- Agricultural Water Quality Best Management Practices: Vegetated Filter Strips
What Are they?
A filter strip is an area of grass or other vegetation used to “filter” sediment, organic matter, nutrients, and pesticides from irrigation and storm water runoff to improve water quality. Filter strips are located between crop fields and streams, drainage canals, or other waterways.
How They Work
Filter strip vegetation slows the flow of runoff water. This allows suspended sediments to settle out and enables runoff water and soluble pollutants and pollutants attached to sediment to infiltrate the ground. The plants, decomposing plant matter, and associated microorganisms help trap sediments and take up dissolved nutrients and other chemicals.
Vegetated filter strips:
• Prevent soil erosion and stabilize field border soils.
• Compete with and protect against invasion by weeds.
• Improve water quality. Filter strips can remove 50% – 80% of pesticides and nutrients.
• Vegetated filter strip areas can be used as turn rows or for field access.
• Enhance habitat for wildlife and beneficial insects.
• Filter strip grasses and other vegetation can be used for forage hay or bedding mulch.
Filter strips require some ongoing maintenance. This may include:
• Mowing and harvesting. For grasses, mowing several times a year encourages dense growth.
• Controlling weeds.
• Supplemental seeding every 2 – 5 years and reseeding disturbed areas.
• Removing debris, repairing gullies, and removing sediment deposits.
• Periodically re-grading the filter strip and reestablishing vegetation when sediment deposition jeopardizes its function.
Installation & Cost
Filter strips should be located along the down slope edge of a field, where sediments, pesticides, or nutrients may move off cropland. Filter strips are usually at least 12 feet wide and often 30 to 40 feet wide. Wider strips provide more filtering benefits. Filter strips work best if runoff flows across the strip as shallow sheet flow, rather than as concentrated flow.
In the mid-west, where filter strips are most commonly used, they are installed along contours with a cross-slope between 1% – 10%. In the flat fields of California’s central valley, the design of filter strips is still under experimentation. Gentle slopes may have to be created with a scraper or road grader. One possible design is an asymmetrical “V” ditch at the field end, with a flatter slope being the 12 – 20 foot wide filter strip and a steep slope completing the “V.” Light disking or another form of tillage and smoothing are used to prepare the seedbed. Seed can be broadcast or drilled in. Seed should be sown in the fall so that the filter will be established for spring crop planting, or planted in the spring and lightly irrigated to germinate.
A variety of plants can be used for filter strips. Grasses or legumes are often planted. The type of plant will depend on the goals for the filter strip. Based on soil test results and the nutrient needs of the filter strip species, fertilizer may aid establishment. Avoid over fertilization, which can favor weeds and contribute to nutrient runoff. For legumes, inoculation with rhizoidal bacteria may be needed.
The cost of a vegetated filter strip is determined by the price of perennial grass seed and any grading necessary due to field end topography. In 2006, average costs of a filter strip run about $750 per acre, depending on seed choice and seeding rate. A more expensive installation would utilize plant plugs.
Planning and Available Financial Support
Advice on planning and installing filter strips is available through Yolo County and Solano Resource Conservation Districts and the Natural Resources Conservation Service. Staff is available to answer questions about the optimum width, location, and vegetation type for a filter strip given individual site conditions. Financial assistance for implementing this practice is available through: The Yolo-Solano Agricultural Water Quality Management Support Program, which can reimburse the costs of materials and time. The Environmental Quality Incentive Program (EQIP) which provides cost-sharing for private land conservation practices.