Erosion control options on big south slope

gardenweb88(9)April 8, 2013

I live in sunset zone 20 and have this huge south facing slope in my backyard that needs some erosion control. Years ago, we had some tall growing grass that the fire department asked us to significantly cut back every year since we let it grow wild. A regrettable decision was made to just remove all the grass rather than deal with frequent cuttings and regular visit from the LA fire department. What's remaining is some patchy grass and some shrubs that randomly sprouted. The soil is pretty dry and rocky.

The slop runs across the backyards of quite a few houses and all houses seem to be having erosion problems. Some neighbors have ice plants growing, but that hasn't helped much. The shifting soil, with ice plants attached, just swallowed up their chainlink fence and flattened the fence.

I've checked out the other posts on suggested plants for erosion control. And it seems wild lilac might be a good choice. Although, I'd have to wait until fall to plant as suggested by websites to let the winter rains help the plant take root.

Any thoughts?

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wcgypsy(10 / Sunset 23)

I'd vote for various varieties of cistus and native salvias.some of the ones that are good spreaders....Bees' Bliss, etc....toyon, rhus, various natives...

    Bookmark   April 8, 2013 at 2:37AM
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Suzi AKA DesertDance Zone 9b

We are in the same boat (without the fire marshal). We live on the edge of wilderness, and fire is always a fear.

I did some research, and this is what I found:

Good Groundcover and fire break: Iva hayesiana Hayes Iva has real merit where you have alkalinity or sodium problems. No flowers to speak of, but a nice green groundcover needing little water in places like Bakersfield, Taft or Palm Springs is something to consider. Iva also tolerates drip irrigation well. Mix with Salvia Pozo Blue or Gracias, Desert Mallow or Hummingbird Sage for more color and contrast.

They do plant it on freeway slopes. We are considering it, but have a lot of clearing to do first. A previous owner planted a lot of junipers (which are massive and I hate them), and a lot of dwarf creeping rosemary, which I love!!

We have 1.4 acres to cover, so variety is nice.


    Bookmark   April 8, 2013 at 10:10AM
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wcgypsy(10 / Sunset 23)

Salvia repens is good for erosion control...also, cistus is used in fire-susceptible areas.....

    Bookmark   April 8, 2013 at 11:13AM
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nil13(z21 L.A., CA (Mt. Washington))

If you are in a high fire severity zone it doesn't really matter what you plant, the fire department is going to make you perform some sort of fire abatement. The problem wasn't the grass. It was the "letting it go wild". Had you watered the grass so it was nice and green and maintained (depending on which fire zone) they would have not bothered you. At least they don't bother me. If you plant natives and don't irrigate in a fire zone (all hillsides in LA are fire zones), you will still have to perform brush clearance where you prune up the bottom of the shrubs and clear out weeds every year. The nice thing about succulents is that they are fre resistant and a little water keeps the landscape 'maintained' as far as the fire department is concerned. However, they don't do a whole lot for erosion control compared to shrubs and grasses. I use vetiver grass for erosion control all the time, but you have to give it some water to keep the FD off your back.

I would guess your best bet would be something that stays really low so you can't prune it up like they want for brush clearance. I would look at Arctostaphylos 'emerald carpet', Baccaris 'pigeon point', and Ceanothus 'Yankee point'. I would also install an irrigation line at the bottom of the slope with the roters pointed up the hill. I use MP Rotators for hillsides. They are low precipitation rate rotors that were designed for hillsides. I've run them on 1:1 slopes for over an hour without runoff. With the crappy rain season we had, you are going to need supplemental water to establish damn near anything this year especially since the season looks over.

    Bookmark   April 8, 2013 at 12:05PM
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calistoga_al ca 15 usda 9

You have been given several good suggestions, but before you make a choice, clear it with the fire people. You should not even think of planting until the fall and the start of the rainy season. Al

    Bookmark   April 8, 2013 at 12:56PM
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nil13(z21 L.A., CA (Mt. Washington))

What is the slope like? Is it deep fill or shallow degraded soil on bedrock? What is the structure (clay/silt/sand)? What is the slope (2 horizontal : 1 vertical or steeper)? where does the erosion go when it happens (concrete v-channel, back patio, over a retaining wall, etc)?

