Erosion Control for Steep Hillside: Last Minute Installation to Hold the Soil
Question from Melissa:
Hi Pat, My elderly parents live in Rolling Hills Estates on the Palos Verdes Peninsula and have a very steep hillside leading down from their backyard. About 1 year ago a “live” sprinkler pipe ruptured and sent a landslide down into the neighbors pool below! After huge expense and going through tge insurance company’s recommended hillside repair service, etc. all plants on the slope have died and my folks now have a bare hillside right before the rainy season! Needless to say I’m not sleeping at night worrying about it. I have read your exchange with Kathy from last June on this subject and was wondering if you have any recommendations of people in the business of doing this kind of work. It is hard to find recommendations and since we feel it wasn’t done correctly last time . . . we are desparate to get it done right now. Any advice would be appreciated.
Answer from Pat:
Wow! This sounds dangerous and right before the rains. Luckily, however, the rains this year are not supposed to be so strong as last year. I don’t live anywhere near you, and thus I cannot recommend anyone to help you, but I have several suggestions regarding what to do. I would make a list of local nurseries and phone each of them to ask if they have heard of similar problems and solutions in your area and for companies doing bank stabilization and also look online. I would do the same for landscapers. Spend a day on the phone and phone a bunch of them and ask if they have handled similar problems.In Rolling Hills and Palos Verdes Estates there is an on-going problem with erosion therefore I would think that inquiring locally might be a help towards getting knowledgeable attention to detail and fast work prior to the rains. I would also ask South Coast Botanic Gardens if they have any advice. Here is a newspaper story regarding the recent landslide in Portuguese Bend: http://www.dailybreeze.com/news/ci_16074876
Beginning in spring, you could have planted something that might have held up that bank but planting should have been done much sooner than this. It is too late now. I have seen huge banks covered with plastic to get them through the rains, but the top of the plastic must be secured into the ground so that water does not course under the plastic. Additionally, covering a bank this way leads to strong water runoff at the bottom, but sometimes it’s the only solution to get a steep bank safely through the winter rains. You will need to find a local company to do this work. Sometimes builders and roofers are helpful with such a situation since they also have to deal with getting areas covered before the rains. Once you get through the rains then you would have time to plant appropriately. When you tell me that the plants all died on the bank, this gives me a clue that they were not watered enough to keep them alive during the summer months. Getting a bank of fresh plant material to take off and grow requires an appropriate drip system properly maintained and timed or else careful light watering by hand with the hose and sprinkler several times a week to make sure the irrigation sinks in and does not just run off or merely moisten the surface. Sometimes people think if native plants were used they need no water, but all need water at first to allow them to become established. You don’t mention what plants were used. If they were summer-deciduous native plants they may not be dead but simply dormant in summer when there is no rain.
A solution that might help now would be to install a concrete revetment product. This means an articulated concrete block bank stabilization system. There are many types on the market. Nurseries are one place to go and local landscapers undoubtedly are familiar with these systems. Geoweb Cellular Confinement System is one such name. I would think there must be a company in the Palos Verdes area who does this work using whatever products they find appropriate. Purchase and installation would cost thousands of dollars and also requires correct hydraulic systems analysis in advance to make sure it will work and be sure you get the right type for the slope. There is even one type of plantable concrete block available for almost vertical and vertical installations. We have several of these walls next to roads where I live. Once installed they can be planted with appropriate plant materials to additionally hold the bank. Lowes Stores and Dixieline and local concrete block companies all sell products such as Keystone Retaining Wall units that are made to hold banks and you can grow plants in the spaces provided by these blocks. Lumber yards, concrete block companies, and other such places can suggest companies to do the installations. If a slope is adequate that the blocks lean their weight onto the bank, weed fabrics installed behind the blocks can stop invasive weeds and help stabilize through winter rains. Cut X holes through when planting so roots can go through into the ground.
