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Save Our Slopes: Can Certain Plants Help?


QUESTION: I live on a hillside and am concerned about all the slopes that slid this year. Right now my slope is covered with ivy. Should I plant something else that might hold the hill together?

--R.F., Woodland Hills.

ANSWER: Catastrophic landslides like those that happened in a few areas of Southern California this winter really can't be prevented by plants. It has more to do with the geology, or how the slope was engineered, and if you're concerned, you should consult an environmental or soils engineer.

Plants can help, by breaking the force of the raindrops and by holding the surface of the soil together, but only up to a point.

Landscape architect Bob Perry, who is also a professor in the landscape architecture department at Cal Poly Pomona and the author of several books on plants for the West, agrees that plants have a limited role in holding hillsides together.

Although they are aggressive and somewhat weedy, he thinks that ivy (English or Algerian) and Hall's honeysuckle (Lonicera japonica 'Halliana') are two of the best soil binders on large hillsides. He likes to use the honeysuckle with Lantana montevidensis on big slopes. Both often need to be cut back severely on occasion to keep them from accumulating too much dead growth underneath.

To keep hillsides from being monotonous, he suggests planting trees and shrubs with these ground covers or billowy, spreading plants like Jasminum mesnyi or the yellow Bank's rose. He'll sometimes mix common periwinkle (Vinca) with these. It's less aggressive but grows better in wetter spots or where there is shade.

On slopes that are smaller, there are many things you can plant, including natives like coyote brush and manzanitas and exotics.

The Sunset Western Garden Book has a list of such plants, though it is perhaps more limited than need be.

The worst thing you can do on a steep hillside is "plant ice plant and water a lot," said Perry. Ice plant is not a very tenacious ground cover. "Cutting away at the toe of the slope with poorly made retaining walls to try and gain more space" is an equally bad idea.

It's also important to keep runoff from the garden from running down the hill. However, though plantings help prevent erosion, they'll do little to stop a landslide.


Q: I'd like to get some earthworms to put in my flower pots. Do you know where I can buy them?

--C.C., Alhambra.

A: Because earthworms naturally find their way into flower pots, I suppose you can add them to pots that they haven't found on their own.

Earthworms are sometimes sold at nurseries, and you can order them from several sources. One is A-1 Unique Insect Control, 5504 Sperry Drive, Lemon Heights, CA 95621, (916) 961-7945.

The smallest container of red hybrid earthworms, designed to cover 250 square feet of garden, costs $18.75, which includes shipping.

What you'll get are "bed-run worms" which include babies and eggs; the largest are still pretty small (about 2 inches in length), but they'll grow to about 6 inches.

They can be added to compost piles, dropped on lawns or flower beds and, I guess, put in flower pots.

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