By: Fouad Jaber, Assistant Professor and Extension Specialist, Texas AgriLife Extension; and Perter A.Y. Amim, Post-Doctoral Research Associate, Texas AgriLife Research
What is a vegetative filter strip?
Vegetative filter strips are areas of existing or planted vegetation along croplands, drainage channels, streams, or other bodies of water. They are usually used to reduce water pollution from point or non-point sources.
These strips usually consist of perennial grasses or timber. They remove nutrients such nitrogen and phosphorus as well as sediments, pesticides, organic matter, pathogens, and other contaminants from water.
Though vegetative filter strips can be effective as a stand-alone best management practice (BMP), they benefit most when used as part of a conservation plan that includes soil testing, fertilizing, terracing, and water and pest management.
How do vegetative filter strips work?
Vegetative filter strips should be designed to disperse runoff across them as uniform sheet flow. This dispersion allows the filter strip to remove pollution from runoff before it reaches nearby bodies of water. These pollutants are removed from runoff water by deposition, infiltration, and biological and chemical processes.
The filter strip slows the runoff as it enters, allowing sediments to settle. The first few feet of the strip usually captures large and medium-size particles such as sand, silt, and soil aggregates. Finer particles such as clay settle more slowly and are deposited farther into the strip. The distance depends on the amount and speed of the runoff as it enters the filter strip.
The filter strip also removes pollutants attached to the sediment, such as phosphorus, ammonium and some pesticides.
Infiltration is the process by which water enters the soil from the surface. As the water slows, more of it filters into the vegetative filter strip.
The rate at which water enters the soil depends primarily on the soil texture. Water seeps into clay more slowly than into gravel or sand. Soils can accept more water if they contain organic matter, plant residues, and a developed root system.
As water moves vertically, sediment is deposited. When soil has a high infiltration rate, dissolved pollutants can be carried through it into underground aquifers or nearby streams by a process called interflow.
Biological and chemical processes
As nutrients and pesticides enter the soil, biological and chemical processes break down or transform these compounds. The new compounds can be taken up by plants, evaporate into the air, or be immobilized in the soil. In some cases, they can become more soluble in the water and end up in streams.
What makes vegetative filter strips effective?
Factors that influence the effectiveness of vegetative filter strips are soil texture, filter strip width, field slope length, type of flow, rainfall intensity, and vegetation management practices.
The soil texture of the field that drains into the filter strip influences how effectively the strip will cleanse the runoff. Smaller soil particles and other materials suspended in runoff are not as easily captured as larger ones.
As the vegetative filter strip slows the runoff, the larger soil and organic particles filter out first. The smaller clay particles usually remain suspended and move down with infiltrating and percolating water into the soil. Vegetative filter strips installed on soils with very low infiltration rates may be limited in their ability to filter suspended clay in runoff flowing through them.
Filter strip width
The most important factor affecting a filter strip’s efficiency is its width. The US Department of Agriculture (USDA) Natural Resources and Conservation Service (NRCS) recommends a minimum ratio of 70:1 of field slope length to filter strip width. The filter strip should be as wide as possible, but avoid removing any more land from production than necessary.
NRCS recommends various widths based on the field slope, soil type, and target pollutants. The minimum filter strip widths for sediments and sediment-bound contaminants are listed in Table 1.
Because removing water-soluble chemicals such as nitrate or pesticides requires more infiltration, the filter strips for these contaminants will need to be wider (Table 2).
To remove disease-causing microorganisms (pathogens), the filter strip must provide enough trapping time for biological and chemical processes to kill the microbes (Table 3).
Tables 4 and 5 list the recommended filter strip widths for treating runoff from concentrated animal feeding operations (CAFO) and from an animal waste management system.
Field slope length
The field slope length is the total area that contributes runoff to the filter strip. As the field slope length increases, so do the volume and speed of the runoff. Areas with longer slopes will require wider filter strips to effectively retain sediments and soluble nutrients.
As the field slope increases, so does the speed of the runoff flowing into the vegetative filter strip. Fast flow increases erosion and carries more sediment into the filter strip. This fast moving water does not deposit sediment as quickly nor does it soak in as well as slower moving runoff. Therefore steeper fields require wider vegetative filter strips.
For slopes greater than 10 percent, filter strips are not recommended for any use. When used to treat water from concentrated animal feeding operations (CAFO) or from an animal waste management system, filter strips are not effective for field slopes greater than 6 percent.
Type of flow
Vegetative filter strips work best when runoff water flows through them as a sheet. However sediment deposited on the edges of the field often narrows and speeds up the flow, which reduces the efficiency of the filter strip. For a filter strip to be efficient, you must occasionally remove sediment buildup or install water spreaders to ensure that the water enters as sheet flow.
Rainfall intensity and existing soil moisture influence how well a vegetative buffer strip will capture runoff. Frequent rainfalls will saturate filter strips and reduce their ability to infiltrate water. Once saturated, the filter strip becomes unable to efficiently trap contaminants.
Field cover or vegetation management practices
Field management practices that leave a good residue cover can make vegetative filter strips more effective by reducing the particulate content of the runoff. A corn-corn or a corn-grain sorghum rotation using disk plow tillage and a beans-corn rotation using chisel tillage can leave less residue cover than a corn-corn rotation using chisel tillage or no tillage.
