Adventures in Repurposing: Cardboard Mulch

For more and more families, online shopping is becoming the norm rather than the exception. But when the delivery truck leaves and the packages are emptied, shoppers are left with an inconvenient remnant: a pile of cardboard boxes. The good news is that if there are any areas in your yard where you grow things on purpose—or where you want to make sure nothing grows at all—you can turn those boxes from burden to blessing by using them as mulch.

Whether you have a large vegetable garden or a small bed of flowers, some form of mulch is probably advisable to help retain moisture, prevent weed growth, and keep topsoil in its place. Mid to late winter is the perfect time for a preemptive strike against the weeds and seeds that are now lying dormant. Since I’m kind of obsessed with using free materials to solve problems, I’ve tried a lot of different mulching experiments. (I’ll spare you the details of the sweet gum ball fiasco of ‘05.) So I was relieved to come across a method that worked so well, was so easy, cost so little, and solved a disposal problem.

The concept of cardboard sheet mulching is simple: Lay down flattened boxes in any area in need of mulch. Overlap the edges so tenacious weeds can’t find their way through a crack. Small boxes fit well between individual plants, and larger boxes are great for pathways between garden rows or for large areas, such as landscape islands. We were thrilled to discover that our seemingly endless supply of Apologia Science shipping boxes fit perfectly between our garden rows.

A quality box will last most of the growing season, but it will decompose over the course of several months. This is a great bonus if you need to add organic matter to your soil. If you plan to leave the cardboard in place, you’ll want to remove any plastic tape or shiny labels, as these won’t break down well. Otherwise, you can simply put the boxes in place, tape and all, and remove them when you’re ready to revamp the area. I like to wet the cardboard before placement, as this makes it conform more readily to the contours of the ground, and it prevents any existing soil moisture from being wicked away by the cardboard.
Once you get your layer of cardboard in place, you’ll notice that things look really, really ugly. Unless aesthetics mean absolutely nothing to you, you’ll probably want to add a superficial layer of secondary mulch. In our garden, we add a 2” cover layer—about ¼ of what would be required as a primary mulch. We’ll use just about any natural material that’s available free or cheap–maple leaves, pine straw, pine bark, hay, or as shown in the photo, compost. Then everything can be tilled into the soil together when the time comes.

If you have an area that’s already sufficiently mulched, you can extend the life and efficacy of the mulch by adding a cardboard underlayer. Simply scrape aside the existing mulch, lay down the cardboard, and replace the mulch, one small section at a time.

Cardboard sheet mulching has provided us with better weed control and moisture retention than any other method we’ve tried, and our earthworm population seems to thrive beneath it as well. It also reduces the soil compaction caused by foot traffic. Perhaps most importantly, we feel good about the fact that we’re taking one tiny step toward responsible care of the world with which we have been entrusted. Reusing the “trash” that we already have rather than purchasing a new product (such as weed control fabric) saves an additional manufacturing process, commercial transport, and dollars that could be put to better use elsewhere.