Simple erosion control for a hillside or garden slope.
Things to look for before you even start on a garden, landscape slope or wild hillside.
Drainage.
Is there water moving across slope or down the hillside? Has erosion been common on this slope? Are there small or large channels on the slope? Where is the water going in the landscape?
Is the soil on the slope stable or is it a sand dune? Gravel pile in the garden? Go up on the slope and dig a hole, (dig the hole where you'll plant a plant later). Is there solid rock under the top one inch of soil? Does the landscape have permanent bald spots?
How good is the drainage? Fill that hole you dug with water and time how long it takes for the water to drain. An hour or less is good; if 2-3 days, then the slope is a problem.
How steep is the slope? Can you walk on it? Crawl on it? Rappel from the top? If it is too steep to walk, can you make cross paths? Can you run a wheelbarrow up it? Can you make a path that would allow a wheelbarrow on the hillside? Staircase? Ladder? You'll need at least a few weedings and waterings to establish the plants; how will that be accomplished? Is the slope accessible at all? Is it possible to bring the slope back into the landscape or garden?
Is there irrigation available on at least part of the hillside? Water of any kind? (You can plant the hillside without water, but it is more difficult, not as pretty and more fire prone.)
How is the sun? Is the slope full sun, part shade, morning sun, afternoon sun. (It's amazing how people have trouble noticing this, but plants sure do.) Some 'a little sunny' slopes could bake a chicken in an hour or so.
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. This
picture is a high tech solution gone bad.
This is a plantable slope.
Though this slope appears quite rocky and without enough soil, it was planted with California native plants, in a mixed planting. Erosion gradually decreased on the slope.
Ok, now that you LOOKED at the slope, time for a plan, idea or a solution :
Most hillsides can be made relatively stable with plants . A planting can stop nearly all erosion and hillside movement in a landscape. Almost. The only way of stabilizing a slope better than plants is a reinforced retaining wall that you need to take a mortgage out to put up(literally). The planting needs to be a mix of groundcovers, shrubs, trees, and perennials with the areas between plants(if there are any openings) covered with appropriate mulch and/or boulders. A varied planting is FAR more effective than a monoculture on a slope. Why? When you have a mixture of plants you have layers of vegetation that the rainfall will hit and when it finally hits the ground the force of it hitting the ground is much reduced. Generally the bigger the plant grows to, the deeper the roots. There are exceptions; pines and some manzanitas have shallow roots; you can use them within the planting but not as the total solution. Some of the pines (Pinus spp.) and manzanitas (Arctostaphylos spp.) can grow on one foot of soil. If you only have a foot of soil on the slope this is VERY useful. If the soil is deeper, a mix of deep roots are needed to tie the top soil(s) to the bottom rock, but the top 1-2 feet of soil needs to be tied tightly together. The shallow rooted plants like monkey flowers (Diplacus spp.), Penstemons (Penstemon spp.), Sagebrush (Artemisia spp.), sages (Salvia spp.) or some manzanitas do this well. (See the sample plantings below.) Also, the type of mulch you use is important. Do not use compost, rice straw, hay straw, large chunks of bark, etc. See the soils page.
2.
Retaining walls. A series of small terrace walls are a much more
pleasant and environmentally benign solution to the slope
problem than a massive reinforced wall. Also, they usually do
not require a permit. Small rock or interlocking walls allow
rainfall to stay on the landscape slope and you can garden on
the resulting terraces. (See
garden wall for more.)
3.
Terraces. Sometimes the nasty landscape slope can be made usable
by cutting simple garden paths across the slope. The resulting
steeper slopes can be readily accessed from the path below. Make
sure you account for runoff across the slope on the path. The
path can become a storm drain if it runs across the slope at an
angle. That is ok, as long as you put some rocks and boulders
along the inside of the path to slow the water and stop erosion
as the water blasts across the slope in a down pour.
The goal is to control the runoff in a way that does not cause further problems. Feeding the runoff off a slope onto your neighbor's slope is not drainage or damage control, it's a lawsuit.
If the cut slope is large, put a 8-10 foot terrace every fifty feet, PLEASE! This may add a little to the grading costs, but will make the restoration of the slope easier and much more functional long term.
DO NOT change the grade around existing OAK (Quercus spp.) or other native trees as you will kill them, as happened in this picture. Here the rock wall should have stopped at least four feet from the trunk.
4. Boulders and logs. What a beautiful garden planting. What a lot of work!
5. Planting by rope. The contractors we've worked with have implemented this solution several times. Rappel from the top and plant enough of a planting on the rock outcropping to make the slope secure.
Dumbscapes, or old erosion ideas that don't work.
A
straw blanket/mat slope before the rain.
GRASS. Planting grass on a slope does not stop erosion. Erosion studies have consistently shown that slopes that were seeded with grass have GREATER EROSION than anything other than bare ground. (Even dead sticks beat grass!). Just because the world is full of idiots, you do not have to do what they do. Don't seed slopes with grass. This myth of seeding grass on slopes to control erosion has been perpetuated for about 100 years and still occurs after fires in some poorly educated sectors of our country. EVERY study that has ever been done recommends against it. After spending a day trying to find an article supporting the seeding of grass to control erosion I could find none. Seeding slopes after a fire or grading does nothing but destroy the ecosystem for perpetuity. Bare, grass-covered or ice plant-covered slopes commonly load up to field capacity (and beyond), while slopes covered with a mix of native shrubs and trees and perennials rarely do(Patric). In a home landscaping seeding with grass makes a weedy slope that is very hard to stabilize and reestablish plants on and it creates a different plant community, ie. Weeds.
