Willows for Soil Stabilization

With winter comes the cool crisp air and the occasional unexpected erosion problem from heavy rains. Repairing an erosion gully or streambed can seem like a headache, however there is a natural remedy that requires little to no machinery to install: Willows.

The willow species, Salix, has over 400 species of deciduous trees and shrubs that include willows, sallows and oisers. In California, common willow species include the Arroyo Willow (Salix lasiolepis) and yellow willow (Salix lucida). With the willow’s speedy root growth and high water use, installing willow sprigs, waddles or walls are a great option to help stabilize soil and provide wildlife habitat. This method of bioengineering is used by many environmental consulting firms and non-profits as a cost effective solution to erosion control.

The best time to collect willows is during the winter months when the willows have gone dormant. Before beginning your project, it is important to survey the site for the extent of erosion, slope stability and the natural vegetation. These factors will help to determine the length and diameter of willows (or other species) required for your project. For example, if you a building a willow wall to stop further head cutting of a gully, willows of different sizes and diameters will be needed. The “post” willows will need to be 1-2” in diameter. The length will be determined by the depth of the gully. Be sure to cut long enough that 2/3 of the post is underground and the last 1/3 is above ground and in line with the sides of the gully. Trim off all branches and, before installation, ensure the willow buds (if any) are facing up. Place the posts no less than a foot apart from each other. The willow “whips” should be about ¾ inches in diameter and long enough to go across the gully with an extra foot on either side to key into the gully. As the whips are tightly weaved through and pressed down, be sure to continue to pack in dirt into the ends of the willows that are keyed into the sides of the gully. Layer the whips until they are as tall as the gully sides. The finished willow wall will help slow down water flow and build up sediment to fill in the gully and reduce further erosion.

From Students and Streams: Willow Walls
Before starting any erosion control or bank stabilization project, it is essential to determine the cause of the problem and an appropriate solution, coordinate with neighbors (if erosion occurs through multiple properties), and obtain the proper permits as local and federal agencies have regulatory roles in structural stream repairs.
For more information on collecting and installing willows for soil stabilization, please refer to the NRCS technical note.