The height of the vegetation on the filter strip also affects its efficiency. When runoff is higher than the vegetation on the filter strip, it will bend the vegetation down parallel to the flow of the runoff. This bending allows the water to flow more quickly across the filter strip and decreases its ability to capture pollutants.
Researchers have compared the quality of runoff from areas with and without filter strips. They found that filter strips reduced the amount of sediment by 61 percent to 97 percent. They reduced nitrogen by as much as 72 percent, phosphorus by as much as 79 percent and pesticides by as much as 97 percent (Table 6).
Where and what should you plant?
Plant vegetative filter strips along lakes, ponds, streams, sinkholes, and drainage diches and canals. Filter strips need to be situated such that the runoff that enters them is slow and shallow.
To prevent runoff from forming channels along the field, have the filter strip follow the contour line of the field. If the flow is already concentrated, use terracing and grassed waterways instead. (See Improving Water Quality with Grassed Waterways, Extension publication L-5532).
Vegetation for filter strips should be native or adapted to local conditions. It should provide uniform cover and have a fibrous root system to help stabilize the soil. The planting could consist of a single species or a mixture of grasses, legumes, and other non-woody plants. They should have stems that are stiff and spaced no more than 1 inch apart.
Table 7 lists vegetation, seeding rates, planting dates, and suitable soils for filter strips in Texas. Local plant species may also be suitable for your specific region. The local USDA-NRCS office can provide an exhaustive list of plants for your area.
A vegetative filter strip is like a pasture or meadow, and establishing one requires most of the same considerations. However, for the filter strip to function properly, you may need to perform additional grading and surface preparation before planting.
Once you have chosen the vegetation, have the soil tested and decide on a seeding method. Follow the soil analysis recommendations with regard to fertilizer or lime. Then seed using either conventional or no-till practices. With either method, the seeds must be spaced properly and have good seed-to-soil contact.
If you choose a conventional seeding method:
- Follow soil test recommendations with regard to liming and fertilization.
- Broadcast lime and fertilizer and incorporate them into the soil.
- Establish firm seed beds.
- Plant seeds to about ¼ inch deep with a drill or other appropriate equipment followed by cultipacking.
Seeds can also be broadcast then cultipacked to ensure good seed-to-soil contact.
The no-till method requires only that you broadcast fertilizer and plant the seeds with a no-till drill. You may need to irrigate to help establish the filter strip. If the plant species establishes slowly, you may plant a companion crop such as spring oats, wheat, or rye to help control erosion and weeds until the primary vegetation establishes fully.
Though vegetative filter strips usually require little maintenance, they must be cared for to work properly. Limit traffic on the filter strip to avoid soil compaction. If flow spreaders are used, keep them level to ensure that they function effectively. Monitor the filter strips to measure their effectiveness and/or determine if they need further maintenance. Table 8 summarizes maintenance needs and solutions.
Why use filter strips?
Vegetative filter strips benefit people and the environment. They control erosion, stabilize stream banks and ditches, improve water quality and wildlife habitats, and beautify waterways.
They also provide economic benefits that include the sale of hay or timber grown on them. Other revenue opportunities include incentive programs like the Conservation Reserve Program (CRP) and the Environmental Quality Incentive Program (EQUIP).
What are their limitations?
Though vegetative filter strips can improve water quality of runoff from urban, agricultural and industrial areas, they do have limitations:
- Land and establishment costs can be high.
- They are effective only if runoff water flows through them as a uniform sheet.
- They probably cannot treat highly contaminated discharges from storm sewers, swales, and channels.
- The irrigation costs to keep them alive may surpass their water-quality benefits.
- They do not directly improve water quality in areas where the soil type does not become suspended in runoff.
The major costs associated with vegetative filter strips are land, seed or sod, fertilizers, equipment, and labor. Though seed or sod and fertilizer are a onetime expense at installation, the expense of land, equipment and labor recur throughout the life of the filter strip.
Though one research study estimates a cost of $62.4 per acre per year, the overall cost of vegetative filter strips will vary according to the land cost, soil fertility, planting and management practices, and the use of the filter strip vegetation.
For further reading
Eck, K. J. Vegetated Filter Strips for Improved Water Quality. Purdue University Cooperative Extension Service. AY-285
Grismer, M. E., A. T. O’Geen, D. Lweis. 2006. Vegetative Filter Strips for Nonpoint Source Pollution Control in Agriculture. University of California Division of Agriculture and Natural Resources Publication 8195.
Leeds, R., L. C. Brown, M. R. Sule and L. VanLieshout. 1994. Vegetative Filter Strips: Application, Installation and Maintenance. Ohio State University Extension. AEX-467-94.
Liu, X, X. Zhang and M. Zhang. 2008. Major factors influencing the efficacy of vegetated buffers on sediment trapping: A review and analysis.
Nakao, M., B. Sohngen, L. Brown and R. Leeds. 1999. The economics of vegetative filter strips. Ohio State University Extension. AE-0006-99.
USDA. 1997. “Filter Strips”. Natural Resources Conservation Service. Conservation Practice Worksheet No.393.
Qiu, Z. 2003. “A VSA-based strategy for placing conservation buffers in agricultural watersheds.” Environmental Management 32(3):299-311
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