Larry G. Steward, Assistant Professor of Horticultural Industries Technologies, Agricultural Technical Institute, The Ohio State University, Wooster, Ohio; T.Davis Sydnor, Professor of Urban Forestry, School of Natural Resources, The Ohio State University, Columbus, Ohio; and Bert Bishop, Senior Statistician, Computing and Statistical Services, Ohio Agricultural and Research and Development Center, The Ohio State University, Wooster, Ohio.
In order to be able to study the ignition potential of materials for mulches to be used in the landscaping process, the researchers selected 13 mulches commonly used and recommended for landscape operations (Sydnor, 1994; Rose and Smith,1996; Williams, 1996; Relf, 1997). These mulches were then evaluated for their ease of ignition under natural field conditions.
Four replicates of the 13 mulches were arranged in a randomized block pattern. There was no blocking effect in any of the tests; thus, the data was analyzed as a completely random design.
The mulches selected for study included the following organic materials — shredded pine bark, shredded hardwood, shredded cypress, composted yard waste, 1/2-inch pine bark nuggets, pine straw (needles), dyed recycled wooden pallets, cocoa shells, oat straw, and mixed grass sod. The following inorganic materials were also included — brick chips and ground rubber tire mulches. Materials were assumed to be equally ignitable.
Aluminum edging strips were used to divide the test areas into circular areas of 0.84m2 (1 yd2) each. The aluminum edging strips were used to prevent mulch from igniting adjacent blocks and contaminating adjacent areas with other mulching materials. Additionally, turf was employed to further separate the test areas.
Mulch depth for each sample was the recommended maximum landscape depth of four inches (Sydnor, 1994; Appleton and French, 1995).
The evaluation was done in an open sod-covered area at Ohio State's ATI facility at Apple Creek, Ohio, so that natural conditions were representative of typical landscape conditions in Ohio. The purpose of this test was to demonstrate what could happen under natural environmental conditions. Further, the mulches were not treated with any fireproofing material (Hickman, 1996).
Mulches were applied to the test blocks in the fall of 2000 from bags or bulk piles. The mulches were allowed to settle for two weeks prior to initiating ignition tests. The mulch materials were subjected to ignition by cigarettes, matches, and a propane torch.
For the cigarette ignition tests, student volunteers were asked to ignite and then discard three lighted filter cigarettes on the surface of each of the four replicates of each mulch sample. The cigarettes and mulches were monitored for 20 minutes to determine if the mulch material ignited. The period of time was measured in seconds from application of the burning cigarettes to ignition of the mulching material. Flames in excess of 15 cm (6 in.) high were noted and then extinguished.
The cigarette ignition trial was conducted twice. The first test was conducted on November 2, 2000, 14 days after the mulch was applied. Weather conditions on that day were relatively calm with temperatures near 50°F. There had been one day since any measurable rain, and the average relative humidity was 68% (Anonymous, 2002b). Wind was reasonably calm, and the test was done in the late afternoon.
The second cigarette ignition test was done on July 27, 2001, to determine if there is any change in the ease of ignition due to natural weathering. Air temperature was 65°F at the time of the test. Wind was reasonably calm on this date also, and the test was done in the late afternoon. The relative humidity averaged 54%. It had been one day since any measurable amount of rain (Anonymous, 2002b).
Match ignition tests were conducted on April 26, 2001. Three wooden matches were lit and thrown onto the surface of the four replicates of each mulch. The time until an active flame occurred in the mulch or until the matches burned out was recorded. The conditions at the time (mid-day) were clear, 55°F, and a light breeze (5 mph gusting to 7 mph). It had been two days since any measurable rainfall. Relative humidity was an average of 58% (Anonymous, 2002b).
The propane torch ignition tests of the mulches were done on November 2, 2001. The flame of the propane torch was in contact with the surface of each of the four replicates of the mulch samples for 15 seconds. The torch flame was then removed, and the time until flames or coals extinguished was recorded up to 60 seconds. Residual flames or embers were mechanically extinguished after 60 seconds.
Ease of ignition following the torch ignition tests was rated on a scale from 1 to 7 as follows:
| 1 | = | No flame or embers @ 15 seconds. |
| 2 | = | Flame @ 15 sec. but no embers @ 30 sec. |
| 3 | = | Flame @ 15 sec. and embers @ 30 sec. but no embers @ 60 sec. |
| 4 | = | Flame @ 30 sec. but no embers @ 60 sec. |
| 5 | = | Flame @ 15 sec. and embers @ 60 sec. with embers extinguished. |
| 6 | = | Flame @ 30 sec. and embers @ 60 sec., with embers extinguished. |
| 7 | = | Flame @ 60 sec. with flames and embers extinguished. |
The environmental conditions that afternoon were clear, 10°C (50°F), and an average relative humidity of 83%. It had been a week since any measurable amount of rain had fallen (Anonymous, 2002b).
Note: Data from the torch ignition tests were evaluated using analysis of variance with mean separations using the least significant differences. Ignition following the cigarette ignition and match ignition tests was rated in a binary fashion as either igniting or not and evaluated using logistic regression tests. Differences were determined using Fisher's Exact Test.