This is a public-interest archive. Personal data is pseudonymized and retained under GDPR Article 89.

Advice on gardening a medit.-climate slope

Hello, fellow Medit-planters.

A couple of weeks ago, I posted a question about how to landscape a
Mediterranean-climate hillside without causing a landslide.  I was
overwhelmed by the response -- more than a dozen of you wrote to me
(most not copying the list, as I had suggested).  The advice was truly
fantastic, with far more responses (and more detail) than I expected.
Thanks!  You should congratulate yourselves, you're a very nice bunch of
people.

Several people expressed an interest in seeing a summary of what I
learned.  It's below.  After seeing how good the responses were, I think
I would be a jerk to keep them to myself.  Thanks again to everyone who
helped:  Nan, Chad, Charlotte, Peter, Sean, Carol, Tony, Rod, Loren, and
I think a couple others whose names I missed.

Mike
San Jose, CA, Zone 9 (min temp 20F)

=========================

Your problem is familiar. I have a flat pad cut out of a hill side with
two
side up going hillsides and two down. In some places the slope is close
to
one in one and in a lot of places one in two. When out area was burnt
over
in June of 1990 the problem you describe was a major concern. The
biggest
problem is to quickly get some sort of surface growth to hold the
hillside
together when the winter rains come.

The cheapest way is broadcast annual rye grass which starts to grow with
very little water and clumps nicely. The problem is to get rid of the
rye
grass after other things grow. There have been some grass selective weed
killers that have appeared in the last year or so that might be very
effective. Also, whole areas of the neighborhood public land were
broadcast
with rye grass and it is still a major weed. The best way of spreading
the
seed is to have it "Hydroseeded" This a commercial process of spraying a
seed and furtilizer plus (usually a dye so that you can see where the
spray
has gone) loaded slurry that gives the seed a helping hand when
germination
is taking place. I used hand spreading of a seed sand mix and it worked
fairly well though it is difficult to see where one has put the seed.

Another, more expensive approach is to cover the slopes with special
course
cloth that holds the top soil together. This comes in about ten and
twenty
foot wide rolls about fifty feet long and is held down with giant tent
pegs. (Prices of about $50 and $100 per roll come to mind.) You can now
put
conventional plants through the cloth.

[My slope is about 200 feet long and 100 feet wide, so I think we'd be
talking about $2,000 just for the cloth in my case.  Does anyone know if
it's possible to hydroseed with wildflower seed instead of ryegrass?
The thought of 200 feet of California poppies gets me excited.  --Mike]

I was forced to use the rye grass because of cost. I have about a 1,000
feet of hill side, some as much as 30 feet high. In the rye grass sown
slopes I sowed many native plants and shrubs + such standbys as oleanda,
accaca, and bouganville. Of course I am in zone 10, not zone 9.

Seven years later, my biggest problem has been blowouts from gofer and
ground squirrel holes. The water gets into the top holes and runs out
the
lower holes which soon greatly enlarge pulling down large quantities of
mud. To battle this I have gone up and covered the upper edges with
large
sheets of black plastic at about the time of the first rains. When the
trees and shrubs are larger I hope that their coverage will soften the
rain
and reduce the runnoff.

I did get some useful advise from the county agricultural advisor. He
was
particularly helpful on plantings to help stop erosion.

===================

Mike:  We had a similar situation when we moved into our current home.
Our
hill faced South-east and while it was steep, it was probably shorter
than
yours.  The house was on a pad at the top of the slope, and there was
about
a 10 foot patio (read: concrete pad) between the back door and the
beginning of the slope.

We wanted to determine what plants to use, but the yard was so large,
that
we needed help in the overall conceptualizing.  We hired a wonderful
landscape architect, who helped us determine which areas of our yard we
would use for different purposes.

We regraded the yard to push the deck out to about 20 feet, and created
a
level lower yard for the lawn (which also covers the leach field for our
septic tank -- you can't plant much above a leach field) for the kids.

