Revegetating with native grasses, wildflowers, and fescues after construction

By David R. Chenoweth

When I was a young environmental scientist, my superiors often stressed the importance of using environmental baseline studies to formulate revegetation plans. Their words and training stay with me to this day and have played a significant role in the success and quality of the outcome at the new Village Lakes residential project in Castle Pines, CO.

Native fescue blend that looks like bluegrass.

Located near a famous Colorado plateau named Castle Rock, Castle Pines is a developer's—and homeowner's—dream. For a visual, all a person needs to do is think about the name "Castle Pines." The area is thick with towering pines, a true Colorado forest located high on the mountain plains. Growing, nurturing, and maintaining a landscape for a residential community is a different story, and this presented a challenge to the development team.

The developers looked for a company to write soil preparation, seeding, mulching, erosion control, and maintenance specifications for the new residential community; Western States Reclamation performed the work. As in the earliest days of my career, my team and I first studied soil maps and mapping unit descriptions from the US Department of Agriculture's Natural Resources Conservation Service county soil survey. We were able to glean general information about soils of the project site. Next, we collected soil samples onsite and sent them to a laboratory to evaluate several physical and chemical parameters that might impact our stripping of surface soils and using them for seedbed-quality material (topsoil). The parameters that were tested are listed in Table 1, along with a brief description of what they indicate.

I strongly believe that all development projects should include an onsite soil survey to identify potential sources of suitable topsoil. The use of quality topsoil will provide faster, more successful revegetation results and reduce the erosion potential of the disturbed site.

The results of the data collection indicated that the soils of the site were generally sandy loams, meaning that the soils were too coarse and low in organic material for use alone as seedbed-quality material. Therefore, we specified the proper soil amendments to provide a good soil medium for successful revegetation.
The soil amendments of the revegetation plan consisted of 4 cubic yards of good compost material and 1,000 to 1,200 lb/ac of organic fertilizer. The desired result was to provide enough soil amendments in an attempt to raise the soil organic matter content to 3%–4%. Research has indicated this is an optimum level for successful revegetation.

Soil organic matter content is also important for long-term erosion control of surface soils. Soil organic matter breaks down into beneficial humic acids, which will create soil aggregation and ultimately "soil peds." Soil peds are much more erosion resistant than soil clods, which break down from exposure to intense rains and erode easily.

Often on other projects I have seen fairly good seed mixtures and mulching specifications prepared, but little if any consideration is given to preserving good topsoil or amending poor soil. In addition, poor erosion control planning has resulted in many projects eroding or washing out before vegetation could establish and provide better overall erosion protection. Successful revegetation cannot be achieved without a minimum depth of good seedbed-quality material.

The next step in the Castle Pines project was to design multiple native grass seed mixtures that would reflect site soil types, slope aspect, and species common to the local ecosystems. The landscape architects were also interested in textures, colors, and heights of grasses and wildflowers that would blend with various landforms and landscape features within the project. Therefore, we decided to use a mixture of predominately native grasses, including both warm-season and cool-season species. While native grasses are all various shades of green for most of the growing season, cool-season grasses tend to turn a straw color in the fall and warm-season grasses turn a prominent orange to reddish color. The two species and colors provide a great color pattern in the fall and complement other landscape features of the project.

The problem is that warm-season grasses are very difficult to establish at times with cool-season grasses. Therefore, careful consideration must be given to the ideal season for planting and the percentage of warm-season seed that is needed to help offset the more aggressive nature of cool-season grasses. Cool-season grasses tend to green up in April and go dormant in late September or early October. Warm-season grasses tend to green up in late May and go dormant in mid-September. Warm-season grasses are generally much more drought tolerant than cool-season grasses. The best planting period to allow warm-season grasses to compete with cool-season grasses is probably from mid-May to late June.

Table 2 lists the warm-season and cool-season grasses that were used in various native grass seed mixtures. The native fescue turfgrass blend is included in Table 3. Because the construction of the project has spanned over three years to date, observations could be made every growing season as to what species were doing the best and what species the client seemed to prefer. Thus, the native grass seed mixtures were adjusted several times during 2000–2003 to accommodate these observations. Table 4 reflects wildflowers used in various mixes and percentages throughout the native grass seeded areas.

