October 10, 2024, 11:28 am | Read time: 3 minutes
Freshly mown grass clippings get really hot in sunny weather. Can the green waste then start to burn? Or does the green waste threaten to explode in the bag?
The temperatures are finally rising. Important gardening work is now on the agenda in many places. This includes mowing the lawn. But what actually happens to the grass cuttings? Every amateur gardener should already have noticed this: Grass clippings get pretty hot pretty quickly. But can they also burst into flames?
Can grass clippings ignite themselves?
Damp grass clippings can indeed become very hot. Sometimes, the green waste ignites on its own. However, a number of factors have to come together for this to happen: In addition to oxygen, moisture is paradoxically also important for the formation of heat. Summer temperatures favor the fermentation process. Microorganisms then make the lawn section really steamy and heat it up to over 60 degrees.
Caution with damp hay
“However, spontaneous combustion of lawn clippings generally occurs rather rarely,” says Rainer Schwarz from www.Brand-Feuer.de. Schwarz also considers it unlikely that a garden waste sack will inflate so much due to the grass cuttings that it bursts or even explodes. The former fire investigator explains: “Damp hay can be a much bigger problem. If hay bales heat up, they should be separated; if this is not possible, the fire department should be called if the temperature is high.”
The experts use a measuring device – a so-called hay probe – to check the temperature and locate hot spots. The dangerous thing about this is that the hot spots are usually not visible to the naked eye. If the hay probe shows things are getting dicey, the experts carefully remove the haystack. Schwarz warns: “Care must be taken when doing this, as the hay can ignite with atmospheric oxygen.”
How do you prevent grass clippings from igniting?
Rainer Schwarz explains: “In general, it’s better to mow the lawn on a sunny day. It’s best to leave the mown grass on the lawn to dry in the sun. Once dry, the grass clippings can be disposed of without any problems.”
Caution: If you dispose of grass clippings in an organic waste garbage can, it should not be placed under a roof or near a building.
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Is it safe to put lawn clippings in the compost?
As a rule, there is no risk of fire. On the contrary, the heat build-up can help with decomposition. Isabelle Van Groening from the Royal Academy of Horticulture: “As soon as grass composts, it is moist and does not generally ignite. Heat is generated during the composting process. With nitrogen-rich grass, this happens quite quickly. Grass is, therefore, a good activator for the compost heap.”
The gardening expert explains that it is best to alternate a thin layer of grass clippings followed by leaves, wood chippings, or other fiber-rich material in the compost. Van Groening says: “This creates valuable compost material instead of a slimy, smelly mass. Alternatively, you can spread the grass in a thin layer of mulch around hedges.”
With mulching mowers, such as the robotic mower, Van Groening believes that finely chopped grass clippings are left to compost on the spot. This saves a lot of time and effort. “And it ensures that the soil retains more moisture during the hot spell in summer.” This keeps the lawn lush and green throughout the garden year, and grass cuttings do not threaten to ignite themselves.