Putting in a terraced retaining wall, how wide does it need to be

jillyluis(6A)October 1, 2012


We are redoing our retaining wall, and making it terraced. The tentative plan is to have the top terrace be two feet wide (of dirt) and plant rhodies or mountain laurel there. I'm concerned though it may be too narrow. It doesn't matter if the plant grows wider, it just matters if the roots will tolerate it (and not ruin the wall)



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Neither plant is at all likely to damage the wall and 2' of width will be adequate for the foreseeable future - the root mass will slowly expand horizontally once the wall is encountered. One caution: mountain laurel and rhododendrons need a highly organic planting medium that will hold moisture but provide fast drainage and lots of air space. "Dirt" is unlikely to provide these essentials.

    Bookmark   October 2, 2012 at 5:07AM
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rhodyman(SE PA, USDA Z6)

The materials the wall is made of may be a problem. New landscaping timbers frequently exude preservative which is toxic to most plants including rhododendrons and mountain laurel. New masonry products made from concrete or with mortar exude lime which will make the soil near the wall too basic for rhododendrons and mountain laurel. Aged landscape timbers and aged masonry products without concrete or mortar will be much better.

    Bookmark   October 2, 2012 at 1:26PM
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Both super helpful replies, and things I had not thought about. I may post this follow up question under a separate heading.

I will plan on heavily amending the first six-12 inches with peat moss and compost.

The alkalinity may be a problem though. My soil pH tends to be 6.5 (although there previously were gigantic rhodies that prior owners removed, so I shouldn't think the soil is all that terrible for rhodies). The back of the top terrace, where the rhodies/mountain laurel likely would go, is pretty old 30-50 years old stone wall. The front of the terrace though is old stone that has been lying around, but they are using mortar in the back of the wall to help hold it together. Hmmmm. I read rhodyman's web page about sulfer and that it needs warm soil to acidify the soil. I'm guessing hollytone wouldn't cut it? I was hoping to put the plants in in a week and half. I live in Mass, and it is definitely getting colder, although the soil is still pretty warm.
So, should I scrap the project until I can check the pH next spring and or because the soil won't be all that warm for much longer? I was really excited about getting some plants in, in the hope of not looking at my neighbor's plastic covered wood pile all winter again.
Thanks ever so much

    Bookmark   October 2, 2012 at 8:38PM
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If the mortar has been weathering for years and years, it's unlikely to still be a problem. The existing soil may be, however. It's never a good idea to add acidifiers (and Hollytone is a fertilizer, not an acidifier) such as sulfur without a test.

I would also avoid peat moss. The brown powder sold by the zillions of bales is too fine, holds too much moisture until it dries out and then becomes very difficult to rewet. Partially decomposed conifer bark and evergreen needles are much better. I've also grown to like Nutrimulch, a product of New England Organics, very much, but see below.

All in all, you're probably better off if you wait until next spring to plant. If you use compost and/or Nutrimulch you'll be better off allowing it to settle and age over the winter. Planting rhododendons or mountain laurels at this time of year is like to stimulate growth which will be very unlikely to survive the wintet.

    Bookmark   October 3, 2012 at 5:31AM
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rhodyman(SE PA, USDA Z6)

The front wall where there will be fresh mortar can be a problem. However, I would use a couple plastic sheets between the fresh mortar and the soil. The plastic that is under the soil and under your mulch will last a long time since sun light never strikes it. Also, incorporate both sulfur and iron sulfate in your soil mix. The iron sulfate is faster acting and the sulfur is long lasting.

Always put in a gravel bed on the bottom of the bed before adding your soil mix. Good drainage is extremely important.

    Bookmark   October 4, 2012 at 10:12PM
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Rhodyman's advice about good drainage is absolutely correct, but a layer of gravel in a restricted area like this one can easily make drainage worse, not better. Just as it does in a container, the gravel decreases the depth of the soil. This means that the lowest, wettest layer of soil becomes closer to the surface and water remains there longer. Decades of gardening advice about using gravel, pot shards, and drainage layers in restricted spaces is just plain wrong. If drainage is a serious problem, perforated plastic pipe properly pitched can be used. Otherwise, you're better off skipping a layer of gravel.

