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A Google search for "banana peel decompose time" gives the result: 2 years.

This claim is repeated in several reputable sources, such as: Popular Mechanics, The Guardian, and BBC.

However, primary sources are not provided: Popular Mechanics refers to an "expert"; The Guardian lists keepbritaintidy,org as the source, but it does not itself provide any primary source for its claims that I can find; and the BBC refers to Zero Waste Scotland, which does not itself provide primary sources either, and also to Recycle It 4U, but this source actually says: "banana skins can last over a month", lists banana skins as 6 weeks, and then says it "can take far longer if the weather is cool".

It seems to me to be a simple study to conduct, so there ought to be authoritative primary sources for this claim. It also seems to me that the claim is sometimes phrased as "up to 2 years", which I take to mean that if you throw a banana peel on the street (rather than say, a forest), in a Northern or high altitude climate with long cold winters (rather than say, a temperate region), and consider the item decomposed only when it has completely disintegrated (some parts may decompose faster than others), then 2 years could be a worst-case scenario.

Other reputable (but also uncited) sources provide different numbers: A government of Australia fact sheet lists "Banana Skin ... 3 - 4 weeks", and this business website states "banana peel won’t decompose for two to five weeks". My personal experience with compost is much closer to these numbers.

There are plenty of other reasons to avoid littering, such as attracting animals, or other litterers, but I wonder if the claim itself is exaggerated mostly for effect.

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    Surely there are conditions in which a banana peel can decompose in less than two years' time, such as in a compost bin, and conditions in which a banana peel can survive for centuries, such as arctic tundra or in a desert. The question itself acknowledges this. As presented, therefore, the claim is too vague to be verifiable.
    – phoog
    Jun 6 at 12:23
  • @phoog I think we can reasonably restrict conditions to being above ground and exposed - buried or encased material could obviously be slower to decompose, but plants that grow (and die) in desert or tundra environments clearly do decompose in much less than centuries. Jun 6 at 18:36
  • I would venture a guess it depends on the environmental conditions a fair amount. "Biodegradable" plastic bags vary a lot in this regard, e.g. in salty water vs dry dirt.
    – Fizz
    Jun 7 at 5:55
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    This probably started out as a claim that banana peels may take years to decompose in a landfill, which is really not that unusual. Decomposition in landfills tends to be very slow unless the item is on the surface due to the lack of air as the weight of garbage above squeezes it all out. See zerowastewisdom.com/post/…
    – barbecue
    Jun 12 at 23:12
  • Another way to tackle this is on a relative basis. Both of the fact sheets supporting this claim also list paper bags as 1 month, which on a relative basis means bananas take about 24 times as long to decompose as paper bags. If the ratio holds across different environments (eg, compost, forest, street, landfill, aquatic, tundra) then it may be possible to check that instead? Jun 13 at 18:33
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Okay, so I can't answer your question perfectly, because the study of time involved for peel decomposition is not very a pointed topic in the literature. Although, decomposition analysis of food and waste management are certainly topics that have received attention at a more general level...

A scientific paper from 2018 in Journal of Physics notes the constituent (carbohydrate) components of banana peel waste1:

the banana peel waste contains more fine fibers than those that are made of wood with high cellulose (60-65%), hemicellulose (6-8%) and lignin (5-10%)

It's important to note the hierarchy of decomposition for macromolecules from easiest to longest:

  • quick – sugars and amino acids
  • medium – (hemi)cellulose
  • long – lignin; cutin

So already it looks on the surface that decomposers have difficulty dealing with various peels and cores compared to simpler targets. Of course, there may be specialized decomposers that, when present and given optimal environmental conditions, could certainly succeed. After all, detritus such as dead tree matter, is largely composed of cellulose; wood is largely lignin, requiring specialized decomposing rotters to metabolize tree stumps, for instance.2

Next, the surroundings and favorable situations for decomposition. You mention Keep Scotland Beautiful; however, you may not be aware that the estimated breakdown time provided is under the assumption of a fully aquatic setting. Aquatic and highly dry environments are not as conducive to some taphonomic processes, compared to areas less extreme. It follows that under a reasonable environmental setting, on land but where humidity lives, in which aerobic decay can be performed, it's possible that peels could degrade between the time required for leaves and foliage (~ 1 year), or possibly shorter, and the oft-quoted 2-year time horizon, but likely not less than the time needed for the fruit itself to rot in full with no remains or skeletonized to the least-degradable portions.

However, unlike the case with wood, the majority of rot in peels do not come initially from termites and fires in forest and grassland settings and other terrestrial biomes. In this sense, peels may not get a similar "head start".

Landfill settings and composting stations are completely different from the natural element. Landfills may be sterilized in ways that make it improbable for decomposers to "do their thing", or simply too voluminous for these to be carried in a reasonable time frame. For instance, studies conducted by University of Arizona researchers begot a book, Rubbish! The Archaeology of Garbage, the brainchild of William Rathje, in what became known as the Tucson Garbage Project, or colloquially "garbology". Primarily intersecting between recent archaeology and consumer spending and disposal habits, the findings also encompassed other practical topics of note. Of interest, newspapers and other paper, which represented 40% of all landfill (by mass, I think?), at least then, is a poor decomposer in these designations:3

"[E]ven after several decades, they remain intact and perfectly legible. The problem, the authors say, is that, landfills are not vast composters; rather, they are vast mummifiers. There is biodegradation, but its pace is measured in centuries, not decades. Even organic materials, such as food scraps, remain unchanged after 30 or 40 years. Mr. Rathje and Mr. Murphy cite an account of an excavation of an ancient Roman garbage dump in which the smell of putrefaction remained unbearable even after 2,000 years."

They also found out that people erroneously underreport the degree of their unhealthy lifestyle and alcohol consumption habits, but that's another topic for further reading.

What ideally would be a final ground for decomposition turns into a mummification temple, sans the brains being pulled out of people's eye sockets. Those involved with the project systematically analyzed 14 tons of waste, and the results were equally mind-blowing and consistent. This completely shattered people's long-standing belief that paper, including newspaper which has accounted for approx 1/3 of papers disposed, was a good decomposer and readily decomposes in landfills. Wrong. Radiocarbon dating is not required for this archaeology; all you need to do is identify the date of an adjacent NYT paper in the strata to know how long that half-eaten Manwich has been lying around there. Could be 35 years. I imagine the same can be said about banana peels.

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