One earthwork that I like to use to control erosion are small swales. Really on steep hillsides it's more like a furrow. Basically, starting at the top, I take a pick and cut a furrow that descends slowly across the hill (you want the water to flow easily but slowly). Then I turn around and go back the other way. I continue cutting zigzag furrows like this until I get to the bottom of the slope(you can also go up the hill starting from where you want the flow to exit). The idea is that the water is going to run down the hill and the traditional sheet grading that works on a fairly flat site fails quickly on steep slopes. The water moves too fast down a steep slope and no matter how perfectly the grading is performed, cultivation and differential settlement will begin to make depressions that concentrate water and that turns into a positive feedback loop that creates gullys. Since you know that the water will do that, it is better to just make an easily maintained pathway, in this case the furrow, for the water to make it down the slope. The added benefit of slowing the water down in this way is that there is more time for the water to be absorbed by the soil thus decreasing the need for irrigation.

    Bookmark   April 8, 2013 at 4:04PM
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hoovb zone 9 sunset 23

Yikes. You don't want whatever is above sliding down into your home. It is shameful what developers are allowed to get away with when it comes to grading.

The grass was probably fine as someone said, it was the lack of maintenance that was the issue, and that slope is so steep there is no way to maintain it easily. I'd build some small terraces that act as safe pathways so the plantings can be maintained. Anything you might plant will eventually need cutting back or removal of dead material. You need a way to safely access it.

You could actually make yourself a spectacular garden on that slope, but you need terraces to do that.

    Bookmark   April 9, 2013 at 12:31AM
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nil13(z21 L.A., CA (Mt. Washington))

You don't need terraces to have a nice garden on that slope. They would be useful but not necessary. Also, if you are in the city of LA and under hillside grading, legally building the required retaining walls will be expensive. The first problem is that no permt required slough walls, what people normally call landscape walls, can only be 4foot from the bottom of the foundation to the top and they can't be surcharged. That means that the area being retained has to be flat for 5feet behind the wall, no slopes until that. The second problem is that you have to found the wall in stable soil. Most of the fill in the hills is uncertified and therefore unbuildable without certified compaction and even then it might not be possible. Or you get a situation where bedrock is 2feet down and you need to key into that so your slough wall can only legally peak out about 1.5feet, which is not worth it. Then if you decide to build a permittable retaining wall, the retaining wall ordinance only allows for 2 retaining walls total with a macimum height (I would have to check the ordinance to be sure) of 8feet each or a single wall of 12 feet. So three walls of 5feet is right out. the LA Building Department is a train wreck. This is why I was asking about the soil conditions and the steepness of the slope.

If the slope of the hill is 1:1, there aren't many options. If it is less than that, you can make grading adjustments that will allow access, like the zigzag furrows I mentioned previously which could also be viewed as switchbacks.

    Bookmark   April 9, 2013 at 11:37AM
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nil13(z21 L.A., CA (Mt. Washington))

If you are in the city of LA, I recommend going to ZIMAS online and searching your address. It will give you a lot of valuable information like fire zone, liquifaction/landslide info, and whether you are subject to hillside grading.

    Bookmark   April 9, 2013 at 11:42AM
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My house is actually at the top of the slope. i'd say the slope is maybe even steeper than 1:1).

I'm not too familiar with what type of soil structure it is. The dirt is a pale tan color that looks like the loose, crumbly dirt you see in the desert. So, I guess degraded soil?...

Parts of the hillside have bedrock just an inch or two below the soil surface. The dirt is loose enough that we have a groundhog problem. We did call an exterminator a few years in a row and that's helped take care of the groundhog problem considerably.

There's no built in erosion control channel at the moment. The rainwater simply etches out vertical channels for itself over a few seasons that we have the gardener fill in every so often. Down past the chain link fence is a sidewalk and much of the eroded dirt just ends up on the sidewalk.

This post was edited by gardenweb88 on Tue, Apr 16, 13 at 20:31

    Bookmark   April 16, 2013 at 8:27PM
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hoovb zone 9 sunset 23

Gophers I think you mean, or ground squirrels. Groundhogs are not common in Southern California.

I wasn't thinking of major terraces, nil. I was thinking of foothold size for safety's sake. Hard to have anything nice if you can't maintain it.

I supposed someone could rope themselves down that incline from anchors up top and do maintenance that way.

    Bookmark   April 17, 2013 at 4:11PM
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nil13(z21 L.A., CA (Mt. Washington))

If it's steeper than 1:1, the only safe way to maintain it would be on rope. Also if it's steeper than 1:1, the OP is a bit SOL. 1:1 is the maximum stable slope that sort of sandy/DG stuff will hold, so toe holds, furrows, and planting holes will all settle with the slightest disturbance. At that point the best choice would probably be ground covers like the ones mentioned that can handle that sort of brutal location and need very little pruning/fire clearance. I have a similar soil depth in places. The best way to plant those locations is to bareroot the plants and plant them in a really wide and shallow (you don't really have any choice when you are that close to bedrock) planting hole (no amendments only topdressing), making sure that when the uphill soil settles it doesn't cover the root flare. Of course, I would wait until the Fall and pray for some rain.

