Is Grass Flammable? (A Comprehensive Overview)

Is Grass Flammable?

Whether grass is flammable or not, depends on a number of conditions. In general, if grass will or won’t burn will depend on some factors such as:

  • How dry the grass is
  • Ambient conditions such as  temperature, relative air humidity,sunlight exposure and wind speed
  • Soil temperature
  • Duration of Contact with a ignition source
  • The nature of the ignition source that is applied to the grass
  • If there is dry or dead grass, the amount of it matters
  • If the grass is dead but has been recently wet by rain or snow it may not burn
  • The flammability of grass also is very dependent on its species
  • What is around the grass, is there other flammable materials, non-flammable materials,  combustible materials, or fire retardant materials

It is also important to consider that grass fire can have very high temperatures, therefore grass fire can burn nonflammable combustible materials in the vicinity.

In the following sections the concept of flammability of a given material will be used as meaning: How easily can the given material catch on fire if an ignition source is applied to the material as long as the material is at standard ordinary conditions (normal temperature, pressure and in contact with air).

A combustible material will be considered a material that can be ignited under ordinary conditions not taking in consideration any external additions. In other words, a combustible material is one that has the possibility of catching on fire but the amount of external additions may be very drastic.

What Are The Most Common Causes For Grass Fire?

Dry grass can start burning if any burning object goes in contact with it. Additionally, unexpected objects such as glass can also cause dry grass to be ignited. Commonly sources to grass fires are:

Cigarettes 

Dropped cigarettes that haven’t been completely extinguished are quite prone to cause a grass fire. Cigarettes thrown from a car on the road can be enough to start a fire. Dried out grassland near roads can easily start to burn if a cigarette is thrown at them.

Disposable barbecues 

Smoldering ash and embers from disposable barbecues can be carried by the wind onto dry grass areas. Abandoned or discarded disposable barbecues that were not properly put out and cooled will retain heat for a period of time and therefore they are capable of heating grass to the point of starting a fire.

Glass bottles 

Less likely to happen but still an important cause of grass fires is glass or any glass material that is left on grass areas. Glass can magnify the sunlight to the point of setting grass on fire.

Recommendations To Avoid Grass Fires From Happening

  • Don’t drop cigarettes or anything that is burning on dry ground.
  • Don’t  drop cigarettes out of car windows, they may land on dry grass by the roadside.
  • Avoid having barbecues in parks and public spaces.
  • Never leave camp fires or barbecues unattended and extinguish them properly after you have finished using them.
  • Position your barbecue on level ground and keep it well away from anything that may catch fire (sheds, fences, trees, tents etc.).
  • Do not barbecue on balconies, the wind may carry smoldering ash towards nearby grassland.
  • If you’re  barbecuing near dry grass, have a bucket of water or sand nearby for emergency use.
  • Be aware that children, animals, balls or anything else may knock over barbecues, increasing the risk of grass fires, especially when in busy parks or public spaces.

Is Dead Grass More Flammable Than Living Grass?

The more dry grass is the easier it is for it to be set on fire. Living grass is made of mostly water, which is the main factor that prevents it from catching on fire. Besides the water, grass is composed of organic molecules (molecules made of carbon) most of which are flammable or at least combustible.

How Ambient Conditions Affect the Flammability of Grass?

One of the factors that affects how likely grass will be to be ignitable is the ambient conditions such as:

  • Temperature
  • Relative air humidity
  • Sunlight (time and intensity of the radiation)

How Does Temperature Influence Grass Flammability?

Temperature affects the flammability of gr

ass mainly for two reasons. The first is that, in general, the higher the temperature the less water the grass will have within its structure. 

The second reason is, overall, the higher the temperature the more easily something can be set on fire. That is because, at higher temperatures molecules move and collide with each other at faster rates. This increases the chances for a combustion reaction to take place.

How Does Relative Air Humidity Influence Grass Flammability?

Humidity accumulation on the grass surface protects it from heat and it may cause the water content on the grass to increase making it less likely to burn.

How Does The Sunlight Influence Grass Flammability?

Obviously grass needs sunlight to survive, but too much sunlight alongside lack of water will cause the water content in the grass to decrease and consequently increase the chances for the grass to catch on fire if enough heat is subjected to it.

In more extreme conditions some of the grass may die, and dead grass is significantly more likely to catch on fire.

