Cardboard is often used as a gardening tool, but there’s a growing debate about whether it’s safe for your plants. With concerns about the chemicals in adhesives, inks, and potential toxins, it’s important to explore whether using cardboard in your garden can do more harm than good.
Understanding Potential Contaminants in Cardboard
- Glue
Cardboard uses two types of glue: one for joining layers in corrugated cardboard (made from organic materials like potatoes and rice) and another used to bond box seams. The first type is harmless and naturally decomposes, but the second may not be plant-based. While the amount of this glue is minimal, if you’re concerned, you can simply remove these parts. - Tape and Labels
Plastic tape and labels often contain non-biodegradable materials and should be removed. Some tape is biodegradable, but its string can take much longer to break down. Labels can also be a challenge as many are treated with plastic films for durability, which don’t break down well in compost. - Ink
Most inks used on cardboard are soy-based and safe for the garden, but glossy or brightly colored packaging, such as cereal boxes, may contain heavy metals or chemicals that could be harmful. These types of cardboard should be avoided in the garden. - Chemical Treatments
Some boxes may be treated with chemicals to resist moisture or reduce flammability. However, this is rare, and unless you’re certain the cardboard is treated, it’s generally safe to use.
Using Cardboard in the Garden
Once you’ve removed any labels, tape, and glossy parts, cardboard is a useful, biodegradable material that can benefit your garden. Here are three ways I recommend using it effectively:
- Composting
Cardboard is rich in carbon, making it a great addition to compost. It helps balance out nitrogen-heavy ingredients like grass clippings and green weeds. Simply layer it with “greens” in your compost, ensuring you turn it occasionally to speed up decomposition. In dry climates, cardboard might take a while to break down, but in wetter conditions, it will decompose faster. - Priming for Sheet Mulching
Cardboard is commonly used as a weed suppressant in sheet mulching, but it can be slow to decompose in dry conditions. To avoid this, thoroughly soak the cardboard before laying it on the ground, and then mulch over it to keep the moisture locked in. In wetter climates, it will break down much quicker, usually within six months. - Weed Control for New Beds
One of the most effective uses of cardboard is to clear new planting areas or line the base of raised beds. It kills off weeds and lawn grasses without needing chemicals or digging. Simply place the cardboard down, add soil or compost on top, and plant directly into it. The cardboard will decompose over time, and the roots will eventually break through into the soil.
Considerations for Cardboard Use
While cardboard can be an excellent tool in the garden, there are some considerations to keep in mind. Some gardeners worry about whether it suffocates soil life or attracts termites. Generally, the impact on soil oxygen is minimal unless the soil is dense and poorly aerated. Termites may be attracted to cardboard, but this is unlikely unless there is already an infestation in your garden.
In summary, when used properly, cardboard is a safe and sustainable gardening tool. Just be mindful of the materials it’s made from, and ensure it’s free of harmful additives before using it to enrich your soil.