Keys to planting the seeds of erosion control
Erosion control blankets and mattings are recognized for their capabilities in reducing soil loss and establishing and/or permanently reinforcing vegetation in areas subject to severe erosive forces. Although there are many different blanket types produced from a wide variety of organic and synthetic fibers and nettings, all share the same ultimate goal — to establish vegetation.
With this in mind, the actual method of placing the desired seeds is a primary consideration when installing erosion control blankets and mattings. Since all seeds require moisture for germination, it makes sense to place the seed within the top one-eighth to one-half inch of soil, where most of the moisture is held.
However, to actually incorporate seed into the soil requires equipment and/or labor that is often impractical on the steep slopes and spatially limited drainage channels where mattings are typically installed. So, most seed applications during blanket installations are done through broadcasting or hydraulic seeding. When using one of these options, the question becomes whether to seed before or after installing the blanket.
Seeding before matting placement is generally the preferred method, since it facilitates direct seed-to-soil contact and has the best possible germination rate.
For example, many wildflower species’ seeds are very sensitive and germinate only under ideal moisture conditions. Thus, the best method is to apply these seeds before blanket installation and to rake them slightly into the soil surface.
Since seeding over a matting after it has been installed may result in reduced seed-to-soil contact, a lower germination rate is normally expected. However, over-seeding is necessary in some cases. On extremely steep slopes with smooth surfaces, where there is a concern for seed migrating downslope before blankets are installed, the rough matrix of the blankets can help catch and hold the seed so migration will not occur.
Also, over-seeding may be helpful in those situations where the seeding was done initially before matting placement, but for some reason successful plant establishment did not occur. Finally, certain turf reinforcement mattings require installation and soil in-filling prior to seed application. However, this is not considered a typical case of over-seeding, since the matting itself is filled with soil.
Increasing the normal seeding rate is often recommended to help account for lower germination rates when over-seeding. The percent increase depends primarily on the openness of the matting structure and the size and density of seeds. When applying very small, dense seeds, such as Bermuda grass, into open-weave fabrics and mattings, increasing the original seeding rate may not be necessary.
However, when working with more dense, fibrous mattings, boosting the seed rate by 10 percent is suggested to ensure that the desired number of seeds reaches the underlying soil. When over-seeding with large, low-density seeds, such as wildflowers and certain native grasses, the normal seeding rate should be increased by 10 percent for open mattings and by 25 percent for more closed meshes.
Furthermore, when applying seed hydraulically over blankets or mattings, water alone is the ideal carrying agent. Tackifiers or fiber mulches will tend to cause the seed to adhere to the matting structures, rather than allowing it to pass all the way through to the underlying soil.
Adequate moisture is the key to seed germination, and the soil holds this key. Thus, striving to maximize seed-to-soil contact will help lead the way to successful seedings with the use of erosion control blankets and mattings.