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.
Torch Tests
As expected, sod and brick chips did not ignite under any of the conditions tested. Additionally, the brick chips did not rapidly expand or explode, even with longer exposure to the flame of up to three minutes. Only debris such as grass clippings on the brick mulch burned, but it was not in sufficient quantity to allow the flame to propagate itself after the torch was removed. The torch killed grass foliage in the sod replication, but the live tissue did not allow the flame to propagate itself after the torch flame was removed.
One of the most ignition resistant of the organic mulches was cocoa shells. Cocoa shells were statistically more fire resistant (alpha = 0.05) than decorative ground rubber, pine needles, oat straw, shredded hardwood bark, and shredded cypress bark (Table 2).
| Table 2. Ignition Ratings of 13 Mulching Materials Commonly Used in Landscape Maintenance Operations. Ease of Ignition Ratings Is the Average of the Torch Trials with Four Replicates in Each Trial. | ||||||||||||||||||||||
| Mulching Material | Ease of Ignition Ratingz | |||||||||||||||||||||
| Decorative ground rubber | 7.00 | |||||||||||||||||||||
| Pine straw (needles) | 6.88 | |||||||||||||||||||||
| Oat straw | 5.00 | |||||||||||||||||||||
| Shredded hardwood bark | 4.13 | |||||||||||||||||||||
| Shredded cypress bark | 4.00 | |||||||||||||||||||||
| Ground recycled pallets | 3.75 | |||||||||||||||||||||
| Pine bark nuggets (1 to 2-inch) | 3.25 | |||||||||||||||||||||
| Pine bark nuggets (1/2 to 1-inch) | 3.13 | |||||||||||||||||||||
| Shredded pine bark | 2.88 | |||||||||||||||||||||
| Cocoa shells | 2.63 | |||||||||||||||||||||
| Composted yard waste | 2.13 | |||||||||||||||||||||
| Bluegrass sod | 2.13 | |||||||||||||||||||||
| Brick chips | 1.13 | |||||||||||||||||||||
| LSD05 | 1.19 | |||||||||||||||||||||
zEase of ignition ratings:
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Composted yard waste and dyed ground recycled pallets responded interestingly and resisted ignition using the propane torch for 15 seconds (Table 2). Cigarettes, on the other hand, ignited both materials as readily as any material (Table 1). Composted yard waste smoldered when ignited by cigarettes but did not burst into flame; however, the recycled pallets, when left smoldering for five minutes, did burst into flame.
The longer the time that a cigarette smolders on the surface may well be the difference. Smoldering mulch (duff) may be as dangerous as flaming mulch to the surrounding plants (Dickinson and Johnson, 2001) and introduces the impact of soil heating that is not seen when the duff layer does not smolder (Miyanishi, 2001). The smoldering mulch might well escape detection and be allowed to burn longer. Further, the longer the time that smoldering mulch would be in contact with the bark of a tree or a shrub might result in greater cambial heating and thus more extensive cambial injury.
Hardwood and pine bark mulch products were generally intermediate in tolerance to torch ignition. While not always statistically significant, pine bark mulch products were more torch resistant than hardwood products (Table 2). While hardwood mulches caught fire, the flames and embers usually died out without being extinguished. Generally, the torch ignitions failed to propagate themselves with ease of ignition ratings below 4 with the exception of shredded hardwood bark.
Pine straw and oat straw usually had to be put out by the investigators as shown by their ease of ignition ratings of 5 or higher (Table 2). Simply speaking, the fires were propagating themselves during torch ignition. These mulches would be expected to be a significant concern in the landscape.
Decorative ground rubber ignited each time it was exposed to the propane torch and produced spreading flames in 60 seconds. Decorative ground rubber always had to be extinguished (Table 2). The flames often spread rapidly and deeply into the material. They were extinguished with difficulty by beating the flames out. Water would spread the burning material. Decorative ground rubber is sometimes recommended for use in playgrounds to cushion falls but, in our judgment, is far too easily ignited for this use. The article Playground Fires Tied to Cigarettes reaffirms our concern (Columbus Dispatch, 1997).
The mulches that were the most fire resistant under all methods of ignition were: cocoa shells, sod, 1- to 2-inch pine bark nuggets, shredded hardwood, and brick chips under the test parameters. These mulches might serve as standards for further testing.
One of the purposes of this study was to employ natural landscape conditions and to identify areas for further study. Some of the mulches were ignited by cigarettes that smoldered on the mulch surface for several minutes but not by 15 seconds of exposure to the torch, even though the torch ignition temperature was presumably much higher. This suggests that the length of time the mulch is subjected to the ignition source as well as its actual temperature will affect mulch ignition. Moisture content of the mulches is another area deserving of study. Finally, the ignition point of various mulches should be defined under standardized test conditions.