Mulch. The type of mulch, placed on top of the ground, is very important in the management of a slope susceptible to erosion. See the mulch page for appropriate types of mulch to use. If you use the wrong type of mulch the plants wont grow very well, weeds could be introduced, and erosion could be increased!
Plastic. Plastic is for bags, soda bottles, and children's toys. If you stuck those items on the hillside they would be about as attractive and effective for erosion control. (After a few years the plastic 'weed barrier', 'mulch' or 'erosion matting' has curled and is sticking up in amongst the weeds.) I removed some of this stuff off of a 'restoration project' (in a shady spot) near San Luis Obispo a few years ago. The ground was practically bald (nothing much was alive) after 2 years, except a little annual rye grass. UGLY! Next to the plastic, there was near- normal recovery. In other places where this plastic matting was used (sunny spots) the weeds had gone crazy.
Hey, it does rain, even in California. A little rain does
this, below.
Jute.
Jute kinda works. Not great, but it can work short term until
your planting is established.
In the pictures there is a series of failures of jute. The final was when it burned with foot and half flames, and the rain washed it away.
If you combine the jute with mulch it can be used to hold a steeper slope than either can hold by themselves. Put down about one inch of mulch, (shredded redwood, cedar, oak, or pine), roll out the jute and pin it down, cover jute again with another one inch of shredded mulch. Make sure you do a FULL planting for this to work. Pop the plant out of its gallon container, and set it carefully to the side. Cut the jute, carefully fill the empty pot with dirt from the hole you dig. Pull the mulch and jute to the side. Set your plant into the hole you made, then put the jute and mulch back into place. Haul the extra dirt away.
Good! A mixture of deep-rooted California native shrubs, and trees, mixed with shallow-rooted shrubs, and perennials, mulched and with no weeds, will control erosion on the slope. Why should you plant a California native plant community on the slope and not grass or ice plant! Because the native plants connect with each other underground, and the microorganisms that live in association with them produce tiny threads that ramify through the soil, coiling around particles of sand and clay and holding them, and also producing glue-like compounds to hold the soil particles. This interconnection, I guess you could think of it as a natural microorganism community underground living in cooperation with the plant community aboveground, which the grass and iceplant, and other alien plants do not possess, is why it is critical to plant California native plants in a spaced plant community to control erosion on a slope.
Straw.(Straw punch, Straw mats) Straw is for animal bedding. On slopes it works for about 15 minutes during the first rainfall. Then the hillside is a weedy, muddy mess and the straw is somewhere else. Also, you have just introduced a massive amount of weed seeds, that you will have to try to get rid of as these alien plants will actually contribute to erosion! As with grass, the erosion is greater with straw than mulch, plants, boulders, walls or anything other than loose dirt.
Concrete. Malibu uses concrete as 'erosion' control. Weird! The coastal sage scrub is beautiful and stable. Some dummy clears the 'brush' and plants grass, the hillside slides, so they cover it with cement that gradually cracks, costs a fortune, looks UGLY, and is dead. And after about twenty years, the concrete falls off of the slope.
Ice plant, 'red apple', and grasses like Red fescue, all behave the same way in a wet year. These plants are not appropriate to control erosion on a slope because 1) they are alien plants and not part of the natural plant community, 2) they have very shallow roots. The slopes load up with rain water to full saturation and then shed/slide off. The top vegetation actually ADDS to the weight of the slope. It feels just like a wet shag carpet, and the roots are about as deep. I wish the news people would get it right; it usually isn't mud slides, it's ice plant or 'grassland' slides.
Fire Concerns:
Many people are concerned, and rightly so, about fire danger. That is why so much iceplant has been planted in southern California. The ice plant does not burn very well, but having it on the hillside actually increases erosion. If you live in a critical fire area, instead of ice plant, you can space the California native plants (some are more fire retardant than others) apart, with mulch and/or pathways in between, to reduce the fire danger and also control erosion on your slope.
Mynativeplants.com is a search engine to provide a plant list for your particular site. The plants love the slopes. Enter the information you found going through the list above and TA DA!, you have a working plant list. Start with that list and 'weed' out the plants you do not think will work, or you just don't like. Try to get down to about 5-10 plant types altogether for most of the slope.
Here are some VERY limited simple plant lists of plants that will generally work.
Some plants for south facing slopes along the coast in sand(where day time temperatures rarely exceed 90F).
Mix and match to make a good slope planting.
Most Ceanothus, the best are: Ceanothus griseus horizontalis and Ceanothus thyrsiflorus repens
Some plants for south facing slopes along the coast in clay(where day time temperatures rarely exceed 90F).
Mix and match to make a good slope planting.
Arctostaphylos edmundsii(all forms)
Arctostaphylos 'Louis Edmunds'
Deer slopes: deer are lazy, give them a path, they'll commonly stay on the straight and narrow.
Baccharis pil. 'Pigeon Point'(cover with chicken wire for first few months)