Hot Surfaces Can Lead To Grass Fire

If a hot surface is in contact with grass, ignition can occur after 10 minutes of contact with a surface at 300 ºC (572 ºF). A surface at 400 ºC (752 ºF) can lead to a fire after 3 minutes of contact. And, a surface at 500 ºC (932 ºF) can lead to fire after less than a minute.

What Types of Ignition Sources Can Cause Grass To Burn?

In general the higher the heat provided the higher the more likely it is from the grass to be ignited. 

How Does The Grass Distribution Impacts Its Flammability?

In the wild grass is often mixed with different species of grass and of other plants. Additionally there may be a mixed distribution of dead and living grass. If a small amount of the grass is dead there are low chances it will create a large fire.

Natural Events May Cause Or Prevent Grass Fire

Although many grass fires are directly or indirectly caused by human action, Nature phenomena such as rain or snow may prevent grass from being ignitable. On the other hand, lightning or sunlight can set grass on fire.

In more complex situations, grass wet from rain which is exposed to the Sun quickly afterwards can heat up from the Sun acting on the moisture on the grass leaves.

Another factor is the wind, wind is not likely to help a fire to start, it may be very important on the spreading of a fire.

Different Grass Species Have Different Flammability

A series of studies performed in the laboratory have shown that savanna grasses can have significant differences in their levels of flammability.

Is Artificial Grass Flammable?

Artificial grass is generally not flammable and some types are but it will usually melt if high heat or fire is applied to it. Potential ignition sources for normal grass (such as a lit match or a cigarette) should not lead to a fire but it is possible for the artificial grass to be damaged in some manner.

Conclusion

Green grass is not flammable while dried out grass is flammable. Grass fires can be small and concentrated if intentionally generated and properly controlled or they can cover wide areas and lead to serious dangers. Moreover, burning grass can be nocive to people and the environment.

Frequently Asked QUestions (FAQ): Is Grass Flammable?

What happens when you burn grass?

The outcome of burning grass varies from a quick and easy to put out fire to a fire that can quickly spread and lead to more severe situations. Grass fire fuel can burn hot and spread fast. Burning grass can also turn the soil more fertile to grass seeds to germinate. Burning grass also affects the habitat of some animals such as voles and even some birds nests. 

Why does green grass not burn?

Normally, for green grass to catch on fire a simple lit match won’t be enough. Green grass has a lot of water within its structure and often it has water deposited on its surface. FOr green grass to catch and maintain a fire heat has to be applied for a period of time.

Is burning grass toxic?

Burning grass can produce some nocive fumes such as nitrogen oxides (NO2 for example), carbon monoxide among others. It is possible that heavy smoke is also produced.

What Is Firegrass?

Firegrass is a type with the scientific name Andropogon ternarius of plant which can reach up to 1.5 meters (59 inches).

At what temperature does grass catch fire?

A temperature as low as 300 ºC can lead to dry grass catching on fire after some minutes of contact. Temperatures higher than that need less time of contact. A naked flame may cause dried out grass to immediately start burning.

Will fire burnt grass grow back?

Usually yes, after a period of time. In fact, in some instances burning grass can turn the soil more susceptible to growing grass given an enrichment of the soil with nutrients. 

References

https://novascotia.ca/natr/forestprotection/wildfire/firecentre/grass-burning.asp (Accessed June 10th, 2022)

https://www.frontiersin.org/articles/10.3389/ffgc.2018.00006/full (Accessed June 10th, 2022)

https://www.london-fire.gov.uk/safety/the-home/grass-fires/ (Accessed June 10th, 2022)

https://www.pbs.org/wgbh/nova/article/invasive-grasses-wildfire/ (Accessed June 10th, 2022)

https://www.fs.usda.gov/treesearch/pubs/9521 https://doi.org/10.2737/RMRS-GTR-153 pages 35 to 49. (Accessed June 10th, 2022)

Simpson, K. J.; Ripley, B. S.; Christin, P. -A.; Belcher, C .M.; Lehmann, C. E. R.; Thomas, G. H.; Osborne, C. P. Determinants of flammability in savanna grass species. J Ecol, 104: 2016. 138-148. https://doi.org/10.1111/1365-2745.12503 (Accessed June 10th, 2022)

https://www.fs.fed.us/database/feis/plants/graminoid/andterc/all.html (Accessed June 10th, 2022)

Morse, T.; Cundy, M.; Kytomaa, H. Vehicle Fires Resulting from Hot Surface Ignition of Grass and Leaves. SAE Technical Paper 2017-01-1354, 2017. https://doi.org/10.4271/2017-01-1354. (Accessed June 10th, 2022)

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