    Bookmark   October 5, 2012 at 4:52AM
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rhodyman(SE PA, USDA Z6)

Rhododendrons are very shallow rooted. That is the reason that azalea pots are much shorter than other pots. I don't understand the concept of wettest layer of soil. The reason for the gravel is to allow the water to move laterally to find the hole in the pot. Water moves well downward, but has more trouble moving laterally. It also allows air to reach the roots which is very important.

We are not talking about a pot, we are talking about a border planting. The gravel layer allows the water to move to the area with the best drainage in case the drainage under the plant is not ideal. Hence the water can move forward, sideways, or backward until it finds better drainage.

Another advantage of the gravel layer, it isolates the pH of the subsoil from the pH in your border planting. This works especially well in the midwest were in many locations the soil is too sweet (low pH) for ericaceous plants. When the water reaches the gravel it reaches the gravel layer. This keep the soil from wicking alkaline water up near the roots since gravel does't act like a wick.

    Bookmark   October 5, 2012 at 2:41PM
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It's extremely counter-intuitive, but a layer of gravel or other coarse material does not improve drainage either in a pot or just below a planting mix in a bed. (At the subgrade base of a retaining wall, with or without drain pipe is another case entirely.) Water has considerable difficulty moving either vertically or horizontally from a mix composed of smaller particles to one with large particles and a much greater percentage of air spaces. The phenomenon is called "perching" and there are many technical explanations online as well as a good discussion of the issue on the container growing portion of Garden Web.

Even though rhododendrons are indeed shallow rooted plants, what can happen is this: With a gravel layer say 8 to 10 inches below the planting mix the lower few inches of that mix becomes saturated. Capillary action can cause the entire depth of planting mix to reach a saturated state before it begins to give up water to the gravel layer.

Shallow azalea pots ,btw, are not intended for long tem growth. They're a florist convention for quick growing azaleas having more to do with aesthetics, tradition, weight,etc.than with long term growth of the plant. That's why they are very rarely used by commercial rhododendron operations which need to grow on their plants for a number of years before they can be marketed.

    Bookmark   October 6, 2012 at 9:16AM
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rhodyman(SE PA, USDA Z6)

So before we line a pond with clay, we should put down a layer of gravel under the clay to insure the layer of clay is waterproof. Very counterintuitive.

Very interesting. There must be a wicking material that can be added to gravel layers to prevent this when gravel is used for drainage. It would be easy to experiment with using transparent containers.

    Bookmark   October 6, 2012 at 10:19AM
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Thank you so much for the super helpful discussion. The plastic sounds like a great idea, but I am worried it would interfere with the drainage. The wall has some drainage going through it. We could add add drainage pipe, but the drainage for that pipe would be complicated
Also, akamainegrower, I looked into nutrimulch and called the 3 biggest garden centers around and they don't stock it. I called the company too, but haven't heard back from them. Any idea where I can find it (I live north of Boston, and would love an excuse to go to a new garden center or go to Maine....but I'm not sure what my hubbie would think of coming back with a trunk full of mulch)
I guess what I am asking, any sense for how bad the new mortar on the front part of the wall will be for the pH, and for how long it may be a problem?
Thank you all so much,
this is tremendously helpful.

    Bookmark   October 6, 2012 at 9:44PM
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Jillyluis, in regard to Nutri-Mulch. Very recently New England Organics became Casella Organics. They have a toll free number: 888-472-9471.

A very cursory search turned up one source in N. Eastham, MA, BSG. Other suppliers may have their own compost and bark blends.

rhodyman: The clay used for pond lining - largely superceded by geotextiles and geomembranes today - is sodium bentonite which has unique properties due to its chemical and physical structure. What's under it matters little as long as its well compacted.

For an interesting look at Nutri-Mulch, go to the Coastal Maine Botanic Garden website. They use more than 500 cubic yards per season.

The most local source and the place I use blends their own under license from NEO or what used to be NEO. Good luck finding it.

    Bookmark   October 7, 2012 at 6:10AM
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rhodyman(SE PA, USDA Z6)

Jillyluis, just poke holes in the plastic for the drain holes.

Even better, if you can find perforated drain pipe and put that in the holes or leading to the holes, that would improve drainage.

    Bookmark   October 7, 2012 at 10:46PM
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