Happy Thanksgiving! Let us be grateful for the recent rain. Let us be grateful for a climate with minimal plant diseases. Let us be thankful for all the other gardeners out there working to provide habitat for pollinators and other beneficial insects. And, let us be grateful for soils teeming with life, filled with organic matter that hold water without drowning plants.
As the season for delivery of online orders arrives with its plethora of cardboard boxes, I have been wondering about using cardboard for mulch or weed barriers. Don’t! According to Dr. Linda Chalker-Scott’s research for Washington State University Extension, cardboard mulch is not a beneficial thing to use in the landscape, even if covered with wood chips or compost.
For one thing, most cardboard is coated with an adhesive to prevent leakage or air penetration. It is often processed with chemicals than can off-gas. Chalker-Scott reports “…it (cardboard) restricts water and gas transfer between the soil and atmosphere.” This means that water cannot penetrate through the cardboard into the soil and ponds up on top. Although cardboard also prevents moisture from evaporating from a soil, this is not good when a soil drains poorly because it creates anaerobic conditions. In faster-draining soils, when the soil eventually does dry out, it is almost impossible to rewet with cardboard on top.
Here’s another important reason not to use cardboard. Termites love it. It is what termite researchers use to attract termites in feeding studies. Imagine using it next to your wooden house. Voles, also known as meadow mice, like to shelter under cardboard. Weed control with cardboard is only marginally effective. While cardboard can work as a temporary weed control method to smother young weeds a few weeks before planting a vegetable garden, it shouldn’t be used long-term. In addition, cardboard is not an attractive landscape option.
Chalker-Scott suggests that for those gardeners who are fans of sheet mulching, which combines things like newspapers and cardboard with compost and wood chips, just use the compost and wood chips instead. They are much better at building a thriving soil without the cardboard in the way. Compost and chips allow good water percolation and air flow and they provide nutrients for soil microorganisms.
You will find most websites say cardboard is great to use in the garden or landscape and that it breaks down quickly. Having followed Chalker-Scott’s research and fact sheets for years, I’ll listen to her.
JoAnne Skelly is associate professor and Extension educator, Emerita at University of Nevada Cooperative Extension. She can be reached at firstname.lastname@example.org.