A video has recently gone viral in which a surface completely covered in white burns extremely easily and quickly and the flames leave behind a splendid green lawn, intact after the passage of fire. The tweet read, in English, “ it is the season to burn pollen ”, and although it was published on April 29, 2022, the original video was uploaded by the local media outlet NueveCuatroUno on May 6, 2020.
The video is recorded in the Cicados park, in Calahorra. There were those who thought it was a trick, that the video was upside down . Nothing could be further from the truth; The video is not upside down, nor is pollen burning in it .
It is not pollen… but it contains pollen
It is widely believed that those masses of fluff that, from the beginning of May, begin to flood parks and gardens, streets and highways, are accumulations of pollen . This image is often accompanied by the expression: “Look at the ground, it’s all full of pollen.”
It is not just a popular belief, some media outlets also broadcast the image on the screen of a large avenue completely carpeted with those poplar “fuzz” to illustrate –according to them– the abundance of pollen that the season brings. “Look how the street is, all full of pollen, allergy sufferers should be careful.”
Actually that fluff is not pollen. The pollen is too small to see with the naked eye ; Only when there are large concentrations in a very localized way, it can be seen as a fine yellowish powder. What look like huge fluff are actually poplar seeds . Each one of them has a large number of branched hairs —called trichomes— forming a structure called pappus , whose function is to facilitate its transport by the action of the wind, and the strategy works. And besides, these seeds do not cause any allergies.
The true poplar pollen floods the atmosphere a month and a half before, although it is not possible to see it with the naked eye. The problem with these pappus, regarding allergies, is that they are natural collectors of dust, spores and pollen from many other plants, which is trapped in the trichomes. The dates on which the poplar releases its seeds coincide with the pollination time of oaks, olive trees, plantains and grasses, species that can cause allergies.
This can have two consequences. On the one hand, as poplar seeds act as natural collectors, they retain part of the pollen and reduce its concentration in the atmosphere, especially pollen from trees . But on the other hand, if one of those fluffs is accidentally inhaled by an allergic person, it can suddenly receive all the pollen load that it has trapped.
Why does it burn so fast?
For a flammable substance to burn, it must reach a sufficient temperature and be maintained for a sufficient time. It is always easier for a material to burn by coming into contact with something that is already burning, even if it is an incandescent spark, although very intense heat can also
trigger the reaction. Water has a very high specific heat, meaning that a lot of heat is needed to heat a small amount of water. So the drier something is, the more likely it is to burn .
Poplar seed pappus are made up of extraordinarily fine filamentous structures of lignocellulose, which form a spongy latticework. These fibers retain a large amount of air inside, and are practically dehydrated ; Thanks to which, they burn with great ease , and the fire advances without difficulty even if there is a massive accumulation, since the same fluff maintains air, whose oxygen continues to feed the fire.
In fact, there are studies evaluating the possibility of using poplar fluff as a raw material to manufacture biofuels.
Why doesn’t the grass burn?
As we have indicated, for something to burn requires that the temperature be high enough and for enough time. Lint burns very fast, and the combustion temperature is relatively low .
Too much speed and too little temperature for the grass, much denser than fluff, and with a high water content, to catch fire.
Of course, it also doesn’t get hot enough for long enough for other park elements to burn, like wooden benches or tree bark. As can be seen in the video.
However, this phenomenon does not always happen in such an innocuous way as it did in 2020 in the Cicados park, in Calahorra. Sometimes several factors can come together that make the difference. If the environment is dry and poorly cared for, if it is hot and if the accumulation of poplar seeds is very high, a fire can start. This happened on May 22, 2018, in the Navarran towns of San Adrián and Viana, where firefighters had to put out two fires caused by the inflammation of poplar seeds .
REFERENCES:
Diethart, B. 2005. Populus alba. In PalDat – A palynological database. “The Serene”. 2020, May 6. «What a pass!»: “hypnotic” poplar fluff fire in Calahorra. NineFourOne.
Hu, Y. et al. 2008. Seed hairs of poplar trees as natural airborne pollen trap for allergenic pollen grains. Grana, 47(3), 241-245. DOI: 10.1080/00173130802237681
Janković, B. et al. 2019. Characterization analysis of Poplar fluff pyrolysis products. Multi-component kinetic study. Fuel, 238, 111-128. DOI: 10.1016/j.fuel.2018.10.064
The burning of poplar fluff causes two small fires in Navarra. 2018, May Diario de Navarra.
