The global Impact of Grass Cover
It seems logical to establish vegetative cover on critical bare areas, and grass is the only cost-effective solution. Approximately 650 genera and 10000 species populate the earth - sparingly in waste, forest-covered and barren lands but densely in open savannahs, mountain sides, fields, roadsides and reclaimed land. In fact grass growth thrives everywhere on the earth's surface with the exceptions of the areas at the poles under ice. Grass species have hybridised naturally and within species there are distinct variants that have adapted to chalk land, high altitudes, sand dunes, shallow rocky soils, arable land and the water's edge. In addition, there are interspecific hybrids of which Agrostis and Festuca play a major role in the establishment of sporting surfaces
The true value of grass
If we pause to consider the value of grass cover, its benefits are so numerous that they highlight the disastrous impact that would arise from destroying it.
• Grass seed is plentiful in most soils
• It stabilises the soil on natural and disturbed land
• It controls dust and binds the soil surface
• It restores organic matter after arable cropping
• It cools the soil surface and reduces extremes in ground temperatures
• It provides a vital link in nature's ecosystems
• It gives pleasure in enhancing the value of our homes, cities and the countryside
• It provides an acceptable surface for relaxation and sport
• It acts as a buffer against wind and water erosion
Much has been written on grass. 'Grass is the forgiveness of nature', it can be abused and trampled, it can be removed and replaced, yet it is immortal and in its own time it always returns to resume its role in covering the earth.
Stabilising critical areas
On steeper gradients and rocky terrain there is difficulty in gaining access with mechanical equipment. In these instances hydroseeding is a viable solution. It involves spraying the seed in the fluid organic mulch and this is achieved from an agitated tank with pumping equipment capable of reaching distances in excess of 20 metres.
Where ground is particularly unstable the earthworks design is vital and reinforcement or the use of rock-filled gabions must be considered. The surface of steep slopes exposed to high rainfall intensity or persistent dry conditions can make establishment difficult. The use of straw and organic mulches offers good protection and water retention during the period of germination and establishment. In addition, the use of geotextiles and biodegradable matting provide a means of stabilising and securing growth.
The placement of grass turf specifically lifted from a grass nursery and pegged in position also provides an instant means of stabilising the soil where immediate protection is required. In addition to the bulk of forage grasses that have been used successfully, Flat meadow grass (Poa compressa) has adapted to dry and shallow soils.
On mine spoil, land fill and slime dams the soil type can be unsuitable for germination. Liming becomes an essential need where low pH values are prevalent. Furthermore, some soils are prone to crust at the surface. Maintaining a moist surface with use of straw mulches or bio-degradable netting becomes vital to secure the acceptable micro-climate conditions.
Low protective fencing may also be necessary to contol wind erosion. The choice of species is also important - especially where toxic levels of certain minerals are present. A cocktail of species are usually tried but Crested dog's tail (Cynosurus cristatus L) tolerates acid conditions and Cocksfoot (Dactylis glomerata L) recovers well in hot dry weather.
The reclamation of contaminated land today is a major concern with the need to develop brown field sites. The extent of contamination both in depth and area can be a detailed exercise in addition to the location of the source - if it still exists. Besides the removal of contaminated soil there is a further need to monitor leachate and restrict mineral rich water reaching natural water courses. The construction of settling ponds is a further requirement.
Stabilising coastal sand dunes has become vital in isolated parts of the world and along parts of our shoreline. In addition to initiating grass growth there is the need to control the influence of the wind within the first metre above the ground. The erection of temporary netting becomes essential. Planting rhizomes of marram grass (Ammophila arenaria) protected with straw mulch or biodegradable hessian netting has proved successful in many parts of the world. In reclaiming land where desert encroachment is persistent spraying bitumen and PVA emulsions have proved successful.
Promoting sustainable drainage
In the current move to drain surface water in a more controlled manner to prevent soil contamination and reduce the impact of flooding, grass cover plays a major role. On flat and depressed areas, grass cover with relatively shallow rooting cannot cope with flooding or incessant rainfall and gradually deteriorates. Ideally, the retaining and diverting surplus water flow above stabilised slopes to suitable collection areas is vital in the long term success of ground cover.
In areas where surface water flow off site is restricted or flood water brings contamination, the attenuation or temporary storage of surplus water serves to reduce the peak flow and discharge this water more slowly at a manageable rate over a longer duration. Contamination can be managed at its source. In urban areas built up with restricted infiltration, this measure becomes an essential part of the preliminary design in order to prevent the harmful effect of untimely flooding. The incorporation of ditches and swales in the reticulation of storm water becomes an essential need in the conservation of sensitive grassed areas.
Preliminary drainage in the form of lateral drains, together with supplementary slit drainage installation on sports pitches, also offers a degree of containment or attenuation. Together with flatter graded surfaces and matted grass cover these implementations provide considerable retention before run-off commences.
In low lying areas the passage of surplus water flow must be adequately planned to ensure long term stability. Water catchment areas must be defined and, using hydraulic models, water flow can be pre-determined for set rainfall intensity over a required return period. Preparing for a 1:100 year storm requires costly preventative measures. Where damage to surrounding areas is of low risk consideration of a one or five year return period enables more cost-effective measures to be applied.
In summary, grass species have adapted to the ends of the earth. Grass cover flourishes where conditions are optimum, but persists and becomes invaluable in the most unfavourable conditions of soil type, gradient, contamination and drought. Utilised in the conservation of land, grass cover provides a sustainable solution wherever the soil is to be covered by natural means. Knowing its shortcomings and the adverse affects of climate, measures can be taken to establish adequate grass cover that serves to stabilise, enhance and restore nature's natural cycles.