Temporary Grassing Options for Winter Soil Stabilization

Introduction

Soil disturbance due to construction, building, or renovation projects does not pause during the coldest parts of the year in the Mid-Atlantic, and there is still a need to stabilize the soil to minimize erosion losses and reduce potential water pollution. Sod installation on a properly prepared soil is almost always an effective alternative for immediate soil stabilization and the potential for some level of site usage with limited foot or vehicular traffic. Sod installations are generally successful even during the winter months, as long as the turf receives adequate moisture to prevent desiccation. However, the installation of sod can be cost-prohibitive in some situations, and in instances where additional land use disturbances might be in play in the future, it is ideal to establish grass intended to serve as temporary, winter soil stabilization instead of permanent sod.

So, what are the grassing options for temporary winter soil stabilization, and what criteria must be met for their successful establishment?

Consider seasonal temperature and moisture patterns in selecting the best soil stabilization options

Soil disturbance due to construction, building, or renovation projects does not pause during the coldest parts of the year in the Mid-Atlantic, and there is still a need to stabilize the soil to minimize erosion losses and reduce potential water pollution. Sod installation on well-prepared soil (with appropriate chemical and physical properties addressed before planting) is almost always an effective alternative for immediate soil stabilization and the potential for some level of site usage with limited foot or vehicular traffic. Sod installations of both cool- season and warm-season grasses are generally successful even during the winter months, if the vegetation receives adequate moisture to prevent desiccation. A typical Mid-Atlantic winter typically offers sufficient moisture, allowing for successful sod establishment. Still, Mother Nature is obviously unpredictable in terms of when, what type, and how much moisture is received. For large-scale plantings, sod installation can be cost prohibitive. However, the installation of a mature vegetative carpet certainly has advantages in both short- and long- term soil stabilization, where this establishment strategy can be implemented.

Since sod installations are often not feasible, the alternative strategy involves seeding temporary grassing options for short-term winter soil stabilization, followed by the establishment of long- term, permanent vegetation (Figure 1). Adequate moisture remains critical for success, but the grassing options in this effort exhibit significant differences in seed germination rates based on temperature. What are the best seeding options for grasses used for temporary winter soil stabilization, and what are the primary criteria for their successful establishment?

Temporary cool-season grassing options

Annual ryegrass (Lolium multiflorum Lam.). In practice, annual ryegrass is likely overused in public efforts to gain temporary winter soil stabilization. Its use is most likely due to public familiarity with its application for winter overseeding of warm-season lawns and athletic fields. It being a common component of seed mixtures referred to as ‘contractor’s mix’, its rapid seed germination rate (and soil coverage) are strengths that annual ryegrass provides in this mix. Still, as an annual grass, it will eventually die during extreme environmental periods when high heat and/or drought occur, leaving voids in the grass canopy. Annual ryegrass is also not exceptionally cold tolerant and during extreme winters, there can be significant stand loss.

However, under ideal growing conditions, it is extremely aggressive, and it is important not to use too much annual ryegrass as it can be so competitive when trying to establish a perennial grass.

This grass is also called Italian ryegrass, and although the names are similar, it is distinctly different from cereal/winter rye (Secale cereale L.), a small grain crop.

Annual ryegrass prefers fertile, well-drained soils. Under ideal moisture and temperature conditions (soil temperatures of 60°F or above), it will germinate in 3-7 days. It will continue to germinate even as soil temperatures approach 50°F, but germination rates will decline and stand densities for the winter can become problematic as soil temperatures continue to drop. For this reason, annual ryegrass has limited application for winter soil stabilization species at colder temperatures and should only be used under warmer soil conditions. One strategy that is commonly employed builds upon its purpose as a component of ‘contractors mix’: use annual ryegrass as a ‘nurse crop’ with a slower establishing turfgrass like Kentucky bluegrass at a seeding rate up to 10 lbs. of pure live annual ryegrass seed per acre being applied with a standard seeding rate of bluegrass. It will provide some rapid soil stabilization and at that level will not be too competitive with the bluegrass that is intended to be the perennial turf.

The use of annual ryegrass also presents concerns for farmers when it is used around production agriculture fields. It has become a serious winter annual weed in small grain crops in the Mid- Atlantic. For all these reasons, the use of less competitive annual grasses that germinate at colder soil temperatures is primarily recommended rather than grassing with annual ryegrass.

Cereal (Winter) rye (Secale cereale L.). This small-grain crop is widely adapted to various soil environments but is noted for generally performing better on poor soils than other small grains. It is once again important to note the distinction between cereal rye and the large group of Lolium sp. These are commonly known as ryegrasses. Cereal rye has germination rates of 4-7 days and is noted for its ability to germinate and grow at lower temperatures than other cool-season species. Therefore, it is an important species either as a stand-alone or mixture component for temporary cover when seedings are made well outside ideal establishment periods. If temperatures are in the high 30s to low 40s (degrees F), then the best seeding option is likely cereal rye. It can grow 3 to 6 ft tall by the following spring and will die after completing its growth cycle by June in most parts of the Mid-Atlantic. It is often recommended that persistent perennial species be ‘dormant seeded’ (detailed later) with the cereal rye and management strategies (particularly spring mowing of the cereal rye) employed to promote the development of a permanent turf cover after the cereal rye dies in late spring.