Mulching was specified to help control erosion and to cut down on the loss of soil moisture. Initially, hydromulch was specified to keep from contaminating the seed mixtures with undesired native and introduced grass species as well as cheat grasses that would result from using native hay or straw mulch.

Although we knew that weed invasion should not be as bad on an irrigated native grass area as on a non-irrigated area, weed control was going to be very important and had to be carefully planned to avoid damage to newly germinating grasses. Therefore, a combination of mowing and herbicide applications was specified.

Mowing was used initially to help cut off weed seed heads and keep the canopy of the weeds down so native grasses would not be shaded out. Mowing was specified at a 4-inch or greater height. Shorter mowing can impact the reproductive parts of native grasses, especially warm-season grasses. Also, shorter mowing can cause sunscald and dieback of native grasses during the hot summer months. Spot treatment of weeds with a wick applicator was also specified to help reduce local areas of noxious weeds such as Canada thistle. Broad band applications of herbicides were only specified after native grasses had reached mowing height, which is felt to be the stage during which damage from herbicides will not occur.

During 2000, Castle Pines and DHM Design personnel requested bids for the initial phases of the project. It was determined that the project should be split between Valley Crest completing all irrigation and landscape plantings and Western States completing all soil preparation and mulching.

The first two years of installation of seeding had its own problems, as on most projects. The project construction schedules consisting of road construction, infrastructure placement, backfilling, and grading simply would not allow seeding to occur each year within a six- to eight-week schedule, which was the best for establishment of warm-season and cool-season species together.

The seeding of the first phase of the project was pushed from midsummer to late summer and early fall. We were running out of time to get the grasses established before soil temperatures would be too cool for seed germination. Generally, it is too cool to germinate warm-season grasses by the second week of September and too late for cool-season grasses by the third to last week of September.

Another challenge was that the temporary water supply from the metro district was restricted for new landscape use. We did have some winterkill of cool-season grasses, and little or no warm-season grasses germinated. Therefore, the problem of giving the warm-season grasses a chance to establish was partially overcome by two means. We would either increase the seeding rate of the warm-season grasses or seed the warm-season grasses the first season and interseed the cool-season grasses the following growing season.

Because the construction of the project has spanned over three years, we adjusted the native grass seed mixtures several times to better establish the blend of grasses desired. The wildflowers were not seeded initially, because weed control—consisting of mowing and herbicide applications—would have killed them. Wildflowers were planted only after it was felt the grasses were established and weed control would not be needed.

Douglas County has a specification that does not allow hydromulch to be used on native grass areas; it can only be used on turfgrass areas on relatively flat surfaces. For this reason, we switched to straw mulch, which ultimately created problems. Viable wheat seed always exists to some degree in wheat stalks. Therefore, volunteer wheat being introduced into the seeded area can create a competitive problem with the native grasses that were seeded. This is exactly what happened on a portion of the Castle Pines project. The volunteer wheat proliferated to the point that it shaded out and robbed soil moisture from the native grasses, creating a seeding failure.

Temporary erosion control measures such as erosion bales, silt fence, and touch-up mulching were important, because the seed mixtures consisted predominantly of grass species that were difficult to establish yet long-lived and very drought tolerant.

The general manager of the Castle Pines project hired me halfway through the project to complete a monthly walk-through to educate him on how the project was advancing and whether any corrective measures needed to be taken. This level of involvement by Castle Pines has now resulted in a showcase project of how beautiful native grasses, wildflowers, and native fescues used as a turfgrass alternative can look. In fact, many visitors mistake the native fescue manicured turf mixture for bluegrass. Water-meter readings have proved that after the first year of grow-in, we have cut water consumption in approximately half of what would be required for bluegrass turf.

Today, the Village Lakes development in Castle Pines is a sparkle in one of Colorado's true gems of a community. The landscaping, a result of the revegetation studies that were completed early on, has truly enhanced the beauty of the natural surroundings and helped make Village Lakes one of Colorado's most desirable places to live.

David Chenoweth is president of Western States Reclamation Inc. in Broomfield, CO.

EC - March/April 2005