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Tuesday - February 09, 2010

From: Fairhope, AL
Region: Southeast
Topic: Grasses or Grass-like
Title: Plants for 60 degree slope in Mobile, AL
Answered by: Joe Marcus

QUESTION:

Dear Mr. Smarty Plants,I live on an eroding 60 foot bluff in USDA Zone 8A, along Mobile Bay (AL). To address erosion problems we are using a gabion-style product called "Green Terramesh," which is a series of wire cages (like large crab traps) lined with a biodegradable woven blanket, which is filled with dirt fill and one inch of topsoil. The cages are stacked on top of each other and secured together, and the face can be planted. Engineers refer to this as a mechanically stabilized earthen wall (MSE). Since the MSE will be 50-60 feet high and 100 feet wide, it requires a professional installation and engineer. My question is what type of plants would you recommend for the face of the wall? The slope will be 60 degrees. Ideally, I'd like a mixture of plants (maybe grasses) and textures (maybe shrubs or prostate juniper). We'd like the entire surface to be covered, not just sporadic clumps (to help cover the wire, which is unsightly). The growing wall face will be very visible from below, and we'd love to accent it with a variety of cascading blooms from the top. We can sprig the wall face during construction with small trees or shrubs, or place sod, sprinkle seeds or hydroseed, but don't want to shade out the other varieties or vice-versa. Prior to our erosion problems, lantana grew along the bluff, attracting butterflies. There was also Algerian or English ivy, wild blackberry, honeysuckle, and a number of small trees. Since the slope face is huge and steep, there will be no way to mow it, weed it or trim it. I don't know how the cascading blooms from the top would do when they met the grasses growing up and the grasses met the shrubs or vice versa. Nor how it will look 10-15 years from now (hopefully still standing) with no landscape maintenance. It will be important to establish a strong root base, especially to support vegetation can cannot be cut back. The woven blanket lasts for about a year or two at most. The wire (which looked like large hole chicken wire), however, will still be there to hold suitable vegetation beyond the angle of repose. Living along the Gulf Coast, we get some stretches of drought but more often receive very heavy, hard downpours especially during the spring and summer. About 2/3rd of the bluff face will be in full sun (western exposure); 1/3rd is partial shade, because a couple large Magnolias growing on a plateau shade the bluff face from afternoon rays. The wall face will be about 20 feet behind the Magnolia branches, so we might need a different vegetation from the full sun area, but it would be attractive if we can use some of the plants all the way across to tie everything together visually. Since the MSE is so visible from below and alongside steps that go up the bluff, we'd like to create some attractive contrast along the wall, with a variety of plants, grouped en masse, maybe like a layer cake. Whatever we select would have to do well growing close to vertical (on the 60 degree slope face). Is this too steep for shrubs or small trees? Will short fibrous root systems cause a plant to topple if it can't be cut back? I�ve drawn a blank in finding a local horticulturalist with experience in making recommendations for what to plant a 60-degree-MSE in Zone 8 or even locate photos. I�ll email you a photo of the Green Terramesh sketch if that would be helpful. Any suggestions would be much appreciated. Thanks.

ANSWER:

The overwhelming issue involved in your question is an engineering one which is outside the scope of Mr. Smarty Plants' service. We very highly recommend employing a licensed landscape architect to assist you in developing a planting plan for your project.

In our opinion, the one inch of topsoil specified for the MSE will be one of several important limiting factors in determining a suite of plant choices. Grasses will top the list with some forbs and possibly a few woody species also useful. Lantana urticoides (West Indian shrubverbena) might work as one of the woody shrub species in the planting, but we doubt that you will have enough good soil there to adequately support junipers. The area shaded by your magnolia tree will naturally develop a somewhat different flora over time than the part in full sun.

Lack of maintenance will mean that, long-term, the nature of the landcape on the planting is likely to change dramatically. No matter what you plant the landscape will, without maintenance over time, lose its original character. Early on, you can expect a lot of weedy species to appear and the overall effect may not be very attractive for some years. In time, however, as the planting matures, more desirable species - some of which may themselves be volunteers - will dominate and the planting will be more natural in appearance and thus more attractive.

The slope of the berm is not too steep for shrubs or small trees if other conditions (soil structure, soil fertility, drainage characteristics, competition, sun exposure, etc.) allow for their growth there. The steepness of the slope is not necessarily prohibitive. Plants with short, fibrous roots are excellent candidates for your slope as they will tend to stay in place and will play the essential role of limiting soil erosion. The root systems of the plants on your slope will not cause the plants to topple, nor will they keep them from toppling.