Before you begin planning a slope garden, consider the grade of your hill.

For a steep slope, landscape designers recommend a retaining wall, rising part way up that will support the garden and add a decorative touch.

There’s no rule of thumb, but generally anything more than a 2-to-1 pitch needs stabilizing, says Ron Davidson, a landscape designer at Northern Lights Landscaping in Duluth.

“If it drops two feet and goes out one foot, that gives you a pretty steep slope,” Davidson says. “With anything steeper, I would recommend stablilizing.”

Another way to visualize a 2-to-1 pitch, he says, is to take a square and dissect it into two triangles.

If bushes or a tree are on the slope with established root systems, however, the wall wouldn’t be necessary, Davidson says.

Tiering the slope is an option, landscapers say.

“You can do a shelf system with small retaining walls and have individual planting beds,” says Dave Madsen, a landscape designer for Engwall Landscape.

If the slope is more gradual, natural tiers can be created with rocks. A row of boulders at the bottom also helps support the slope, cutting down on soil washing away.

Erosion is a particular concern in a newly planted garden.

“Gardens may wash away in rain when new; that’s one of the challenges of planting on a slope,” Madsen says.

After plants are established, their root systems will hold the garden in place, according to Madsen and other landscapers. Mulch, such as shredded cedar or cypress wood, and the strategic placement of large rocks also helps.

Planting a slope is “one of the best erosion controls there is,” Madsen says.

To install a slope garden, first remove the existing sod on the hill. Some landscapers plant before adding rocks, mulch or a fabric weed barrier. Madsen puts down a fabric weed barrier first, then mulches. Then he creates holes in the fabric and plants through the holes.

“It helps with the weeds and helps to retain the soil,” Madsen says of his method.

If the slope isn’t steep, the garden can be planted like a flat bed. Make sure the plants get the sun and soil conditions they require.

Madsen includes shrubs and small evergreen bushes that help hold soil in place. “There’s no set rules on slopes,” he says. “You can plant just about anything you would in a flat garden.”

Duluth garden designer Kurt Herke uses rocks — football-size and larger — to create planting areas in a slope garden.

“I create pockets for the plants to grow,” he says. “Rocks help stabilize a slope, hold the dirt in place and they’re visually appealing.”

Herke uses boulders at the bottom of a slope to hold the garden in place. He also uses rocks to create boundaries between shrubs and perennials or to create a terrace area for plants. Herke tries to avoid straight lines as well as a too-scattered look.

Herke says slope gardens are typically well-drained, whether in sunny or shady locations, so he uses plants that like well-drained soil.

“Iris is one of my favorite ones to put in a slope garden,” he says. “They do well in this climate and on a slope.”

Herke generally uses short plants, under three feet tall, in a slope garden. Plants that work well in a rock garden — such as dianthus, rockcress and some varieties of sedum — also work well on a slope.

“I take some clues from settings in nature,” he says. “I tuck a fern next to a rock to make it look like it’s always been there. Or I put in stone with lichen on them.”

This story appeared in The Daily Herald on page D12.