make a pocket into the side of the slope and fill it with well draining soil). The soil can drain to the side. In this case a layer of gravel might help to chanel the moisture out to the side of the pocket.

    Bookmark   April 22, 2008 at 1:40PM
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(quote)I spoke to a landscaper at a lawn and garden show in Memphis at the Agricenter one year who told us to plant them this way with the gravel in a shallow hole and sort of halfway above the ground. (quote)

I would HAVE to follow his advise as hew lives here, and makes a living with it.

    Bookmark   April 23, 2008 at 10:37AM
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brandon7 TN_zone(7)

Or you could get a second or third opinion if you have any reason to doubt what he recommends. Many landscapers just recommend what they've heard. Many in the nursery/landscape business commonly give bad advice. Most of the workers have little to no horticultural education and spend very little to no time researching techniques or information. I read posts on Gardenweb every time I log on about how some "professional" told the poster something that turned out not to be correct. The tree forum is a good example. I can't even count the times I have read where the nursery recommended leaving the protective cover, and even the metal basket in some cases, in the hole when planting balled and burlapped trees. Even landscape installers commonly do this. Then the homeowner is stuck with a stunted or dead trees a few years later.

My experience has been that the best information is information obtained from multiple sources. Extension service publications are also usually extremely helpful. Another valuable source is college textbooks (used ones work just fine, but newer editions are likely to have more up-to-date info). I have found a few really smart people in the nursery business, but they are unfortunately the exceptions. If one source is wrong, you can usually determine that with a few second opinions. So far as drainage issues go, the next time you are at the library, check out a book on horticultural soil science. I would bet that after you have read it, you'll know way more about the subject than 9 out of 10 nursery "professionals".

    Bookmark   April 23, 2008 at 2:57PM
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decolady01(7a/6b AL/TN)

My spouse built 10 raised beds for me. They are 4'x8'x1', filled with creekbottom soil. No gravel underneath. This will be the 3rd year for 2 of them and the 2nd year for the other 8. We put them down on top of the clover and Bermuda grass. The grass has not grown up through the beds. We have no gravel underneath and the drainage is the way it ought to be. No standing water in the beds, and no water just running through. This is working very well for us. Oh, and I grow organically.


    Bookmark   April 23, 2008 at 5:50PM
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