Drainage, erosion control, and mulches

These are massive topics which could probably each take a whole semester long of class in and of themselves. In general, as a maintenance gardener, you will probably not be installing drain tiles, earthmoving with a loader, or hydroseeding and juting a hillside. These are jobs in the construction phase of a project. However, you may inherit or take on the care of such a landscape when the contractors are done. It is nice to know the basic principles behind the tools and technology, and how you can be a good caretaker to such a usable space.

Basic terms and concepts to know, in order of appearance:
infiltrate, hydrophobic, hydrophilic, permeable, leach, sediment, berm, swale.


Nobody wants to be picnicking in a swampy pile of mud wallow, or have a hillside of a backyard of water gush into their living room during a winter storm. It is problematic when all of your plants die from being waterlogged too long, and definitely a bummer when the 100 feet of retaining walls all collapse on the brand new flagstone patio. Hence drainage and erosion control…

Water is a heavy and dense material. It comes out to be about 62 pounds per cubic foot. One cubic foot of about seven or eight gallons of milk. That much weight.

Water when traveling at high velocity, or when combined with abrasives, can cut through extremely hard substances. Watch a video of a waterjet cutter slicing through metal and thick stone, go on a walkabout of a riverine canyon, or feel the smooth granite as you are swimmin’ in a sierran stream.

Water goes where it wants to go. Usually this is in a serpentine swirling fashion from high to low elevation, from sky to cloud to mountain to sea. It travels atop the land in streams and rivers and lakes, and travels below the earth in groundwater and aquifers. It pops up occasionally as springs and geysers. It infiltrates easily materials with large pore spaces, like sand. It takes longer to infiltrate materials with tiny tight pores like clay. But then, gravity pulls the water quickly out of the sand, whereas it clings for longer time inside the clay. Water avoids places that it does not like, places called hydrophobic (water fear or tending to repel water). It prefers places called hydrophilic (water loving). This is related to the electrical charges of materials, and the clingy surface tension aspect of its liquid being.

As water moves, it picks stuff up and drops it off at intervals. Stuff becomes dissolved in the water, and when trapped, hidden stuff can emerge or precipitate or reveal itself once again.

Once you start to pay attention to drainage, you will see it everywhere. Many of our hard surfaces we build are not permeable. Just think of all the concrete sidewalks, asphalt roads and parking lots, houses and buildings. Water does not go through them and penetrate back into the earth, water sits on top of them. Not permeable. So to keep water from forming huge puddles everywhere there is a slope on that impermeable surface to keep water moving. We direct the water to a grate. Which leads sometimes to a catch basin, then to a drain pipe, and on down the line to the sewer treatment plant, or sometimes straight into the river or the lake or the bay or the ocean. We dont want that water cutting and dissolving and destroying our handiwork and flooding our homes.

Drains can be above ground easy to see, or underground. Page 272 -279 in your reader.

Erosion control

Erosion is how the land and wind and water interact with each other. Wind and water take hits on the earth, and the earth gives up bits of herself to tumble down down down. The combination punch of wetness then southerly or norwesters or hurricanes named Maria pulverize crush and set in motion the rocks and sands that make up mountain ranges and foothills. Some continents are ancient and been subject to eons of erosive forces; they have poor soils in terms of nutrients. The nutrients have all been washed away. Leached is the proper word. In flat lowlands where the river widens and water floods the earth, water deposits the stuff gathered on high and sprinkles it all over. You do this for a million years or two and you gonna have a pretty thick ol buildup of sediments and ‘soil’. So erosion is a natural process that we recognize and work with.

When we build roads in hilly areas, sometime we gotta dynamite through it to make that pass. This leaves a steep road cut, oftentimes easily susceptible to erosion. Rocks falling and hillsides collapsing into repose. Settling into their natural more comfortable posture. When I worked out at Indian Basin in southeast San Francisco, a wetland restoration project created a series of sloped channels next to the water. This was meant for the tides to come in and come out of. Without some sort of erosion control, water would have destroyed all our efforts. If you knit and weave together the bits of earth particles, then it is less likely to erode, and hold strong. This is done by covering the earth, as well as by planting it. The roots act like anchors holdfasts tentacles and webs. In the reader, page 280-285, then pages 286-288. Bare soil is always vulnerable to erosion and to colonization by any ol seed that is nearby. Specifically – weeds.