Wheat (Triticum aestivum L.). This small grain is another short-lived, rapidly establishing annual grass that can be used as a temporary cover in cool-season grass establishment. It is one of the cheapest temporary grasses for this purpose. Germination is somewhat temperature-dependent, with rates as little as 7 days at temperatures of 65°F or 14 days at 50°F. However, it still germinates readily to temperatures as low as 45°F, but when soil temperatures are less than 45°F, it is inferior to cereal rye for cold-weather germination and establishment.

Barley (Hordeum vulgare L.). Barley is another small grain crop, like wheat, that can be used for temporary vegetation either as a stand-alone species or in cool-season seed mixtures. Barley is an erect, bunch-type winter annual that establishes within 7 days under optimal soil temperatures of 65°F. It prefers finer-textured soils high in Calcium, pH levels of 6.5-7, and does not perform well in dry conditions. For late season uses for winter soil stabilization, barley will germinate at soil temperatures as low as 45° F, but it is less competitive than cereal rye at lower temperatures. Barley reaches a maximum height of 3 ft by the end of its life cycle, significantly shorter than cereal rye.

Similar to other small grains, it may be necessary to mow this temporary vegetation in late winter or early spring to encourage the successful establishment of the perennial grasses added to the site, which are intended for permanent vegetation.

Seeding rates, site preparation, post-planting care, and introduction of perennial vegetation

The seeding rates and application timing periods provided in Table 1 are recommended by the Virginia Cooperative Extension for the Virginia Department of Transportation's winter soil stabilization and/or companion planting efforts across three geographic regions of Virginia. A standard rate for cereal rye, wheat, and barley is 50 lbs. of pure live seed per acre; however, seeding levels can be increased to 75 lbs. PLS/A if needed for thicker coverage or when trying to stabilize steeper slopes.

*Use 75 lbs/A level when establishing on slopes or trying to achieve greater soil coverage for the winter months.

As far as site preparation, the principles presented in the VCE Agronomy Handbook, Part III (https://www.pubs.ext.vt.edu/content/dam/pubs_ext_ vt_edu/424/424-100/spes-299-C.pdf) should be followed in terms of soil testing, pre-planting soil preparation, and achieving soil-to-seed contact. One area of preparation that often cannot be provided to the same level as that for ideal establishment periods is extensive soil tillage. Under suboptimal planting conditions, some level of soil preparation is required; however, overly aggressive soil preparation may increase erosion rather than help minimize it with the temporary grass. And while Mother Nature will likely provide the moisture for these situations, one thing that can be done to help conserve moisture, moderate soil temperatures, and reduce soil erosion potential is to mulch the seeded areas with up to 1 bale of straw per 1000 square feet of seeded area applied over the seed (where possible, the use of a tackifier agent will help keep the straw in place).

When using temporary grasses for winter soil stabilization, it is anticipated that they will decline in health and eventually die as environmental stressors arrive the following year. The ideal scenario for the successful establishment of the permanent vegetation is that it will now be introduced into the declining stand of winter annual grasses, with minimal competition from the temporary vegetation. Very often, especially on sloped areas, the seed of the improved, perennial grasses is applied simply by seeding ‘over the top’ of the temporary grass. On sites where mowing is possible, the temporary grass would then be cut to a height of 6 to 8 inches to reduce its shading effect on the new seedlings. The ‘straw’ created from the mowing event would be left in place to provide additional mulching benefits for the establishment of permanent vegetation. Of course, seeding cool-season grasses and legumes into the temporary cool-season grass canopy has its own challenges with soil temperature and moisture because now the situation to consider is ideally there are soil temperatures that are warm enough to promote cool-season turfgrass establishment, but not so warm that the new seedlings cannot survive the arrival of summer heat and moisture stress. Also, keep in mind that there is likely a large seed bank of summer annual grasses (such as crabgrass, goosegrass, and foxtail) and numerous other weedy species that will respond to the warming soils.

For the best chance of establishing cool-season turfgrass, monitor soil temperatures and once they are consistently in the 45- 50°F range at a 2-inch depth, that is a satisfactory time to introduce the perennial cool-season grass seed for permanent vegetation purposes. Keep in mind that the cool- season seed can also be introduced to the site in the dead of winter in a process known as ‘dormant seeding’. The concept is that the seed will germinate when environmental conditions (temperature and moisture) are suitable. The primary concern is that the longer the seed remains on the soil over the winter, the more likely something is going to happen to it to reduce germination potential (eaten by birds or rodents, seed washes away in a heavy rain event, etc.).

For warm-season grasses that will be seeded as permanent vegetation, soil temperatures of at least 55°F and above are required for germination and establishment. And once again, the practice of ‘dormant seeding’ warm-season grasses has equal merit (and challenges) for successful permanent vegetation establishment as for cool-season grasses, but the persistently warming temperatures will favor the warm-season grasses.

Summary

Whenever possible, plan permanent vegetation establishment during optimal times to promote both the long-term success of the vegetation stand and benefits of soil stabilization. However, when schedules change and ideal planting conditions are no longer possible, apply the grassing and timing strategies presented in this publication to achieve effective winter soil stabilization.