Using a Native Understory to Control Weeds in Riparian Restoration
By Heyo Tjarks, Restoration Biologist
Editor’s Note: This article was originally published in Cal-IPC News, Spring 2012.
Due to the construction of dams and levees throughout the Central Valley for agricultural and urban development purposes, current ecological conditions on most of its floodplains do not favor the establishment of native woody or herbaceous species. Dams and levees have altered the natural hydrology (e.g. flood frequency, duration and amplitude) and geomorphology (e.g. sediment transport, bank erosion, and river meander) to which native riparian vegetation is adapted and reliant upon for reproduction and successful establishment. Because of these alterations, native vegetation is often outcompeted by aggressive weeds. Restoration projects on the Sacramento River over the past few decades have established approximately 8,000 acres, to date, of riparian forests with native woody species. However the herbaceous understory is frequently dominated by annual grasses or other weeds including yellow-starthistle or milk thistle.
Within the last decade, River Partners has strived to increase the overall biodiversity and habitat structure for the benefit of wildlife within our restorations. Our goal is to design native plant associations that will develop into sustainable communities through ecological succession under the current and projected future conditions. One of our major advances toward this goal has been achieved through an aggressive approach of understory weed management and the establishment of an herbaceous layer consisting of native perennials. Through experimentation, River Partners has successfully germinated and established several native herbaceous species in the field, including mugwort (Artemisia douglasiana), gumplant (Grindelia camporum), telegraph weed (Heterotheca grandiflora), evening primrose (Oenothera hookeri), western goldenrod (Euthamia occidentalis), creeping wild rye (Leymus triticoides), and blue wild rye (Elymus glaucus), to name a few. We use an approach which combines modern agricultural equipment and techniques, up-to-date horticultural knowledge, and adaptive management practices. With this combination, we are able to; 1) design, plant, and establish large acreages (up to 800 acres at a time) with multiple native understory species, 2) effectively control non-native invasive weeds throughout the project sites, and 3) create beneficial wildlife habitat.
Before creating a field design, River Partners biologists conduct an evaluation to assess the site conditions (e.g. soil types, topography, depth to water table, current vegetation, historical vegetation, and inundation intervals). Once these variables are understood, it is possible to select a suite of species which are capable of self-sustaining growth within those site conditions. In addition, species are chosen and arranged based on their benefits to wildlife (i.e. habitat structure and food source) as well as they’re ability to compete against non-native and invasive species. Once a suite of species are chosen, River Partners collects and processes native seed from remnant vegetation within the project area or as close to the site as possible in order to ensure the genetic adaptation of the local ecotype.
Next, we design the field layout. Design considerations include matching species’ growth requirements to the microsite conditions across the project area and arranging species within this framework to produce habitat (i.e. structure and food sources) required by the focal wildlife species. Finally, the project area can be prepared for planting using modern farming techniques including discing, ripping, floating, pulling planting berms, and installing irrigation. Typically, a project’s life is three years, in which the site can be prepared, planted, maintained, and established as a self-sustaining community.
For example, our approach on the San Joaquin River National Wildlife Refuge (SJRNWR) is to install woody trees and shrubs along the planting berms during the first year of the project, followed by aggressive weed control during the first two growing seasons. Our weed control approach involves flood irrigating the aisle-ways between planting berms in order to promote the germination and growth of non-native and invasive species in the seed bed. Then, aisle-ways are disced or sprayed with herbicide in order to exterminate the weeds before they set seed. Hand labor using backpack herbicide sprayers is used to remove weeds along the planting rows where discing or broad herbicide application would damage the planted native woody species. This process is repeated multiple times throughout the first two growing seasons in order to exhaust the existing seedbed. Ultimately, this sterile seedbed approach reduces competition for native grasses and forbs that are broadcast or drill seeded at the end of the second growing season.
After seeding native grasses or forbs, the understory is actively managed during the third and final year via irrigation, mowing, and herbicide applications. Adaptive management strategies and timing are critical at this stage. For example, it may be necessary to mow the aisle-ways if significant weed pressure still exists. If so, it is important to mow before the weeds become so tall that mowing creates a large amount of mulch which will smother any smaller, native species. However, it is equally important to cut the weeds low enough to reduce the competition for sunlight with the native species. Thus, choosing the optimal timing and blade-height is key to a successful mowing regime. In order to facilitate the use of herbicides for weed control, River Partners separates the aisle-ways into native grass mixes and forb mixes. By planting an alternating pattern of forbs and grasses, it is possible to add diversity and structure to the restoration, while also allowing the use of selective herbicides to combat weeds.
This approach has resulted in a dense cover of native herbs: 65% and 71% absolute cover of native herbaceous species and less than 4% absolute cover or weeds on two fields surveyed in 2010. These results are typical of many of our projects in this region and more recent projects implemented on the Sacramento River. This method of understory establishment has been employed by River Partners since 2004. Since then, we have restored approximately 1,700 acres of riparian habitat on the SJRNWR alone. This approach has not only been successful at combating non-native invasive weeds, the planted understories have also been resilient to disturbances including fires and long-duration flood events. Lastly, the method has also created beneficial wildlife habitat. Over the past decade, River Partners has documented several threatened and endangered species utilizing and breeding in our restoration projects. These species include the least Bell’s vireo, western yellow billed cuckoo, valley elderberry longhorn beetle, and the riparian brush rabbit.
The above article originally appeared in the December 2012 issue of the River Partners Journal.