Choosing Native Grass Mixtures
The best-case scenario for any disturbed site is the restoration of the native ecosystem using only native plant species; but is that achievable? In recent years, seed of native grass species has become more readily available allowing the option of sowing native grass and forb seed, as opposed to seed of introduced species, for restoration projects in British Columbia. There are challenges however to achieving this goal and there may be factors that make introduced species more feasible in certain situations.
Following are several factors to consider when evaluating the potential use of native grass seed blends.
1. Environmentally sensitive areas: In sensitive remote locations, extreme care should be taken to avoid introducing any foreign species. Ideally a vegetation survey should be completed, prior to disturbance, and only species found on site be used in a seed blend to revegetate the area. In the absence of a vegetation survey all aspects of the ecology and geography of the site need to be considered when selecting species for inclusion in blends. Often the area being disturbed is far removed from any native grassland, but the project calls for the use of grasses to provide rapid cover. The success of the project hinges on the suitability of the native species selected for use. In some circumstances shorter lived introduced grass species may be considered for use as a temporary cover allowing nature to eventually reclaim the area.
2. End use of a site: Native grasses may not persist in areas that will be mowed regularly or receive heavy foot traffic. However, in marginal soil or in harsh environments some native grass species may be the best option. Agronomic species may be better suited to areas that are intended to provide forage for animals whereas native species may be more desirable in areas that are more passive and off the beaten path.
3. Budget: Germplasm of native species is collected where the species occur naturally and can sometimes be collected from multiple sites to improve adaptation of the progeny. The germplasm is then multiplied in areas that have a history of successful seed production. Unlike agronomic, or other introduced grass species, these native collections have not been improved through years of plant breeding where seed yield is a selection criteria. Seed yield can be less consistent and seed cleaning can be more challenging for native grass species. Production challenges contribute to the cost of seed of native species. When comparing to seed blends of introduced grass and forb species the cost of native seed can be many times higher
4. Erosion requirements: On steep slopes such as 1:1 or greater, quick establishing species are required. While some native species establish quickly and consistently, other species have high seed dormancy and may not germinate for years or take up to two years to fully mature. On steep slopes where erosion control is critical, a blend of shorter lived agronomic and native grass species may be the safest choice to ensure a consistent vegetative cover. The challenge in these situations is to include the rapid establishing species at a level where they can be effective but not inhibit the establishment of the native grass species. This is achieved by paying close attention to blend composition and sowing rates.
5. Seed availability: Native seed species are not always produced on a large scale and some of the native forb seed is hand collected. We try to maintain an inventory of seed of native grasses and forbs, but not all species are available all the time. Allowing for substitutions may be necessary, and for large projects where significant volumes of seed is required, it is prudent to begin sourcing seed of native species well in advance.