Before you start
Stands of native grasses can be established either specifically for seed production or for other end uses. Stands for seed production should be designed to produce the maximum yield of harvestable seed in the minimum period of time whereas other considerations are important when establishing stands for other purposes. The following section deals with 'end use' establishment whereas establishing stands specifically for seed production is dealt with in Section 6.
Planning
The goals must be clear before commencing any restoration or revegetation program. Goals should be developed in the context of land-use and/or environmental suitability. For example, is the aim to recreate wildlife habitat, to restore agricultural production or to create an aesthetically pleasing landscape? How extensive is the revegetation activity? Are whole paddocks being revegetated or are you concentrating on small selected sites in different parts of a complex landscape? Different revegetation objectives may require different approaches.
Selection of appropriate native grasses for a particular site will depend on the proposed use of an area, the site characteristics (including local flora), availability of seed supplies as well as climatic conditions. Establishment of a native grass stand is therefore more involved than establishing a monoculture of an exotic species. The best-bet option is to:
- Start with a weed free site, where possible
- Ensure species used match site conditions and final land use
- Use the highest quality seed available
- Use appropriate sowing rate to optimise establishment success
- Manage weeds until grasses are established (flowering)
Pre-sowing management
The choice and mix of species to be reintroduced into an area will primarily be driven by local site conditions. For example, factors such as soil characteristics (topsoil depth, soil texture, elevation and slope, soil fertility, pH and salinity) will influence the choice of species.
Past cropping, fertiliser and herbicide histories can provide useful indicators of potential weed problems. It is also the local conditions that will determine the pre-sowing activities required, such as cultivation and/or spraying of weeds. For example, a heavily weed infested area may require a clean seedbed up to twelve months ahead of sowing. On the other hand, sowing into existing degraded pastures will require little pre-sowing treatment provided there is not a large soil seedbank of weed seeds present. In hostile mine site environments, major earthwork activities will be required prior to seeding.
Stockpiling topsoil, sometimes for periods of years, is a common practice when revegetating land surfaces following major landscape modifications such as mining or highway construction. The stockpiled topsoil is then spread on the substrate surface and used as a seedbed for sowing the selected seeds. The seeds of very few native grasses will survive storage in a topsoil stockpile whereas weed seeds usually have efficient dormancy mechanisms that allow them to come through such treatment unaffected. Therefore, the population of weed seeds increases proportionally during topsoil storage and usually causes substantial problems if native grasses are sown subsequent to spreading. Many native grasses are well adapted to becoming established in inhospitable environments and it is sometimes preferable not to use topsoil at all but to sow directly into the substrate following landscape modification.
The germinable weed seed load of topsoil can easily be estimated by taking samples of the topsoil, spreading them in trays and watering them in a glasshouse or other suitable environment and counting the number of seedlings that appear per unit of tray surface. The trays should be watered for at least four weeks to obtain a reasonable estimate of the seed load. In addition, if the seedlings are harvested and the trays dried down before watering again for another four weeks, additional seedlings will nearly always appear. The suite of species that appear in autumn/winter will usually be different from those that appear in the spring/summer. Figures of the order of 50,000 seedlings/m² are common and represent a formidable competitive hurdle for sown native grasses. If the number of seedlings is less than 100/m², then sown native grasses would have a much greater chance of becoming established. It is not necessary to be able to identify the individual species to get a good idea of the seed load of a sample of topsoil.
All native grasses can be separated into either warm or cool season species. Warm season grasses, also known as C4 species, do most of their growing and seeding in the summer months, and produce a 4-carbon compound as the first product of photosynthesis. Cool season grasses, also known as C3 species, grow and seed in the cooler months (winter/spring) and produce a 3-carbon compound as the first product of photosynthesis. Warm season grasses are better adapted to higher temperatures, higher light intensities and lower CO2 concentrations than cool season plants. They are susceptible to frost and are winter dormant. Cool season grasses are frost tolerant, provide green feed during winter, have reduced growth in summer and survive high temperatures and low rainfall by entering a drought induced dormancy. When temperatures are low and moisture not limited, growth is resumed, and we often see some grasses flowering apparently 'out of season'. The species mix you sow will need to take into account the seasonality of rainfall for your area. Establishing a community of both cool season and warm season grasses in a restoration program provides the opportunity for year-round growth if seasonal conditions permit.
Determinate flowering
Perennial grasses that usually have their major growth period in the cool season followed by a period of summer dormancy, such as Elymus scaber (Common wheat grass), often have determinate flowering, particularly in southern Australia where summer rainfall is limited. The flowering period of such grasses is usually short in spring and plants make little growth during the summer, even if rain falls at that time. In regions with higher regular amounts of summer rain, flowering may be less determinate, particularly if there is a dry spring and some rain in mid-summer. Under such circumstances, flowering may even extend into autumn, depending on the particular rainfall pattern in an individual year. The growth and flowering season of perennial grasses is usually determined by the interaction of their growth and reproductive responses to temperature and day length and the rainfall patterns in any particular year.
Some perennial grasses that have their major growth period in the warm season, such as Themeda triandra, also have determinate flowering, in that each plant flowers only once per summer. Individual plants may flower at different times, however, thereby giving an appearance of indeterminate flowering of the population. If there is a wet spring followed by a dry early summer, flowering will be concentrated in the early summer and even if good rains fall in mid- to late- summer, no more flowering will occur. On the other hand, if there is a dry spring followed by good rains throughout the summer, some plants may be flowering at any time over a period of several months.
Indeterminate flowering
Many frost-tolerant perennial grasses show indeterminate flowering and produce inflorescences throughout the summer, provided soil moisture is available, as in the case of Microlaena stipoides. In such grasses the total mass of herbage produced in the summer may be much higher than in the winter period and such grasses are sometimes referred to as 'year-long green'. Rainfall patterns have a marked effect on flowering and if there is a wet spring followed by a dry summer, they may appear to show determinate flowering. In other years, depending on the pattern of rainfall events, such grasses may have one flowering period in the spring and another in the autumn with few flowers in between.
Some warm-season perennial grasses also have indeterminate flowering, as in the case of Bothriochloa macra. These species are able to produce inflorescences (Inflorescence - a group of flowers borne on a stem) throughout the summer, depending on the availability of soil water. The first inflorescences are produced later in the spring than those of year-long green grasses. Some species produce many more inflorescences in the latter part of summer, irrespective of the availability of soil water in early summer, as instanced by Sporobolus creber on the Northern Tablelands of NSW.
Sculptured seeding
Sculptured seeding is the name given to the matching of seed mixes of native grass species to varying site conditions. It provides an ecological approach to revegetation activities based on an understanding of the natural vegetation in the area. A major objective of this approach is to establish a diverse plant community that is capable of natural regeneration, plant succession and dynamics. Significant areas within the Canadian Prairies and the American Great Plains region have been 'revegetated' using this technique.
Sculptured seeding recognises that landscapes are not homogeneous, and that land is an endless series of mosaics. Even when we have cultivated a paddock to provide a uniform seedbed a degree of patchiness will still exist within the landscape. This may be a gradual change in pH, an abrupt change in soil structure or a patch that receives more moisture than surrounding areas. An understanding of how a landscape is organised, no matter how fundamental, is the first requirement before 'redesigning' landscapes. This approach implies that conventional revegetation activities such as sowing monocultures will simply not be appropriate for all revegetation activities. It is the restoration of plant community function through the re-designing process that is the ultimate goal. Figure 2 demonstrates how an appropriate mix of species can be designed to reflect the composition of species that occurred naturally prior to revegetation activities.