There are a variety of coverings blankets to choose from. The natural materials like jute don’t hold up as long as synthetics like polyethylene plastic. But perhaps that is a good thing. What is important is that your timing is good, and that plant roots which hold the soil are established before the rainy season arrives and threatens to wash it all away. Get that soil all bound up. And remember, with large swathes of land, you don’t want to make an impermeable covering. Its not like putting a rain suit on the earth. That would just create a sheet and volume of water you have to direct and channel to avoid more problems. You want to create a colossal sponge with porous tunnels bringing that water deep deep deep deep down.

People always ask – what is a good plant for erosion control? Ideally that would be a plant with somewhat deep and spreading roots, persistent over time. One adapted to local climate and soil conditions. Around here the weeds that seem to fit that bill are many, and if you wander around looking at all the steep neglected lots throughout the city you will see them. They are cotoneaster, algerian ivy, monterey pine, french broom, pampas grass, red valerian centranthus ruber. If you have an aversion to these plants, then yank em out and plant some ceanothus, california buckwheat, lupine, bunch grasses, and huckleberries. But don’t leave the soil all naked.

A somewhat more labor intensive but more well thought out approach contours the land before you start covering it and planting it. If you create berms and swales and terraces then the water and sediment does not just rush on down and away. It will pause and swirl and take a break. And in that time some of the water will have lost its go go go speed momentum and sink down into the earth. And in the process it will irrigate your plantings, or water the little white crown sparrow’s berry poop plantings.


Reader page 294-296
Your professor Gus Broucaret always gives out this somewhat funny handout in class. I will post it here as well at the end for its relevance to mulches.

The reader lists a whole bunch of materials used as mulch. Many of these are unusual as mulches and I have never encountered them in the field – fiberglass, crushed stone, sand, lawn clippings, sawdust, sphagnum, or bog(?). However the basic concept is clear, cover the earth. The blanket keeps the water moisture in the ground, and additionally can provide some bit of weed control and suppression. Plus it makes the yard look nice by tying the design altogether. Many times in our ornamental landscapes we will cover first with a layer of weed cloth (woven plastic), then put the mulch on top. For more natural gardeners the preferred first layer is cardboard, not plastic. This is called sheet mulching. The earthworms love to eat wet decaying cardboard cellulose. Again, the same idea. You are not trying to block nature’s activities permanently. Just block em long enough for your plants to take hold and thrive and be able to take care of themselves.

A few helpful hints for maintenance gardeners using specific materials as mulch:

gravel and pebbles: Small pebbles will roll everywhere, especially on a slope. Then the leaves and dirt and dust of the world get in there too. So just keep that in mind…

decomposed granite (d.g.): We never use d.g. as a mulch. We do use it hard packed on pathways and around trees in tree wells. It is permeable to water and tight hard enough that weeds do not grow well in it.

pumice: Red rock was fashionable for mulch back in the day, laid over plastic bags in a rose garden with those curved wave shaped concrete edgers. These days it has mostly fallen out of fashion, but it may come back. Its other use is mixing the smaller piece 1/8” or 3/16” size into the soil to help drainage and aeration and maybe keep gophers at bay.

wood chips and bark: You can buy neat bags of these at the nursery or get it for free from either the municipality green waste stream or from the local tree workers chipping the stuff into their trucks. It may sometimes carry pathogens, disease. It may become food for a number of local fungi that like to eat and colonize wood chips. In these parts, the common mushrooms on chips are poisonous Hypholoma aurantiaca common name is redlead roundhead, the smelly red cage stinkhorn Clathrus ruber of the penis family Phallaceae, and the super potent hallucinogenic wavy cap Psilocybe cyanescens which love the hardwoods pieces of eucalyptus.

straw and hay: Hay is the good and more expensive stuff used for fodder – livestock food. It is cut when it is green and chock full of nutrients. Straw is the left over stuff after grains have been cut and harvested. It is often used for bedding in the horse stalls or chicken coops. You can feed it to the herbivores too but it is not as high quality a food. Straw and hay are not the same thing, not interchangeable. But, both straw and hay get bundled into bales. They are commonly used as mulch in vegetable beds. Oftentimes you get wheat or barley coming up next to your planted swiss chard and tomatoes.