How to Plant Grass Seed on a Slope

Photo showing grass growing

You love the way your landscape slopes down to the street, and you dream of the day a green lawn carpets the space. But getting grass to grow on a bare slope can be challenging. Here you’ll find tips and recommendations on how to plant grass seeds on a slope to help fulfill your dream of a beautiful green carpet.

What is slope?

Slope is how far the land surface rises or falls. As part of the land’s topography, slope is an essential measurement for understanding drainage and how water moves across the property.

“Numerically speaking, slope is the change in elevation (rise) over a given distance (run). In other words, the number of feet the land rises or falls over a horizontal distance,” according to the University of Tennessee Extension’s Which Way does it Fall? How to measure slope in your yard. “Slope is often described in terms of percent. For example, an area that rises 12 feet over a distance of 100 feet has a 12% slope.”

Why is slope important?

There are a couple of reasons the slope is a critical factor when it comes to sowing grass seed and landscape maintenance. 

  • Slope is also important when planting (or sowing) grass seed and mowing the lawn. Several organizations, including the Integrating Transportation and Community Plannings, say a steep slope is 15%. The University of Tennessee Extension says grassy slopes should be less than 20%.
  • On a slope greater than 15%, the challenge of planting grass seed is keeping it from being washed out along with soil erosion. Keeping grass seed in place even with a 15% slope may be difficult.
  • Steep slopes can be dangerous when mowing, whether with a walk-behind or riding mower. It’s easy to slip on a steep slope when walking behind a mower. Riding mowers may tip over. 

What if the slope is too steep?

If your slope is too steep and dangerous to mow, consider alternatives. Usually slopes are planted with low-profile plants that are under 4 to 5 feet at maturity. This is especially true if you want to see the scene at the bottom of the slope.

Terrace the slope by adding stones, lumber or low retaining walls horizontally every 3 to 4 feet as you go down the slope. The terraces don’t have to be the same size and shape. Some can be rounded or flowing. Add plants to the terraced sections. 

Select plants that help control erosion with strong, deep roots. “Gro-Low,” a native fragrant sumac (Rhus aromatica) was bred to control soil erosion on hills and slopes.

Look for low-maintenance plants so you don’t have to climb down a steep hill to take care of them. Low-growing evergreen shrubs, such as the native creeping juniper (Juniperus horizontalis), are a good choice.

Deep-rooted prairie plants and grasses add flowering perennials to the mix. Sedges adapt well to many environmental conditions. 

Ground covers, such as lilyturf (Liriope spp.), bishop’s cap (Epimedium spp.), hellebores (Helleborus spp.), and Japanese spurge (Pachysandra terminalis), are just a few that do well. Avoid invasive species: Wintercreeper (Euonymus fortunei) and periwinkle or myrtle (Vinca minor, V. major).

Cover the slope with riprap or other type of stone mulch, or cover the slope with shredded bark mulch.

Grass seed in hand
iStock photo

Prep soil on the slope

Preparing the soil on a slope is the same as preparing it anywhere. 

  • Remove grass, weeds and rocks.
  • Rough up the soil or add about an inch of topsoil mixed with compost. This creates a loose surface for the seed to adhere to.
  • Grade the top and bottom of the slope to flatten them a bit. This reduces the chance those areas will be scraped by mowers.
  • Measure the square footage of the area to determine how much grass seed to buy.
  • Buy the best grass seed you can afford.
  • Select a seed that works in your landscape, such as full sun, shade tolerant, warm or cool season variety.
  • Look for seed that results in grass that is drought and disease resistant.
  • Grass with deep roots, such as tall fescue, is one type recommended for northern lawns. Bermudagrass has deep roots ryegrass germinates quickly and can help hold soil in place until deep-rooted grass gets established.

How to seed a 15% or greater slope

When planting grass seed on a steep slope, install a temporary dam, such as a 1×4 or something similar, at the top of the hill. This slows down rainfall and helps reduce soil erosion.

Spread seed on slope

Water deeply – about 6 inches deep – the area to be planted with grass seed a few days before sowing. The moisture helps the seed grab hold and begin the germination process. Insert a long screwdriver into the soil and pull it out. Look for traces of moist soil on the metal portion of the screwdriver. Measure how many inches of moist soil is on the screwdriver and that tells you how deep the soil moisture is.

