Category Archives: mulch

Using Cardboard for Compost? by Julia Frisbie

By Julia Frisbie-

posted May 3, 2022.

I got an email this week from Torgy Torgeson, the other half of the dynamic duo whose spinach I so admire. He writes:

Anna and I have had one or more garden plots in the 29th Street Community Garden for 11 or 12 years. Over the past few years we have observed folks with other plots having used corrugated cardboard boxes as a mulch to keep down weed growth. 

In the May 2022 issue of Consumer Reports there is an interesting and edifying article on the occurrence of PFAS chemicals in “paper” food packaging (The Dangerous Chemicals In Your Fast Food Containers, pp. 36-43). More interestingly, among the papers they tested for PFAS, brown paper bags had the highest concentration, at about 192 ppm vs 5.6 ppm in take-out containers!

My forestry background tells me two things in particular: that corrugated cardboard is simply brown paper bag stock that is formed into the well-recognized laminate we call “cardboard”, and secondly, that paper making is NOTHING BUT chemical processing to digest cellulose….. the more chemicals, the merrier!!  

My other readings tell me that PFAS are quickly becoming known pretty universally as being really bad for people! So, here’s the question. Have you heard or read any material that expressly or casually mentions the down-side of using paper or cardboard for garden mulch or composting?

First of all, a cursory search reveals that Torgy’s absolutely right, PFAS are not good for us. The CDC’s Agency for Toxic Substances and Disease Registry notes that they’re linked to increased cholesterol, testicular and kidney cancer, changes in liver enzymes, pregnancy complications, low infant birth weights, decreased vaccine response in children… and maybe even lower resistance to COVID-19.

To answer Torgy’s question about whether or not I’ve heard or read anything that mentions the potential downside of using paper or cardboard for weed suppression: I haven’t! When I searched, I found a lot of articles about high levels of PFAS being found in purchased compost and mulch (ugh), but nothing about paper or cardboard being used in the garden as mulch. And yet… Torgy’s logic holds up. If we don’t want it in our bodies, we probably don’t want it in our soil. (What more are bodies, anyway, than soil in a temporarily different form? Ash Wednesday’s admonition, “from dust you are made and to dust you shall return” comes to mind.)

But what to do instead? Cardboard is great because it suppresses weeds while slowly breaking down under the soil. One of the first things we did when we bought our first home was to break down our moving boxes and layer the cardboard under mulch to turn our lawn into garden beds. If you took this advice from me, and it was bad advice, I’m so sorry!

One of our wonderful neighbors expanded her garden beds this year by removing sod rather than adding cardboard and compost. I hate to recommend the removal of ANY organic material from our topsoil-poor island, but maybe that’s a safer option. It certainly worked for her, and my ducks were all too delighted to help!

Of course, tillage is an option, and maybe the benefits of one-time tillage outweigh the risks of compacting subsoil. That’s the approach that my CSA farmer friends took. They tilled the first year, but have subsequently used silage tarps in their off-seasons to keep the weeds down. This approach has been popularized by market gardeners like Curtis Stone and Jean Martin Fortier.

If you plan far enough ahead, silage tarps, landscape fabric, or 6mm black plastic mulch could be useful even without tilling first. They don’t break down into the soil, but if you leave them in place long enough (think MONTHS rather than weeks), they’ll smother grass and weeds enough to create a stale seedbed. With care, they can be used for many seasons. But they represent a higher upfront cost, both in terms of money and petroleum… and I have no idea whether or not they leach chemicals into the soil over time.

What do you think? When it comes to expanding your growing space, are you in camp cardboard, camp sod removal, camp tillage, camp tarp-and-time… or camp something else entirely? I’m not sure what I’ll do next time I run out of space and decide to expand my growing beds. I’ve been thankful to be able to use cardboard up to this point, but when Torgy sounds the alarm, I figure we better listen. He’s an adventurous person, not prone to unreasonable anxieties, who has a proclivity for creating interesting things from scratch. To make the growing space for Anna’s legendary spinach, he built raised beds over the top of the existing soil. If you can source safe, high-quality compost, then raised beds might be a fantastic option. But it’s probably also the most expensive of all the options I’ve mentioned today… unless you can build with foraged materials… which might leach chemicals into your soil! AAACK!

When I’m faced with an ambiguous choice in my garden, I try to learn from the plants themselves. Here’s what I see:

  • Doing something is better than doing nothing. No matter how I’ve prepared the soil, the plants keep showing up. They’re not paralyzed by indecision or imperfection; they try one thing, and if it doesn’t work, they try another. Individuals may sicken or die, but the greater plant population changes and persists.
  • Plan for diversity. Plants always seem to show up with friends; no species wants to live alone. Perennials and annuals are especially delighted to live near each other. As I’ve worked the soil this spring, the most delicious patch of it I’ve found has been under two blueberry bushes who’ve been cohabitating with tomatoes, beans, clover, purple deadnettle, and a prodigious patch of volunteer tansy! The point is that any garden you grow will be happier than pure, lonesome grass.
  • Everything is connected. Plants don’t pay much heed to carefully outlined growing beds, property lines, or categories of who’s a weed and who isn’t. They communicate with smells, and trade sugar for minerals along underground networks. We don’t always understand how, but we know that small actions can have big impacts, both positive and negative. The creation of long-lasting chemicals has consequences at the cellular level within our own bodies. So, too, does the creation of green space.

In summary, please plant a garden, even if you’re not sure which way is the right way. Encourage lots of different plants to grow in it. And then help your neighbors do the same. And then let’s lobby the city to preserve existing green spaces and transform easements into polycultures. And on and on, until every square inch of this island is too beloved to despoil, and our whole system is reoriented away from human laws that allow for the creation of long-lasting chemicals, and towards the laws of the natural world instead. If we think we can supersede these, we are fooling ourselves. By acknowledging that everything we create is part of a living ecosystem, we discover both responsibility and belonging.