Wear

Turning tires tire the turf.

Turfgrass wear is above-ground damage caused by frictional abrasion, crushing, and shearing. Poor traction and wheel slippage ("spinning out") worsen abrasion. Vehicles as light as golf cars can be quite damaging (5), especially when the turf is wet. Abrasion may be exacerbated by poor turfgrass cover and the clinging of soil particles to tires, acting as a scouring agent. The importance of leaf abrasion has been minimized by some researchers. This is plausible "from the survival of grass and weeds growing in cracks of city sidewalks" (14). Leaf abrasion is repaired quickly because turfgrasses extend new leaves from intercalary meristems, but stolon abrasion is a long-term injury. Crushing damage occurs following traffic on wilted turf, and is typified by rapid discoloration.

Turfgrass wear tolerance is derived from cell wall components (9) which occur in greater amount in warm-season, C₄, grasses. As would be expected, C₄ grasses are more tolerant to wear than C₃ grasses (15). Among Florida grasses, bahiagrass (Paspalum notatum Flugge) is generally regarded as the best in wear tolerance, as well as being high in root strength. Because they have strictly above-ground stems, stoloniferous grasses such as St. Augustinegrass (Stenotaphrum secundatum [Walt.] Kuntze) and centipedegrass (Eremochloa ophiuroides [Munro] Hack.) are more susceptible to abrasion and crushing than rhizomatous species such as zoysiagrass (Zoysia spp.) and bermudagrass (Cynodon spp.). Total verdure (above ground biomass) is also important in wear tolerance (9) and simply raising the mowing height can increase wear tolerance (15).

Recuperation is as important as direct wear tolerance. Because of its slow growth rate, zoysiagrass recuperates poorly from traffic damage (15), while bermudagrass recuperates quickly. Factors which promote fast growth and root initiation encourage recuperation. Potassium fertilization has not yielded consistent results. Recuperative differences partly explain the greater severity of traffic damage in shaded areas (2), where recuperation is minimal. For this reason, shade tolerant species such as St. Augustinegrass, particularly dwarf St. Augustinegrasses, may have a place in shaded, lightly trafficked areas.

Compaction and Stability

Storm drain to augment natural turf infiltration.

Compaction, the antagonist of percolation, has been studied with artificial simulations. Foot and wheel traffic press together fine soil particles, crush soil aggregates, reducing pore space. This is indirectly harmful to turf because pore space is necessary for movement of water and oxygen to roots. If at least 70% sand is present, compaction is less serious, and an acceptable percolation rate is sustained in bare soil (11). Percolation is greatly decreased once vegetation has been established, thus probably no more than 3% clay is appropriate (4). Medium sands (0.25 to 0.50 mm particle diameter) provide minimally adequate (up to 15%) water availability, and good drainage (3). Mechanical aeration is effective to open up compacted soils. Most native Florida soils are sandy marine sediments not subject to compaction problems, but may provide insufficient vehicular stability.

Surface stability of sand is weak because sand particles have little cohesion and little surface traction, although cohesion is improved at or near field capacity (6). Compaction by linear wheel movement or pressure rolling of wet sand actually increases the cohesion of sand, and is beneficial. Grasses quickly increase soil shear resistance because of their roots (13). Root organic matter is the main determinant of surface traction in sand, and above-ground plant material per se is reportedly not so important (6). This must be qualified, because above-ground biomass contributes indirectly to root growth. Turf which is worn above-ground (reduced verdure) may be unable to replenish root biomass.

The shearing action of turning wheels is a serious potential problem for vehicular turf (5). As the tires turn, they exert a high friction coefficient against the ground, which contributes to shearing. Slow moving turns and power-assisted wheel turns on stationary vehicles also stretch the ground in opposite directions, contributing to shear. Turfgrass leaves can thereby be broken or ripped from the stem, and the shearing often extends into the soil, ripping the roots. Loose, unconsolidated soil, such as dry sand, slips and shears more than strong soil. If this process continues beyond the capacity for root recovery, accelerated failure will lead to vehicles getting stuck.

The Human Component

Proper placement of valve box.

Catalytic converter damage.

While wear, compaction, and physical stability are the most important problems in vehicular turf areas, other potential problems occur and can be solved. Catalytic converters on low-clearance vehicles occasionally kill the grass, and may necessitate higher soil moisture before an event. Petroleum leaks from vehicles may be damaging, if vehicles park repeatedly in the same spot. Sprinkler heads and other utilities (e.g., valve boxes) which cannot be located away from trafficked areas must be set very close to grade, and safely reinforced, or else they will be damaged.

Poorly designed utility turf accumulates garbage.

A psychological problem with grass parking areas is that motorists don't know where to park, resulting in erratic parking patterns and traffic jams. For infrequent events involving unfamiliar guests, human parking guides are essential. For other situations, clear signage may suffice, especially if it is supplemented by familiar visual cues, such as topography, bollards, and shrubbery. Turf presents a challenge to the human wardrobe. Narrow heals can puncture the surface or snag on grass stolons or subsurface debris. Smooth-soled shoes can slip on wet grass. Moisture, soil particles, plant materials (e.g., seeds), and insects may be collected on shoes or other apparel and carried into buildings. Maintenance of adequate turf cover is the best prevention, but it is also desirable to provide a passive method for cleaning of footwear in the entryways to buildings. There is probably no transportation surface that has been used more widely by more humans for more millennia than grass, yet it requires reacquaintance. This is another opportunity for improved signage and public relations.

03 May 1998

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