A few weeks ago, I attended a forum on pollinators in our gardens. Among other things, we learned about their role in our ecosystems and methods for planting native plants to attract and feed them. This is a very interesting topic that we will discuss at another time (let me tell you, our native insects are not doing well!). The speakers were all experts in their fields, scientists and researchers working at American colleges. But something bugged me about one of the presentations on setting up pollinator gardens: the use of cardboard as mulch.
The speaker suggested putting old cardboard boxes that have been dismantled on the ground to prevent weeds from growing. It’s a quick way to create a new plant beds where there were none without dealing with the weeds already growing there. They suggested planting in the spaces between the sheets or making holes and adding organic mulch over the cardboard to hide it. Sounds like a good idea, but does it really work?
The Advantages of Cardboard as Mulch
Cardboard can effectively prevent weeds from growing. It blocks the sun from reaching the leaves of the plants underneath, hindering photosynthesis and the plants’ ability to grow. It’s also difficult for seeds to germinate on the cardboard. However, if organic mulch is added, they may still germinate. This isn’t surprising! Weeds are very difficult to control and tend to settle in the most unlikely places: cracks in the sidewalk, on a stone wall, I have even seen one grow on a wire fence where a small pile of organic piled up. In other words, there will always be unwanted plants in your garden and cardboard is a very effective material to slow down their progression.
Another point in favor of cardboard as a mulch is that it decomposes and will feed the soil in your flower beds. Cardboard is made from trees after all. You don’t have to worry about its toxicity either, as the inks used in its production are vegetable-based. Be careful, however, not to use waxed cardboard which will take a long time to decompose and prevent water from passing through.
What bugs me about the use of cardboard in our gardens is that it can interfere with the flow of oxygen in the soil. The uptake of nutrients by plants requires that there be oxygen in the top layer of soil. Respiration and water absorption are also affected by the amount of oxygen in the ground. And let’s not forget the microorganisms that are beneficial to our plants and need to breathe. In short, cardboard can affect the fauna present in the soil and slow down the physiological processes of plants. One point against cardboard!
It’s also possible that cardboard will reduce the amount of water that makes it to the ground underneath it. Ouch! Another big blow against cardboard. I’m not sure they’ll get up after this…
Other Uses for Cardboard
Well, it seems that cardboard as mulch is not such a good idea after all. That doesn’t mean that cardboard is useless in the garden. For example, you could use it in the pathways of your vegetable garden and add a layer of mulch like straw, leaves or ramial wood chips on top. This will prevent weeds from growing and since there are no plants, the lack of oxygen in the soil won’t be a problem.
You could also use it to prepare a plant bed in advance. By placing cardboard on the ground for several weeks or even months, the weeds will eventually die. Be careful, because some plants take a very long time to die, perhaps even years, to die off. It depends on the type of weed you want to get rid of. This technique is used in agriculture but with black plastic sheets.
The Alternative for Laidback Gardeners
So how do you create a new flower bed? Start by covering the chosen area with a dozen sheets of newspaper or heavy cardboard. Then add 20 cm of planting soil. Without light, weeds and grass will die and the newspaper or cardboard will slowly decompose. Add organic mulch while you’re at it. And that’s it! You have A Nearly Instant Flowerbed for Laidback Gardeners!