Holding Your Ground
Slope rations for planted areas are a critical consideration for the establishment of plant material. For planting, a slope ratio of one foot of drop for every four feet of horizontal distance, called “four-to-one” (4:1) slope, or flatter, is best. Most of the time, the area beyond the graded platform for hardscape structures is steeper than that. Here are some general rules for deciding what to do with steep slopes.
Any slope steeper than “three-to-one” (3:1), is difficult or dangerous to mow. Trying to establish turf grass on slopes so steep is difficult, too. For steeper slopes, use vigorous ground covers, or a mixture of vigorous shrubs and native grasses to hold the soil. Another option is to use large masses of Juniper shrubs. They have long, low, spreading forms that do a good job of holding slopes on commercial properties, where professional maintenance crews can use a custom spray program to reduce typical Juniper pests and special pruning techniques to keep their spikes dense and weed-free.
For slopes steeper than “two-to-one” (2:1), it is difficult for trees and shrubs to establish root systems strong enough to hold the soil. There are very few grasses or shrubs that can work on these severe slopes, and your options for plant material are, at best, invasive. Kudzu does a wonderful job of holding slopes over 2:1, but it will out-compete everything else to the detriment of the local environment. Lespedeza will hold a steep slope, but there is no way to eliminate this scruffy plant progeny from the seed bank after it is planted. You can use native plant material on a 2:1 slope. Choose quick-growing Pine or Cedar seedlings interplanted with a heavily-seeded native grass and forb seed mixture. Vigorous vines might work, but would require lots of hand-weeding every year. Consider terracing to artificially create flatter shelves along the natural contours of the site. With terraces you have better planting options.
Anything steeper than 2:1 will exceed the natural angle of repose for soil. No mulch or grass can hold a slope steeper than 2:1.
That means the hillside can potentially fail and slide as a result of gravity, unless it is reinforced in some way. Expensive structural fixes are needed for steep slopes. Typically, on construction slopes greater than 3:1, erosion control blankets are required to prevent gullies and erosion.
For homeowners, armoring an extremely steep slope with flat stones is an option, but this is back-breaking work, and stone is expensive. As slopes approach a 1:1 ratio—a sheer cliff. The only option is to build a retaining wall.