But you're right, you definitely need a way to maintain slopes like that. You just have to be careful of where you concentrate water flow when you cut little foot hold terraces for access.

    Bookmark   April 17, 2013 at 5:46PM
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nil13(z21 L.A., CA (Mt. Washington))

oh and
@$&^@#(!#(* gophers!!!!!!

    Bookmark   April 17, 2013 at 5:47PM
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You're right; they're ground squirrels. We got a letter a few months ago from LA County saying the pesticide they were using has been banned from the market, which leaves them with a less effective killing agent.

    Bookmark   April 17, 2013 at 8:54PM
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napapen(ca 15)

I vote for swales also. Across the slope. They will collect rainwater and contain it in the swale. It will seep into the soil and you will have a source of the natives you can plant on the edges of the swales


    Bookmark   April 19, 2013 at 6:04PM
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The recent rains the past two days have caused more erosion on the steep slope. Standing on the a concrete slab in my backyard I can see that the erosion is starting to eat away at the dirt underneath the concrete slab I'm standing on.

Can anyone provide a ballpark estimate of how much money it will cost to remedy the situation and how long it will take? It seems like there's an immediate need to add more dirt to replace the dirt that's been lost underneath the concrete slab.

I keep reading that it's best to wait until fall to plant erosion control trees, but I'm not comfortable waiting that long to do something about this situation. I'm tempted to go out and buy some fast growing ground cover like California Grape and plant throughout as an immediate start. Would that just be a futile waste of time and money? (The though of a field of unattended wild growing grapes rotting in the sun and attracting rodents and pests isn't appealing though).

Also, reading that erosion nets are cost effective options.

This post was edited by gardenweb88 on Wed, May 8, 13 at 4:32

    Bookmark   May 8, 2013 at 3:43AM
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nil13(z21 L.A., CA (Mt. Washington))

we probably won't have anymore rain until fall, so it's not that pressing. you need to get the grading worked out before you go planting stuff. Read what I wrote earlier about steep slopes and concentration or diffusion of the flow and what happens over time. A nice thick layer of mulch will help with erosion issues tremendously and help keep weeds down. You can plant at this time of year but you will have to irrigate the new planting through the summer.

California grape is a terrible choice if you want drought tolerant as it is native to the moist foothills of oregon to central california. Just because a plant is native doesn't mean it is appropriate to a specific microclimate. You have been given plenty of appropriate choices upthread.

With regards to the concrete slab, it sounds like water sheets off of it towards the slope. If this is the case, you will always be battling undercut erosion unless you underpin the slab with concrete that goes down a couple of feet or preferably into bedrock. This is simply a matter of how water flows over things and not having the ability to have enough soil to prevent exposing the underside of the slab.

    Bookmark   May 8, 2013 at 1:49PM
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Here's a picture of the slope from the top. This picture doesn't show the concrete slab where rain runs off and undercuts the erosion. But, you can see the exposed concrete where the fence poles were embedded.

I don't know if it's too steep for swales or furls. The slope is alot steeper than some of the pictures i found of slopes that have utilized furls or swales.

    Bookmark   May 8, 2013 at 7:54PM
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nil13(z21 L.A., CA (Mt. Washington))

The same thing that is happening to your slab is happening to those post foundations. The post foundations need to be covered with mulch so that you cannot see concrete and maintained that way to stop that. Bringing the soil level up a bit wouldn't hurt either.

That slope looks like 1:1 from this camera angle to me. Without knowing exactly what type of soil you have but being familar with soils in the area I would say it's too steep to do anything but mulch heavily and plant. I would fix the eroded bits by tossing eroded soil from the bottom of the hill up the hill to fill them in. Make the slope as evenly 'flat' across the slope as possible to avoid concentrating water as much as possible. It actually doesn't look that bad. Then I would lay at least six inches of mulch on it.

    Bookmark   May 9, 2013 at 11:22AM
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Do you recommend any particular type/ composition of mulch for maximum benefit (erosion control mix or some special blend)? I'm concerned about mulch attracting pests which would make their way into our home. Would synthetic mulch be acceptable?

Would you recommend an erosion netting be layed out before the layer of mulch?

Our backyard and the sloping hillside are pockmarked with ground squirrel holes. We've been having exterminators come out every few years to take care of the areas in our backyard on level surface, but they've always refused to service the hillside because of the grade. I'm guessing we need to call in a specialist exterminator for the hillside holes prior to layering the mulch.

