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Can Cardboard Be Used Safely in the Garden?

October 26, 2024

Cardboard is often considered a useful tool in the garden for various purposes, from mulching and weed suppression to composting and soil improvement. But, is it safe to use? In this post, we’ll explore the types of cardboard, potential toxins, and the best ways to incorporate cardboard safely into your garden.

This is an extension of a previously article on the use of paper and cardboard in the garden. We had a few inquiries and questions focusing on cardboard as sheet mulch. Here’s the original article if you’d like to read: safety of paper and cardboard in the garden.

Understanding the Types of Cardboard

Not all cardboard is created equal, and understanding the differences can help you make informed decisions about its garden safety. Let’s break down the types:

  • Uncoated, Plain Cardboard – Basic brown boxes are usually made from natural, untreated paper fibers. This type of cardboard is the safest for garden use, as it’s generally free from harmful chemicals.
  • Recycled Cardboard – While recycled cardboard is often safe, it may contain residues from adhesives, inks, or binding agents used in previous packaging. However, as long as it is uncoated and free of heavy printing, it can be used safely.
  • Coated or Waxed Cardboard – This type of cardboard is treated to be water-resistant, making it less suitable for the garden. The coatings often contain petrochemicals or other non-biodegradable substances that can leach into the soil.
  • Printed or Colored Cardboard – Boxes with heavy printing, glossy finishes, or bright colors may contain inks and dyes, some of which could include harmful substances like heavy metals or other chemicals. It’s best to avoid these types of cardboard in garden beds, especially around food-growing areas.
Our video on the types oof paper to use in the garden

Can Cardboard Contain Toxins?

While many cardboard types are generally safe, there are potential concerns:

1. Inks and Dyes

Older inks, especially, may contain heavy metals like lead. While most modern inks are soy-based and considered safe, it’s best to avoid heavily printed cardboard.

2. Glues and Adhesives

Cardboard boxes are often held together with glues and adhesives, which may contain chemical binders like formaldehyde. While the levels are typically minimal, it’s a good idea to use cardboard with less adhesive or remove any tape and glue residue before use.

3. Coatings

Waxed or coated cardboard contains petrochemicals that can be harmful to soil health. These substances prevent decomposition and may release unwanted chemicals into the soil.

Best Practices for Using Cardboard in the Garden

If you want to use cardboard in your garden, follow these best practices to ensure safety:

1. Choose Plain, Uncoated Cardboard

Basic brown cardboard boxes without printing, coatings, or wax are your best bet for garden use. This type of cardboard breaks down naturally, enriching the soil without releasing toxins.

2. Remove Labels, Tape, and Staples

Before using cardboard, remove all tape, labels, and staples to minimize contamination and ensure faster decomposition.

3. Shred or Moisten Cardboard for Faster Breakdown

To speed up the decomposition process, tear or shred the cardboard into smaller pieces and moisten it before adding it to the garden. This will also make it easier for beneficial soil organisms to break it down.

4. Use Cardboard for Mulching and Weed Suppression

Cardboard is highly effective for suppressing weeds and retaining soil moisture. Lay sheets of cardboard over garden beds or paths, cover with mulch, and allow it to decompose slowly, enriching the soil as it breaks down.

5. Incorporate Cardboard into Composting

Cardboard is an excellent source of carbon or “brown” material for compost piles. Mix it with “green” materials like kitchen scraps, grass clippings, coffee grounds, or manure to balance the carbon-to-nitrogen ratio.

Master Gardener Tip:
Cardboard is considered a ‘brown’ material in composting, meaning it’s rich in carbon. To achieve the right balance in your compost, always add a sufficient amount of ‘green’ materials, like fresh grass clippings, kitchen scraps, coffee grounds, or green leaves, to provide nitrogen. This balance speeds up decomposition, prevents the pile from becoming too dry, and creates nutrient-rich compost for your garden.

Master Gardener Steve

Benefits of Using Cardboard in the Garden

Cardboard offers several benefits that make it a valuable resource for gardeners:

  • Weed Suppression – Blocks sunlight, preventing weeds from growing.
  • Moisture Retention – Helps retain soil moisture, reducing the need for frequent watering.
  • Soil Improvement – Adds organic matter as it decomposes, improving soil structure and encouraging earthworm activity.
  • Sustainable Recycling – Repurposes waste material, reducing landfill contributions and supporting sustainable gardening practices.

FAQs About Cardboard in the Garden

Q: Is it safe to use cardboard around food crops?

Yes, plain, uncoated cardboard is safe for use around food crops, as it breaks down naturally and does not release harmful substances. Just make sure to avoid coated or heavily printed cardboard near edible plants.

Q: How long does cardboard take to decompose in the garden?

Cardboard typically takes a few months to break down, depending on moisture, soil conditions, and thickness. Shredded or torn pieces decompose faster.

Q: Can I use cardboard in raised beds?

Absolutely! Cardboard is often used as a base layer in raised beds to suppress weeds and retain moisture, providing an extra boost of organic matter as it decomposes.

Conclusion: Is Cardboard Safe for the Garden?

The answer is yes—cardboard can be used safely in the garden, provided you choose the right type and prepare it properly. Stick to plain, uncoated cardboard, and avoid coated, glossy, or heavily printed varieties. With the right approach, cardboard can be an effective tool for mulching, composting, and improving soil health, all while promoting sustainable gardening.

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