Planting on a hillside

Hillsides or any type of slope can add an extra challenge to planting a garden. The existing turf or weeds may be the only thing holding the soil in place. Removing them to clear out a garden bed may cause erosion before the new plants can establish large root systems.  Killing lawn and then planting and mulching a slope may become a mess after the first rain, as the mulch runs downhill and washes gullies into the garden. The roots in the soil are what keeps it from washing away. How do we protect our soil during this transition.

I have several suggestions to try to control erosion during garden establishment. Using a combination of methods will probably give the best results. More gradual slopes will need less intervention. Killing exiting vegetation in a way that does not disturb the root system is better than digging them out. The less soil disturbance the better off the soil will be.

Erosion mats are the traditional way of dealing with erosion on a slope. There are plastic or biodegradable mats that can be pinned down and then the plants will grow up through the mats. The downside of mats is that sometimes birds or snakes can get tangled in them and weeding is harder when the roots are tangled in the mat. Plastic mats do not degrade and need to be removed which can be tricky to untangle them from all the plants. Erosion mats may still have their place on very steep slopes but I use them as a last resort.

If you are converting a turf slope and have plenty of time, kill sod at the top of hill first and work your way down. Start with killing a strip of sod at the top of the hill, plant your plants, and wait for them to become established. Once the top strip is established, start work on another strip of sod below the first strip. Working your way slowly down the hill season by season until your planting reaches all the way to the bottom. I find that starting at the top of the hill is better because the seeds from the native plants will fall down and start spreading into the lower parts of the slope.

Another erosion solution is to make swales. This works especially well on gentle slopes. When the soil is cleared, build gentle ditches parallel to the hillside. Plant your plants in and around the swales. This will slow down the rainwater and help keep the soil from eroding. Over time the swales will fill in but your plants will be established by then. 

In addition to the swales, use logs half buried parallel to the slope. The logs can be held in place with spikes if they tend to role downhill. As rain water flows down the slope it is stopped by the log and encouraged to infiltrate into the soil. This series of logs and ditches form a washboard of swales and berms. We want the surface water to slow down and infiltrate as it hits each obstacle.

Reroute water runoff during establishment. Downspouts or surface flow may be heading towards the new garden. If the water can be temporarily diverted to go around the new planting it will decrease the amount of flow the slop needs to deal with. A trench could even be dug along the top of the slope to detour water to one side during establishment but then filled back in after the plants have developed root systems. If a gutter drains at the top of the slope, hook up an extension tube to the downspout to bring the water all the way to the base of the slope until the new garden is established.

Temporarily reducing the amount of stormwater headed towards the new bed is for the long-term benefit of the slope. Less volume and a slower water flow is less likely to cause erosion. Over time the ditches will fill in, the logs will rot, and plants will take their place to hold the soil. Slow, steady, thoughtful progress is the best way to tackle gardening on a slope.