I have also seen whole banks sprayed with Gunite (sprayed-on concrete) such as the kind used for swimming pools. An entire bank below a house and deck and above a swimming pool was sprayed with Gunite on a steep hillside near my home. This can be an inexpensive and suitable solution, though not a beautiful one. Another spray protection for stabilization of earth is a polyurethane coating made by a company called SPI. Several companies manufacture ornamental rock walls to retain slopes. One product for this purpose is called Shotcrete. Matrix is a company that does this kind of work.
Fifty-four years ago when my late husband and I first moved into our home, I once faced a problem of similar magnitude right here on my own property. My husband’s step-father was John Lloyd Wright, the son of Frank Lloyd Wright. Like his famous father, John was also an architect, and he designed our home which was built into a hillside where there had previously been a steep bank. John ordered that the site of the foundations of our house be bulldozed out of the hillside and all the earth pushed westward towards the ocean to make a patio surrounded by an earth berm, 3 feet tall on the inside but approximately 30 feet tall on the outside with a steep grade. We stabilized the inside of this berm with large rocks. During the summer I personally planted iceplant or hotentot fig (Carpobrotus edulis) all over the exterior slope of the earth berm. I climbed up there by laying a tall ladder onto the ground and moving it along as I worked. Carprobrotus is not favored today since its seeds can invade wild lands especially near the ocean. Also, it is said to pull down steep banks. However it never pulled down my bank. I watered carefully with the hose several times a week until the roots struck into the ground and held the bank. After the rains came it held the bank perfectly. By spring it covered the bank. I never again watered it and it never was a problem.
Thank you Pat for your speedy reply!
I understand clearly that we need get the slope protected now for the rains and wait till Spring to plant. That is what we had to do the first time as the original pipe rupture happened in October and so no work could be done on it until well into Spring. Thank you for reminding me . . . I had forgotten in my “hysteria” of worry! When the hillside repair people planted the slope (after soil engineers did their work and the slope was packed and graded), they used a spray on top of substance that held the dirt and also contained seed, which I believe was a wildflower mix . . . which probably did go dormant in the summer and we stupidly thought it was dead.
Now my Mom’s gardener has removed everything – too bad! It is unfortunate we were not advised more about what was done and a shame we were so ignorant. Hard lessons learned on many levels!
Once I get the work underway to secure it through the rainy season, I can then spend more time finding the absolute correct folks to landscape design and do the work. Contacting the South Coast Botanic Gardens will hopefully help, as well as other local nurseries.
Thanks again for your help. It is truly appreciated.
It sounds to me as if you need to contact the company that did the work and tell them what your gardener did and ask what to expect and what to do now. Whatever was used should have come with instructions on how to maintain and keep the bank. Perhaps your elderly parents forgot about this. Wildflowers are usually not enough to hold a bank such as you describe. Deeper rooted plants are also necessary. If your gardener only pulled out annual plants the seeds may come back again in fall. For example, banks that are not overly large or hugely steep are often held with a seed mix that is a combination of clumping gazanias and sweet alyssum. Then plants that are longer lived are planted with deeper roots to hold the bank over the long haul. In the case of gazanias and sweet alyssum, these should have included instructions regarding irrigation. In general I always tell folks to stay away from annual grasses. (Make sure no annual grasses are included in a seed mix if you plan to use a seed mix.)
On the other hand, perennial grasses are available for holding steep banks and these can be an excellent permanent solution to your problem. Creeping red fescue (Festuca rubra) is a perennial grass variety that is sold under various brand names such as ‘Slopesaver’, or ‘Slopesaver 2′, but all Festuca rubra must receive regular irrigation. It gives you the look of long green hair covering an entire steep bank. These banks never need mowing and look good and stay green year-round. Festuca rubra is excellent for holding steep banks and can be installed from seeds or planted from sod. Many named brands of Festuca rubra are available. All can be planted on steep banks. Sometimes Festuca rubra is used to cover the ground under trees on flat or sloping ground. Festuca rubra is shade-resistant but cannot survive without irrigation.
For more drought-resistant types look for seed mixes or sod that includes the drought resistant native California grasses with grayer color. If you have my organic book, look on page 132 and 133 for a discussion of all these grasses and others. Various sod companies sell drought-resistant grasses that give a touseled look on banks or flat ground. One is called “No Mow” and is fairly drought-resistant.
