Is Cardboard Killing Your Garden?

Is cardboard toxic? After all, it usually comes with glue, ink and tape, and there are chemicals used in the production process too – so, as gardeners, do we need to rethink how we use it?

It’s a hot debate right now, so I reckon we’d best get to the bottom of this. We’ll explore the concerns about cardboard and whether they’re justified, then look at what we can do to use cardboard safely, and effectively, in the garden.

Potential Soil Contaminants From Cardboard

Glue

Let’s start with that often-mentioned sticking point: glue.

There are two types of glue used in cardboard. First, there’s the glue used within the cardboard itself, like the stuff used to join up the layers in corrugated cardboard. This type of glue is completely harmless because it’s made from starchy crops like potatoes, rice and tapioca. It naturally decomposes just like any other organic matter, so it’s more than safe in the garden.

But then there’s the glue used to join sheets of cardboard together, for example to form a box. This isn’t necessarily plant-based, but the amount used is so small in the grand scheme of things I personally don’t worry about it. That said, if you want to be squeaky clean and avoid even traces of anything unnatural then you might want to just tear or cut this bit out.

Tape and Labels

Most tape is plastic and won’t degrade easily, so it should definitely be removed along with any staples used to hold boxes together. Some of the big online retailers now use biodegradable tape to hold everything together. But while most of the tape disappears in time, the strings within the tape can be significantly slower to break down, so for this reason I’m inclined to remove as much of this tape as I can, just to be sure. And by the way, if you happen to know what the strings are made of, please do share this in the comments below – I’d love to know!

Some labels will just rot down along with the cardboard, but many are treated in some way, for example with a plastic film, to ensure they remain weatherproof and don’t come off in transit. And, of course, they’re usually glued on. As a rule, there aren’t that many labels on cardboard, so it takes little time to peel off any you come across. If you find they’re glued on firm, try wetting the cardboard to make them easier to remove.

Ink

The overwhelming majority of inks used on cardboard – and for that matter newspaper – are soy-based, meaning they’re perfectly safe for using around the garden. But caution is needed for colorful or glossy cardboard like cereal boxes, as well as glossy paper such as that found in magazines, because they often contain heavy metals and potentially a host of other nasties.

Heavy metals are in fact found in most soils naturally, but if they build up too much they can be dangerous for our health, so it’s a good idea to avoid adding more to your garden unnecessarily. Plus, plasticky, glossy cardboard doesn’t break down well in the soil or compost, so the best place for this is in the recycling pile.

Chemical Treatments

Most plain brown cardboard is not treated, but some boxes may be treated with chemicals to, for example, repel moisture or reduce fire risk. But this is pretty rare, so it’s not something I really worry about – though, of course, if you know for sure that cardboard has been treated, avoid using it.

Using Cardboard in the Garden

So your cardboard is now stripped of labels, staples, tape, and you’ve made sure there are no colored inks or glossy bits in there. All that’s left are plant fibers and starches derived from plants, so it’s all fully degradable and safe to use around the garden.

There are three ways in particular I like to deploy cardboard in my garden - but even some of those can prove controversial.

1. Composting Cardboard

The obvious use for cardboard is as a compost ingredient. For the fastest compost we want to aim for a mix of ingredients with a combined carbon to nitrogen ratio of around 30:1. Cardboard is almost entirely carbon, which makes it fantastic for balancing out more nitrogen-heavy ingredients.

Cardboard is especially useful during summer when other sources of drier, higher-carbon ‘browns’ such as fallen leaves are in short supply. The simplest way to use it is to just mix it in roughly equal proportions with ‘greens’ such as seed-free weeds, spent crops and grass clippings. Add your cardboard and greens in thin layers to get a good mix and, if you want your mature compost sooner rather than later, turn it from time to time to introduce more air and speed the process along.

2. Priming Cardboard For Sheet Mulching

Compost from cardboard’s a no-brainer, but using cardboard for sheet mulching to suppress weeds can be controversial.

One reason is that cardboard can be very slow to fully decompose in drier climates. If cardboard is able to dry out, it’s going to take a lot longer to disappear into the soil. However, in a wetter climate like mine, cardboard can rot down under mulch within just six months or so.

And cardboard readily repels water – at first. It takes time for water to seep into it and for the cardboard to become pliable and soggy. For this reason, it’s absolutely essential that cardboard destined for use on the soil is really thoroughly soaked once it’s laid in place. We need to prime it by watering it, then mulch over it to lock the moisture in. Be sure to really thoroughly water when the weather’s dry so that the cardboard never dries out again.

3. Using Cardboard For Weed Control

For me, the absolute best use of cardboard is to clear new areas for growing, or as the base for new raised beds. It’s hugely effective at killing off most weeds and lawn grasses without the need for herbicides or heavy digging – and you can just pile your soil and compost on top, and plant straight into that.

There’s little chance of the cardboard beneath drying out like this, and it should have started softening and decomposing by the time the roots from the plants grown above reach down to it. It’s how I started all of my garden beds, using cardboard to clear weeds without the sweat and toil of digging out the weeds.

Plant roots and, of course, soil life needs to breathe, and there’s some debate about whether cardboard might actually suffocate the soil and the life within it. But while adding cardboard might make it harder for oxygen to penetrate particularly heavy, wet clay soils, generally this impact is really very minimal.

And then there’s termites, which favour cool, damp areas and the ready supply of cellulose found in wood fibers to feed on – making cardboard mulches ideal territory. But unless termites are already an issue in your garden, laying down cardboard is very unlikely to attract them in from elsewhere.

For me, cardboard will always be a welcome source of free and abundant material with so many uses in the garden. Let me know how you use cardboard in your garden in the comments below.