Design Tip: March 4, 2010

Graded Slopes

Images submitted by reader

A newsletter reader asks...

Could you make a recommendation for a ground cover or plants to control erosion? Everytime I look at it I can't see an easy solution. Any suggestions?

And the answer is...

I see this problem almost daily. A subdivision has been built in a hilly area and the builder has simply created flat building sites by terracing the lots. The graded hills between the lots are either located entirely on one lot or the property line goes right through the middle of the slope. Without vegetation, the hill begins to erode or even collapse.

If the hillside in question only has small gullies created by runoff, you can probably go ahead and start the process of landscaping it. However when I see sink holes beginning to develop in the hillside, the slope may need to be regraded and stabilized or any landscaping you install may end up being dislodged by the unstable slope. My advice in this situation is to contact a Certified Landscape Architect or a Civil Engineer. Some hillsides are so steep or tall that the installation of a retaining wall may be in order.

If landscaping will fit your needs, my first advice is to locate the property line. If you are the downhill neighbor and the property line is at the top of the slope, then the hill is all yours. If it's at the bottom, it belongs to your uphill neighbor and a meeting will be in order. If the property line goes through the slope itself, once again meet with your neighbor because landscaping half a hill probably won't solve your problem. After determining the location of the property lines and you have reached a consensus with your neighbor (if necessary), take measurements, determine the amount of sun the area gets and create a plan or idea on paper.

Before you do any earth moving in this area - planting, grading, etc. - make sure to have the utilities marked by calling 8-1-1.

Amend your soil using topsoil and organic supplements. It's obvious that what has been left after grading is the hardpan red clay. After amending, you may want to cover the hillside with a jute cloth (jute netting) before planting. This open-weave material is made from biodegradable plant fibers and is used to temporarily hold the soil on a hillside until the plants become established. Simply cut holes in it when planting.

I don't usually recommend covering an entire hill with a ground cover plant. Weeding this will eventually become a nightmare, especially on the slope. Trees, shrubs and perennials have very good root systems that can hold the soil. For the situation pictured above, I would suggest some small-growing trees at the top or mid-way on the slope. Don't plant them in a line. Plant them as specimens, in a zigzag pattern or in groups of 3 or 5 the length of the slope. Install low-growing shrubs in a similar fashion next to the trees. Perennials or annuals (including ground covers) can be planted at the edges at the top and/or bottom. Create a border by either trenching or installing some type of edging to keep the turf from invading the bed. Apply a generous layer of mulch which should be 2-3 inches deep when settled to protect the roots and cover the jute. Stay away from pine straw as it slides easily. Shredded pine or hardwood may be the better choice.

Here are some plants that I would avoid for this situation: Here are some plants that I would recommend for this situation: This is just a partial list of well-adapted plants. If you feel overwhelmed by the project, you can always hire a Landscape Architect or Landscape Designer to assist you.

Please email me if you have any questions or topics you would like to submit for later articles.

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