Are pistachio shells biodegradable? Can they be composted?

  • Date: May 14, 2022

The nutritional benefits and unique, subtle flavor make pistachios a popular snack. Consumption has increased in many countries, including the US, in recent years. Producers do not have to remove shells to process the nuts, so they don’t – it saves them both time and money. But this means that after snacking you will most likely be left with a mountain of hard shells that you don’t know what to do with! And let’s face it – they usually end up in the trash. If we can, we should try and divert them from landfill. But what else can we do with them? Are pistachio shells biodegradable and can we add them to our compost pile?  

A bit of background – what are pistachio shells made from?

As you probably know, pistachio shells are made from a really tough material. In fact, it is so strong that researchers in Iran (a big player in the pistachio world) have incorporated pistachio waste into building materials! Although they have an almost wood-like appearance and texture, they are not made of wood. Experts have shown that, on a microscopic level, the shells can be seen to have a porous layered structure abundant in triglyceride fats and cellulose.

So, are pistachio shells biodegradable?

If something is biodegradable it means that it can be broken down naturally by microbes. Luckily, pistachio shells are biodegradable. It is estimated that they will take approximately 3 years to biodegrade. Although best to reuse them, if they do end up in the landfill pile they won’t be there for decades or even centuries like some materials!

Are pistachio shells compostable?

Composting is a great way to dispose of waste.  It prevents organic waste (anything from hair to a pair of jeans) going to landfill whilst providing a nutrient-rich compost to use in your garden. Everyone’s a winner!

Pistachios are biodegradable. So, they must be compostable, right? Although this is largely true, it does not always apply. To be classed as compostable, the biodegradable material must ultimately be able to contribute to nutrient-rich compost. Luckily, pistachio shells fit the bill. They make a wonderful addition to any compost pile.

Compost pile ingredients generally fall into two distinct categories: carbon rich or nitrogen rich. Carbon rich materials are known as ‘brown’ ingredients and they include items such as dried leaves and twigs. Nitrogen rich ingredients are referred to as ‘green’ and include food scraps and freshly cut grass.  Pistachio shells contribute to the ‘brown’ element of the compost pile; they are rich in carbon. in your pile, try to stick as closely as possible to the suggested green to brown ratio. Although composting is not an exact science, following the basic principles will create the most favorable conditions for the microbes to do their job. If you are new to composting, you can find some of the more frequently asked beginner questions here.

Pistachios, and other nutshells, also improve aeriation in the pile. Gaps between them allow air to circulate. If there are no air pockets, microbes will have little or no oxygen to carry out their role. Not only will the composting activity decrease or even come to a complete halt, it will not smell so good either!

Things to consider when composting pistachio shells…

Pistachio nuts often come salted. Although they taste good, high levels of salt are not ideal for the compost pile. A couple of salted nut shells would not be a disaster, but any more than that could contaminate the soil when the finished compost is added. Salt is also known to kill bacteria and other microbes – the last thing you want in a compost pile. No microbes equals no composting! This does not mean you need to give up the salted variety of pistachio though. Simply give the shells a good clean with water to remove any excess salt before adding.   

Some preparation is also required if you want your pistachio shells to compost optimally. As with other stubborn compostable materials – such as crab shells or bamboo – it helps speed up the process if they are crushed. This simply provides a larger surface area for microbes to act on. Use whatever you can get your hands on – a hammer or pestle and mortar will work well. You can also soak them in water for 24 hours or so just to soften them and send them on their way. However, if you neither crush them or soak them they will still eventually biodegrade – you will just require a bit more patience…

The bottom line

Diverting waste from landfill is a simple way we can reduce our impact on the environment. And composting, is one of the best ways to do this, benefiting both our garden and lowering our carbon footprint. Pistachio shells biodegrade and can be composted – but they do take time. Crushing them up helps them biodegrade optimally. Just ensure you remove the salt beforehand, or you might do more harm to your garden than good.