  • Apply seed using the instructions on the grass seed product. The label tells you how to set your seed spreader to apply the correct amount. 
  • Crisscrossing the area with the spreader is usually recommended. 
  • Lightly rake the seed to help cover it. 
  • Don’t walk on the seeded area.
  • Lightly water the newly planted area for 5 to 10 minutes to dampen the soil and 1 or 2 inches deep.
  • Water about 10 or 15 minutes twice daily. Morning and evening are best.
  • Cover sowed seed with clean straw, cheesecloth, or burlap to protect it from birds, rainfall and erosion until the grass is actively growing. The straw can be left to decompose or raked out of the lawn. Remove the burlap or cheesecloth once the grass is established, usually 4 to 6 inches high.
  • Once the grass is established, the roots should control soil erosion.
Man on truck hydroseeds a slope to make it easier to grow grass on the downward slamt
U.S. Army Corps of Engineers | Flickr

Hydroseeding a slope

You can rent a hydroseeder that sprays mulch mixed with grass seed onto the soil. This can make fast work of seeding a slope. Stand where you can hit all areas and spray.

You’ve probably seen this method used to seed areas after road construction. Because water is part of the process, the seeds instantly have moisture. Follow the hydroseeder and product label instructions for follow-up watering.

Tip: Take proper safety precautions when working with power equipment. Wear protection for your eyes and ears, gloves, and closed-toe shoes.

Seed mats or blankets for slope

Although more expensive, mats or blankets embedded with grass seed and fertilizer may be one of the best and easiest ways to get a green lawn on a slope. Most of the mats and blankets are made of material that decomposes. The idea is to lay down the blanket or mat, water it, and eventually the mulch will decompose. Hold the mats in place with lawn staples.

One advantage of these blankets or mats is that grass seed is evenly distributed, resulting in a lawn without clumps or empty spaces. If you have a large area to cover, consider contacting your local landscape supplier to see if it has options that cover more square footage than available at lawn and garden stores.

Germination usually occurs within 7 to 21 days. Regular watering, about 1 to 1 ½ inches a week, is needed to keep the seedlings from drying out. Hold off mowing for 4 to 6 weeks after laying the mats.

Mats or blankets should decompose in about 90 days, sometimes less, sometimes more. The mulch that carries the seed to the soil may be a color that is camouflaged as the seeds germinate and grow.

Some mats or blankets are made with a netting that may be slower to decompose, and many people end up pulling it out once the grass is established. This can be risky because you could pull up seedlings that are still growing. Be careful with the netting, however, so it doesn’t get tangled in a lawnmower. If you leave the netting in place until it decomposes, increase the mower’s height.

Overseeding, slit, or slice seeding a slope

Machines that overseed or slit seed grass also can be rented. They make shallow slices or trenches in the soil and inject them with seeds. Each of these machines is useful when working to repair bare patches or renovate areas of poor growth.

This method often eliminates much of the soil prep except to break down clumps of soil and remove rocks. Sometimes called power seeding, it’s more efficient than roughing the ground and tossing down grass seed. The equipment ensures good seed-to-soil contact, which helps with germination.

Core aeration frequently accompanies overseeding or slice seeding. You can rent aerators. This machine pulls about a 3-inch core from the soil and leaves it on the surface. The cores resemble dog droppings. However, as they decompose over a couple of weeks, they add trace nutrients to the soil. Meanwhile, the holes allow water and other nutrients to reach the roots of the grass.

FAQ about growing grass on a slope

What’s the best time of the year to plant grass seeds on a slope?

Usually, fall is the best time to sow grass seed. However, if the slope is bare, sow seed as soon as possible. Leaving bare soil for very long results in weed seeds blowing in and taking hold. If you opt for seed blankets, mats, or hydro- or slice-seeding, that can be done about any time.

How do I know the best grass seed for my area?

Contact your local county extension office for grass seed recommendations for your slope. Also, your area’s home and garden centers will have suggestions. They usually only carry seed that is best for your area.

When to call a lawn care pro

Caring for grass growing on a slope can be difficult, especially when it needs mowing. Once your grass is established, connect with a local lawn care pro who can give it the utmost care to help it thrive.

Main Photo Credit: CSIRO | Wikimedia Commons | CC BY 3.0 with text overlay

Jo Ellen Meyers Sharp

Jo Ellen Meyers Sharp award-winning garden writer, editor, and speaker. (She speaks at libraries, garden clubs, public gardens, home and garden shows, Master Gardener groups, and horticulture industry events.) Known as a hortiholic, she frequently says her eyes are too big for her yard. She blogs at hoosiergardener.com.