This post was edited by gardenweb88 on Fri, May 10, 13 at 0:43

    Bookmark   May 9, 2013 at 2:57PM
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nil13(z21 L.A., CA (Mt. Washington))

I would just use arborist wood chips from a tree service. That should be the least expensive. Which pests do you imagine a little mulch is going to attract? Personally, I wouldn't worry about it. Fears of rats in the ivy and such are typically overblown. No food source no pests. Synthetic mulch is not acceptable at all and is probably prohibitively expensive. If you mulch, there is no need for erosion netting.

It would be best to perform some ground squirrel control first. I'd just call around and explain the situation. It will be a constant battle though

    Bookmark   May 10, 2013 at 11:20AM
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evilscott(USDA=10, Sunset=24)

Some plants to consider:

Baccharis pilularis, Ceanothus purpureus, Ceanothus 'Blue Jeans', Cupressus, Arctostaphylos, Rhus,

    Bookmark   May 13, 2013 at 3:41PM
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I've had a slope slightly steeper than yours in the back of the property 150' wide also facing south. I'd had numerous plants including fig tree that was well established into the sandy/rocky soil. Starting anew, i'd recommend something that spreads fast, extremely hardy and has a strong root system. rosemary the prostratus variety would be highly recommended for this type of setting from experience.

good luck.

    Bookmark   May 13, 2013 at 6:17PM
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nlin0273(z9-10 CA)

I almost want to say creeping thyme, also.

    Bookmark   May 14, 2013 at 6:59AM
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nil13(z21 L.A., CA (Mt. Washington))

Creeping thyme will require a bit of supplemental irrigation on a south facing slope in z20 with fast draining soil.

    Bookmark   May 14, 2013 at 11:21AM
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"Long term studies have shown a well designed hillside garden planted in native plants has no measurable erosion. California native plants provide a low tech solution to slope management."

Check this out:

    Bookmark   May 16, 2013 at 9:18PM
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ljrd3(z9 / 10 CA)

i just had about a dozen small (1 gal max) CA native shrubs/ground covers installed on a small but steep south facing slope. There are just a few - so I can hand water, but how often? Thanks.

    Bookmark   January 6, 2014 at 6:42PM
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South Orange County is planted in acacia rodolens. It is used some in Temecula also. Common vines when used on slopes will tend to hug the ground. Light ground covers such as myaporum and small leafed iceplant are being used. All will need some irrigation to keep it alive in the summer.

The key is to have cover that will slow the rain drops and a build up of mulch on the ground that will slow the run-off. Still, gravity will win in the end.

    Bookmark   January 7, 2014 at 12:12AM
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For the first year treat these as potted plants untill the roots grow into the surrounding soil. The water must go directly into the potting soil that came with the plant.

Use 12th of a gallon per day if the temperature is in the 70"s. Use 1/6th of a gallon per day if the temperature is in the 80's. Use 1/4 of a gallon per day if the temperature is in the 90's. Delay one day for each 1/4 inch of natural rain. The second year use the same total amounts, however, you can begin to water less frequently using proportionally more each time and also watering a greater area around the plants.

    Bookmark   January 7, 2014 at 12:25AM
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Evelyn256(Zone 9a SoCal)

My backyard is a dry, rocky slope and no where near as large as yours, but I was able to plant a garden there. It only had two pine trees on it when we moved in. I chopped out the rock and managed to set some stepping stones in middle and on sides, so I could navigate to plant. I put mulch and water via sprinklers. The mulch keeps sliding down, but I try to encourage it to stay. Here's an After shot. I'll see if I can find a Before...

    Bookmark   January 7, 2014 at 12:52PM
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Evelyn256(Zone 9a SoCal)

Here's a Before picture...

    Bookmark   January 7, 2014 at 12:53PM
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Evelyn256(Zone 9a SoCal)

Another view. The Calif. Pepper Tree seen quite small on right in Before picture grew quite fast. It's huge now after just two years and provides some shade for the plants.

    Bookmark   January 7, 2014 at 1:00PM
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Quite beautiful. As time goes on, you will find both the pepper and the pines killing out other plants with competion for sun and root space, and with compost drop that will favor the trees.

I would suggest letting the trees have their space and letting the lesser plants change over time.

    Bookmark   January 7, 2014 at 11:34PM
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ljrd3(z9 / 10 CA)

Steve - Thanks For the advice on amount of water - so water daily for the first year (except when it rains?)

    Bookmark   January 8, 2014 at 1:38AM
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ljrd3(z9 / 10 CA)

sorry - re-read the your post re: watering new native plants - see the per day instructions - duh! thanks again.

    Bookmark   January 13, 2014 at 10:39AM
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