How to shred cardboard for compost

How to Shred Cardboard for Compost

Cardboard is an excellent substitute for carbon rich, brown matter when you can’t find dry plant material. Using shredded card in compost is a well known solution. But handling large amounts of packaging and boxes can be tricky. I sometimes find myself facing a huge heap of cardboard boxes that need cutting and shredding in preparation for the compost bin. When you’ve done this a few times you naturally start to wonder what’s the ideal solution.

So how do you shred cardboard for compost? Ideally, you need to remove any plastic from cardboard packaging before putting it in a compost bin. Flatten the cardboard on a large surface before cutting into strips with a box cutter. Cutting along the corrugated lines will make work easier. Wetting the card beforehand can also be a big help.

Shredding cardboard can be a very time consuming and manual process. But if you want to get the benefit of this abundant material for use in your compost, you need some kind of method for dealing with it.

So I decided to investigate the best way to shred cardboard, or whether there was an alternative to all that hard work.

On a side note, cardboard is not a perfect material for composting. Dry leaves are far better than cardboard. Foliage is rich in nutrients, whereas processed card will add fewer benefits to the end result. However, cardboard breaks down well, and adds structure to any compost mix. It makes a good “brown” material when nothing else is available.

Shredding Cardboard for Compost

At certain times of the year you find yourself with a lot of wet, “green” compost material and very little dry “brown” matter such as dry leaves, branches, and twigs. And as you probably know, good composting requires a balanced mix of both. In addition, when your in an urban or city environment with less trees or leaves to use as a source of carbon, cardboard is probably going to be your ideal material.

Cardboard is very good for improving the structure of compost because it helps introduce air pockets. Oxygen is an important ingredient because the composting microbes need it to do their job.

But in my experience card needs shredding. If it’s not shred, large sheets will stick together in a big damp mass. In this state it takes a long time to decompose, so it sort of defeats the purpose. And big chunks of card can be difficult to turn.

Shredding the cardboard makes sure that there is sufficient structure in the pile for air to circulate properly.

I know shredding can be tedious. So below I’ve tried to give you some ideas to make your life easier, and a few alternatives that you may want to try for yourself...

General Shredding Tips

Avoid glossy or colored cardboard. Even though modern inks are often vegetable based and therefore fairly harmless, the inks used on colored or glossy card can potentially contaminate your compost as I’ve heard they contain heavy metals.

Also, try to remove plastic such as tape and package document envelopes before you shred cardboard. Plastic is usually not biodegradable. And once it's been shredded it’s even more work to remove these offending items!

Some people say staples are a similar concern, although I tend not to worry - after all they add some iron to the mix!

Tip - If your compost is too dry, you need to reduce the amount of evaporation. You can use sheets of cardboard to cover the tops and sides of your compost heap and reduce moisture loss.

The Classic Shredding Method

Begin by preparing any large format cardboard by hand. If possible give yourself some space. A well prepared work area will make the going easier. After you’ve removed any plastic, lay your card on a large flat surface at a comfortable height for easy working (an old door on a pair of sawhorses for example).

Slice the cardboard with a decent box cutter, as far as possible cutting along the lines of corrugation, not against them (By the way, I find cutting cardboard with scissors is highly frustrating).

Divide your cardboard into more manageable 8 inch (21cm) strips.

You now have two choices.

  • ​Continue to shred into smaller strips of about 1 inch (2.5cm) by hand using the box cutter
  • ​Feed the card through a shredder.

Personally, I prefer the electric shredder method, and this is why you begin with 8 inch wide pieces so that they feed into the mouth of the shredder. However, a word of warning. I recommend you use a heavy-duty shredder for this (my standard shredder broke down when I tried it with card, but then again, it was only a cheap model).

A good example is this heavy-duty shredder on Amazon which can handle 20 sheets at a time and it will even deal with staples!

A shredder results in cardboard pieces which are light and airy. This is great for composting and in particular for vermicomposting. The shredded paper has a high surface area making it easy break down as the composting microbes ​get to work.

Whichever method you choose, your card is now ready to go on the compost pile!

The Wet Card Technique: Shredding without a Shredder

This idea is pretty simple. If you soak cardboard it becomes easy to tear. Depending on how much card you want to shred and the size of your packaging, you have a few different ways of achieving this. You can even leave the plastic parts in place, since most of the time they become easier to remove after soaking.

For example, if you live in a rainy climate you can leave card outside to begin decomposition.  The soaked cardboard will become easier to break up.

Alternatively you can use a large tub to soak card for up to 2 weeks. Cover the cardboard with plenty of water. When it softens you can easily shred the soaked card (to do this I've seen some people use a drill with a paint mixing attachment to shred the card).

Lasagna Method

Have you ever stumbled across the concept of lasagna gardening?

Lasagna gardening is a slow compost process that requires little effort. The method begins by laying down sheets of cardboard which then get composted into the underlying soil and vegetation. You then add alternate layers of nitrogen rich green material, and dry carbon rich waste.

You can use this same method in your compost heap by creating alternating layers of cardboard, green and brown organic waste (Sprinkle the layers with water as you go).

You can also spread some soil or old compost over the card to help speed up the process (the soil contains some useful microbes to get the process started). Compost piles are teaming with fungal activity. This cardboard lasagna construction will get broken down by the fungus over a period of a few months. It’s is relatively slow operation and will take at least 3 to 6 months.

You can even leave the plastic tape on your cardboard packaging and remove when everything has broken down.

Quick & Easy Chipper/Shredder Solution

This is probably the quickest solution if you can’t be bothered to do all that cutting and preparation. Using a garden chipper / shredder is instant and effective. It’s instant because you can process your card immediately without any pre-treatment. And a good shredder makes very short work of your cardboard!

The method is easy. Remove any plastic tape etc., then roll your card up into a big cardboard log. Feed this into the chipper and you have shredded card in a flash! (Here’s a demonstration )

There’s just one caveat to this method. You will probably struggle if you have a low powered electric chipper. 

This is obviously an investment, but it has the advantage of dealing with all of your other yard debris. Here’s an example of a powerful and efficient chipper / shredder that should make short work of any cardboard and big branches!

High Fiber Composting

high fiber composting materials

High fiber composting is an idea that was developed in response to the kind of waste material people most often have access to. Two common types of compostable waste were identified:

  • ​Food waste
  • ​Cardboard and paper

This method is well adapted to people who have small yards and who don’t have access to a steady supply of plant-based “browns”. It’s also a convenient way to compost kitchen scraps.

Food waste on its own is too moist and can quickly become a smelly mess if there’s too much of it in the compost mix. Shredded cardboard is used as a “brown” substitute and provides the necessary structure to ensure good air circulation.

Putting Cardboard in your Compost

Cardboard is a very handy source of carbon. When shredded it breaks down more quickly and improves the density and structure of any compost. It’s also a quick and efficient way to deal with compost that is too wet or foul smelling (this can happen when the compost contains too much nitrogen rich greens).

Hopefully the above has provided you with some tips on how to recycle your cardboard waste in your compost bin.