THE COASTAL GARDENER:

With winter nearly upon us, the danger of slippage and erosion to our coastal hillsides is a real concern. Over the past few winters we have seen slopes fail in parts of Laguna Beach, Laguna Niguel and Anaheim Hills, all with catastrophic consequences.

There are several things to consider when dealing with a slope or hillside. Is the soil on the slope naturally created or is it the result of grading and construction? Is there a history of erosion on the grade? Does water move across the incline, or is it moving down? If there are erosion channels on the slope, are they small or large? Where does the water go that hits the slope as well as water in the areas above and below it?

When you dig a hole on the hillside, is there solid rock near the surface? Sandy soil on top of clay soil is a problem, as is clay on top of sand. Either on top of a rock layer is also a problem. We shouldn’t be surprised when these hillsides fail.

Imagine building a two-layer cake, then tipping the plate about 20 degrees.

Now imagine the top and bottom layers of the cake are of a completely different texture and porosity, one dense and the other light. Now add a little water. Maybe even shake the tray a bit. What happens?

Why are people surprised? These slopes need trees and plants to stabilize them and bind the soil layers together.

Most hillsides can be made relatively stable with the right plants. A proper planting can stop almost all erosion and slippage on a slope.

The only better solution to a steep slope is a reinforced retaining wall. This, however, would require considerable engineering, and building it might mean taking out a second mortgage on the house.

The most important rule when using plants on a hillside is to use a mixture of plants and sizes — groundcovers, shrubs, trees and perennials. A varied planting is far more effective at holding a slope than a monoculture of one plant.

Unfortunately, many people still mistakenly think they are stabilizing their slope with a large planting of ice plant, ivy, honeysuckle or myoporum. These single plantings are a big mistake and often fail.

Generally the bigger a plant grows, the deeper it roots. The object is to bind the soil on a slope at various depths. A few trees will bind the soil deeply, shrubs to a medium depth, groundcovers and perennials more shallowly.

On a slope, a variety of plants growing to different sizes is essential.

Additionally, when you have a mixture of plants you also have layers of vegetation that the rainfall will hit. When the water finally hits the ground its force is greatly reduced.

Any areas of the slope that are not covered with plants should be occupied with well-healed boulders or appropriate mulch.

I find the best mulch for slopes is a liberal layer of shredded redwood or cedar. They are long lasting, good for the plants and, most importantly, do not wash down the slope after the first rain. Don’t use fine composts, planting mixes, straw, chunks of bark or coco mulch on a slope as a mulch; they will quickly wash downhill.

When planting the slope be sure to use water-thrifty California Friendly plants. Retaining water on a slope is a difficult chore and too much irrigation will encourage slippage and erosion. When planting the hillside do not till the soil or dig planting holes any larger than is necessary. Cultivating a slope is just asking for trouble; it’s far better to leave the soil structure intact. Most California Friendly plants are well adapted to local conditions and require very little, if any, soil amendments.

There are several old, outdated slope strategies that just don’t work, yet I still see them repeated over and over again. Planting grass on a slope does not stop erosion and many studies have consistently shown this to be true.

With very few exceptions, ice plant-covered slopes are a terrible idea. Ice plants are physically heavy and, with time, grow on top of themselves until their weight eventually pulls them, and a large sum of soil, down the slope. “Ice-plant slides” are all too common.

Jute netting can work for a brief time, while a mixed planting of trees, shrubs and water-thrifty perennials fills in. If jute is used it should be in combination with shredded redwood or cedar over the top.

With a little planning and perhaps some professional assistance, slopes and hillsides can be not only attractive, but safe and stable as well.

ASK RON

Question: Are there any treatments for keeping my evergreen holiday wreath and garlands fresh for a longer period?

Jennifer

Newport Coast

Answer: Your holiday greens will stay much fresher if you apply an anti-transpirant to them before you hang them. The brand to ask for is Cloud Cover. Anti-transpirants are clear liquids that seal moisture into the plants, extending their life considerably.

ASK RON your toughest gardening questions, and the expert nursery staff at Roger’s Gardens will come up with an answer. Please include your name, phone number and city, and limit queries to 30 words or fewer. E-mail stumpthegardener@rogersgardens.com, or write to Plant Talk at Roger’s Gardens, 2301 San Joaquin Hills Road, Corona del Mar, CA 92625.

RON VANDERHOFF is the Nursery Manager at Roger’s Gardens, Corona del Mar.