We then terraced the slope from the patio down to the lawn.  We created
three levels, each at least 4 feet deep (far deeper in some areas).
Each
level is heavily amended and planted with perennials.  The top two
levels
are the highest water levels (all DRIP IRRIGATION -- I highly recommend
Robert Kourick's Drip Irrigation for Every Landscape and Climate.  It is
an
excellent book that recommends a very sane method.  If you are really
concerned about supersaturating your slope, drip irrigation is your best
bet as it delivers water to exactly where you want it and when you want
it).

Because hardscape is so incredibly expensive, we surfaced our entire
patio
built walls to retain the terraces, and steps down to the lawn out of
broken concrete.  I called concrete demolition companies, companies that
install pools, etc. in the yellow pages.  95% of the time, they were
happy
to deliver the concrete they were taking out instead of paying the dump
fees.  We learned to look for 4" to 6" thick UNREINFORCED concrete, in
chunks that were at least 12" to 18" in length or diameter.

We set the concrete in a bed of sand and/or decomposed granite and left
room between the chunks for planting thymes and other "crack" plants to
soften the look.  Most people are amazed when they find out it is
recycled
concrete.  Flagstone for this project was estimated at about $14,000.
Concrete to cover the same area AND build walls costs us $250.

Our terraces follow the contour of the hill.  Our citrus will go in on
an
area that is still sloped, but is a gentle slope and we are pre-plumbed
to
deliver gray water from the washer to the citrus -- gravity fed!


===================

A tricky little problem you have there! I don't know your scene (we live
in a different country), but you are correct in thinking you need
professional advice. What you need to know about is not just the top
couple of feet of ground, but what lies beneath it as far down as the
water is likely to go (to where it might start things sliding!).

If you lived here in NZ, I would say "go to the Geological Survey", who
would certainly be able to advise you on soil and subsoil types for your
area. Try the Geology Department of the nearest university, if they
don't have the information you need themselves, they should certainly be
able to point you to a suitably-qualified person to advise you.

Actually, unless you are planning *major* earthworks, with careful,
minimal, watering where needed, I would not have expected you to run
into those kinds of problem. Terracing here is usually done with "crib
walling" provided it is not more than about 5-6 feet in height at any
one "step". Anything more than that _does_ usually require local
authority permission plus engineering advice being provided.

[I responded with some more information, including a mention of our soil
type, which is basically clay mixed with broken stones.  Turns to muck
as soon as it gets wet.  The response back to me is below.  --Mike]

It certainly sounds as if great caution is going to be needed with your
soil type! We too live in an area where the sub-surface rock is very
much shattered by earth movements (which continue - another 4.5 shake
yesterday!) but we are more fortunate than you in that - given the
conditions - our local rock type remains surprisingly stable on slopes.

I would have thought that gentle terracing (not going too high! -
preferably not more than 4 feet per step) using crib-walling blocks or
similar means of support, coupled with precise watering using
permanently installed pipeline with drippers to apply small amounts of
water exactly where it is needed, may be OK, but it definitely sounds as
if local expert knowledge is needed!

If you
are planning fruit trees on anything more than a "home garden" scale,
planting them on sloping ground is NOT a good idea anyway, because of
the difficulties of attending to the trees (for pruning etc.), and
access for harvesting the crop, which will involve a lot of up- and
down-hill work! It can also be difficult - even dangerous - setting up a
ladder for tree access on narrow terraces!

Sorry if this sounds negative, but you asked for comments/advice!


=============

        Mike, I think you may be worried for nothing. The extra water in
summer 
will not translate to winter landslides. Anyways 1 in 4 or 1 in 3 is not
a steep
slope. My parents yard in Orinda is more like 1 in 2 with terraces and
only 1-2 
feet of soil over sandstone, no slide problems there. We did have some
seepage 
problems at the Base of the hill where it runs into a drainage dam like
say the 
garage or a patio. Your trees will stabilize the slope and use up any
excess 
moisture thereboughts. If the soil you add to any terraces drains well
(as it 
should) then it alone in 1 or 2 foot terraces should not be an issue. If
you 
still think drainage is an issue then place drain pipe in trenches
covered with 
rocks under your walkways and lead the water to a drain that exits to
the 
street.

