Correcting Erosion on a Steep Bank in Encino

Question from Denise:
Our hillside, steep and large area was covered in ivy and was just fine and holding. We are not sure what happened, but due to either improper draining of the pool, a broken pipe, or years of slow erosion, the last years caused the lower part of the hillside to totally erode, in many areas becoming concave and horrible. We are currently regrading, putting on biodegradable jute, and some supports, but would like your advice as to which drought resistant deep rooted plants to replace the ivy with. We don’t want much color, and just something clean and rugged that will hold on to the earth. If you need I can forward you some photos. Your website is fantastic and very informative. I enjoy reading it. Thank you.

Answer from Pat:
Erosion of steep slopes is a serious and pervasive problem of gardens and landscapes throughout Southern California. Bank stabilization can be complicated and technical, so I am glad to hear you are getting professional advice and labor to correct the problem. Sometimes the problem is slow erosion or fast erosion and other times whole landslides can be caused by several factors, including earthquakes, improper grading of the original housing scheme, soil types, and/or water damage from rain, leaking irrigation pipes, or swimming pools. In many cases, builders bulldozed out platforms of land for houses on the sides of hills, and simply dumped the fill dirt down the edge of the hill to make the slope down to the next platform. If the fill dirt was not sufficiently compacted as it was placed on the slope, it may be an accident waiting to happen. Even when proper compaction was done, problems may arise years later in those cases where builders have cut through the various natural horizons of soil and then mixed the soils to make fill for banks. The various soils that were mixed may slip against each other or they may slowly wear away since they shed water. Correction requires professional help.

Many times a lighter, perhaps sandy top layer of soil overlaps a hard clay or rock strata beneath. This may not be the problem in your case, but when the light soil gets wet, water may saturate this top layer and hit the clay or rock layer below making it slippery. Then the upper layer of fill can just slide down to the bulldozed platform below, just like a person on a slide into a pool. Plants, like your Algerian ivy, may grow on the top soft layer and and hold that together for many years, but Algerian ivy roots are not deep, so the chances are their roots never penetrated the lower layer. Then if someone comes along and cuts into the bottom or toe of the hill, the whole thing will slide down, plants and all. Or simply from rain this can happen, or the result of an earthquake. I have seen and heard of this happening countless times. Natural landslides often happen also, and especially when someone cuts through the toe of an ancient landslide so the support at the bottom is no longer there. No plant can prevent this from occurring. The solution in some cases is to drill holes sideways through the various layers of soil on the bank and fill the holes with reinforced concrete so that the top slipping layer is anchored to the solid layer of heavy clay or rock behind it.

But from what you describe I’m not sure if a landslide occurred. In your case, slippage of land may not be involved but what might be a factor is very dry soil that simply washes off when it gets wet instead of allowing the water to penetrate. Alkaline soils also tend to shed water and most western soils are alkaline. Sandy and clay soils that dry out completely often shed water. Then when irrigation or rain hits the surface it cannot penetrate, it simply erodes particles off the surface. Solutions include soil penetrants and soil amendments, mulches, and jute. Terraces also help correct these problems, since level ground absorbs water better than sloping ground where water simply runs off. Additional techniques for correcting this problem include building water basins for plants and using low-volume, pulsing irrigation systems or drip systems designed to penetrate and not to run off the ground.

As I have said, no plant will cure the problem, but once you have had professional help to properly rebuild, stabilize, amend and revise the irrigation system for this bank, I would definitely suggest you plant the slope with Acacia redolens. You have said you want something drought resistant, easy, not particularly colorful, but sturdy for full sun. Acacia redolens fits these requirements and will do a good job covering and holding soil on a bank such as you have described. Acacia redolens has been used by CalTrans for many years to hold banks made of fill dirt and has withstood the test of time. Though the original plant grew to approximately six feet tall, I suggest you get ‘Low Boy’ or ‘Desert Carpet’ since these newer varieties grow only to about 2 feet tall. These plants bloom with yellow pompom flowers in late winter, but you don’t plant this for color. The foliage is attractive grayish green and is largely pest and disease free. ‘Desert Carpet’ is said to take a little longer to cover but is meanwhile putting out deep roots. Both plants are extremely drought-resistant once established. Both take heat and need full sun. Another possibility is to use a landscape block to stabilize the bottom section of your bank or even to terrace above that.

Thanks so much for your kind remarks about this website. My intention is to make it helpful, accurate, and informative. I’m very glad it’s helping people.