When I wrote to you this morning I omitted one piece of rather important information that in retrospect I feel I should have included:
Since you have now told me that the bank was stabilized under the supervision of soil engineers who properly packed and graded the slope and then had the bank sprayed with polymers and seeds, the work was done correctly. If the bank was seeded as late as October last year and then held through last winter’s heavy rains, it sounds to me as if there was no fault on the side of the company that did the original bank repair. In fact, they did a great job and deserve thanks and not accusations.
The fault is unfortunately in the home-owner not knowing what to do from then on if anything. It is very important now to get back in touch with the original company who did the job, as I already suggested, and clarify this. If the bank was installed last October and held through the rains, perhaps all it needs now is another spraying now to hold it through the coming winter. Or perhaps it’s fine as is. You need to find out. Then you need a clear plan what steps if any are necessary during the following spring and summer and you need to be able to tell your parents’ gardener what to do or not to do in accordance with this plan. Also, if there is a secondary spraying find out if this should be followed up with irrigation and if so, exactly how water should be applied and how much and how often.
It sounds to me as if you need to contact the company that did the work and tell them what your gardener did and ask what to expect and what to do now. Whatever was used should have come with instructions on how to maintain and keep the bank. Perhaps your elderly parents forgot about this. Wildflowers are usually not enough to hold a bank such as you describe. Deeper rooted plants are also necessary. If your gardener only pulled out annual plants the seeds may come back again in fall. For example, banks that are not overly large or hugely steep are often held with a seed mix that is a combination of clumping gazanias and sweet alyssum. Then plants that are longer lived are planted with deeper roots to hold the bank over the long haul. In the case of gazanias and sweet alyssum, these should have included instructions regarding irrigation. In general I always tell folks to stay away from annual grasses. (Make sure no annual grasses are included in a seed mix if you plan to use a seed mix.)
On the other hand, perennial grasses are available for holding steep banks and these can be an excellent permanent solution to your problem. Creeping red fescue (Festuca rubra) is a perennial grass variety that is sold under various brand names such as ‘Slopesaver’, or ‘Slopesaver 2’, but all Festuca rubra must receive regular irrigation. It gives you the look of long green hair covering an entire steep bank. These banks never need mowing and look good and stay green year-round. Festuca rubra is excellent for holding steep banks and can be installed from seeds or planted from sod. Many named brands of Festuca rubra are available. All can be planted on steep banks. Sometimes Festuca rubra is used to cover the ground under trees on flat or sloping ground. Festuca rubra is shade-resistant but cannot survive without irrigation.
For more drought-resistant types look for seed mixes or sod that includes the drought resistant native California grasses with grayer color. If you have my organic book, look on page 132 and 133 for a discussion of all these grasses and others. Various sod companies sell drought-resistant grasses that give a touseled look on banks or flat ground. One is called “No Mow” and is fairly drought-resistant.
When I wrote to you this morning I omitted one piece of rather important information that in retrospect I feel I should have included:
Since you have now told me that the bank was stabilized under the supervision of soil engineers who properly packed and graded the slope and then had the bank sprayed with polymers and seeds, the work was done correctly. If the bank was seeded as late as October last year and then held through last winter’s heavy rains, it sounds to me as if there was no fault on the side of the company that did the original bank repair. In fact, they did a great job and deserve thanks and not accusations.
The fault is unfortunately in the home-owner not knowing what to do from then on if anything. It is very important now to get back in touch with the original company who did the job, as I already suggested, and clarify this. If the bank was installed last October and held through the rains, perhaps all it needs now is another spraying now to hold it through the coming winter. Or perhaps it’s fine as is. You need to find out. Then you need a clear plan what steps if any are necessary during the following spring and summer and you need to be able to tell your parents’ gardener what to do or not to do in accordance with this plan. Also, if there is a secondary spraying find out if this should be followed up with irrigation and if so, exactly how water should be applied and how much and how often.