=================

Sounds like a pretty scary slope Mike!
In your shoes I would talk to people who have had happen to them exactly
what you fear most and ask them how they would do things again and if
they did who they got to do it for them  etc etc.
Word of mouth and personal recommendations would be the way to go.
On slopes that steep I can't understand why it was scraped bare.
If you were to build on a slope that steep here the soil would have
absolute minimal disturbance and most people wouldn't even plant a
garden, but that is an very small number of houses we are talking about
and usually the granite is pretty close to the surface.
You've definately got a challenge to garden on there, good luck!

[Note to folks living outside California:  It's general practice here to
grade down a new tract home lot to bare Earth before it's delivered to
the buyer.  I don't know the reasons, but it does remove the tinder-dry
brush (and trash) that tends to build up on a construction site...
--Mike]

=======================

Glad you know to be careful of your hillside, and are willing to get
professional help.  A good landscape architect should be able to help
you
with your situation.  They are trained in slope stability as well as
plant
material, irrigation etc.  The professional organization that certifies
landscape architects is the American Society of Landscape ARchitects
(ASLA)
and there is a rigorous test they have to pass to be able to put those
initials after their name, so that is one way to verify credentials.
Good
luck with your project.

[The ASLA has a web site by the way, but it's more geared to architects
than the public.  --Mike]

=====================

My backyard has a slope of 45 degrees or so.  And a big soft spot in
which
the yard contouring backhoe kept getting stuck 18 yrs. ago.  I planted
Arctostaphlos "Emerald Carpet" on it and the slope is holding fine
though I
was very worried two years ago about a mole digging everywhere.  The key
to
successful drainage, the contouring man told me, is a little shallow
ditch
which drains the water off in the winter.  He specifically told us not
to
concrete it in, but to just see to it before heavy rains that the water
runs
off to the side of the property.  This makes for great fun in some of
our
deluges!  The slope, however, is intact.  Above the arctostaphlos is
planted
rosemary which we've only lately started to clip with an electric hedge
shears.  This makes for new growth and beautiful blue flowers in the
winter.
Birds of all kinds really love our back yard.  I don't feed them as
there's
plenty of food for them with the rosemary and whatever bugs are on
junipers
and manzanita.

====================

Best bet to keep slides from happening is to terrace it. It involves a
lot of
work ( ask the Ifugaos of the Philippines they terraced whole mountains)
   which if youre willing to do keeps soil and slided from happening.
However
usually depending on the hill side you might want to contact a
contractor.
Although through careful planning and the use of ground covers such as
arctostaphylos ( manzanitas) and other plants that grow along hill sides
naturally, they can help to keep soil in place. Around the Monterey
area, we
have very steep hills. The grasses that used to grow extended roots
several
feet into the soil therefore keeping the hillsides intact. These are
grasses
like Stipa, Deschamsia,etc that grew native. 

================

I'd think that you can get practical advice from your area by talking to

a builder about whatever "rules of thumb" they have for grading
hillsides 
 and more importantly, to locate professionals who can advise on what's 
safe in your area. Or try extension and soil conservation services for 
guidelines in your area.  If your garden is on a slope that's no steeper

than the one your house was put on, or if it's the original hillside,
and 
not steepened by cut-and-fill, it's very likely that you can terrace it 
with making it unstable. The main thing in engineering it is to create 
good drainage at the contact between native soil and backfill, and to 
make sure that all terrace walls are very freely draining.  If a terrace

wall acts like a dam, the weight of water behind it will likely cause 
failure.  I would guess that only the worst-designed slopes will fail 
from summer irrigation [if irrigation is not excessive, most of the waer

should be taken up by the planting].  In any case the major concern is 
winter rains -- especially the biggest expected rain event after soils 
are largely saturated.  In San Jose, it would vary widely, but you're 
probably talking about, say 5 inches or more in 48 hours every several 
years -- and that's what your hillside has to be engineered for.  I
would 
hope that a Mediterranean planting, even when being established,
wouldn't 
take more than the equivalent of one inch prcipitation per week.

You'll find a couple of articles on steep hillside gardens in
California, 
if you browse past issues of Horticulture magiazine.  Only see it in the

local library, and don't have the references.
Other Mailing lists | Author Index | Date Index | Subject Index | Thread Index