Mulching

To Cardboard Mulch or Not to Cardboard Mulch

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.

Cardboard spread in a garden between plants used as mulch
Cardboard used in a garden as mulch. Photo: London Permaculture, flickr.com.

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.

Several people put cardboard sheets on the floor
Installation of cardboard as a mulch for weed control. Photo: Grand River Conservation, flickr.com.

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.

A man holds soil in his hands
Oxygen is needed in the soil for microfauna and a plant’s physiological processes. Photo: U.S. Department of Agriculture.

Yes, But…

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…

Cardboard boxes laid out on the ground to make a pathway
Cardboard can be used to create paths in a garden. Photo: ediblebackyard.co.nz.

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.

Drawing of a person spreading soil on newspaper sheets
An almost instant flowerbed for laidback gardeners with newspaper or cardboard.

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!

Mathieu manages the jardinierparesseux.com and laidbackgardener.blog websites. He is also a garden designer for a landscaping company in Montreal, Canada. Although he loves contributing to the blog, he prefers fishing.

15 comments on “To Cardboard Mulch or Not to Cardboard Mulch

  1. Kathleen Craig

    I used a paper shredder to shred up the cardboard for mulch.

  2. Allen R Linoski

    Not a bad article except for “Well, it seems that cardboard as mulch is not such a good idea after all.”
    The “negatives” presented are, at worst, temporary compared with plastic yet the author summarizes with the above statement, then finishes the article by suggesting its use(???).
    Cardboard over weeds/turf with mulch on top is the best way to convert tonatural planting.

  3. It also can cause problems by attracting mice who like the shelter. They have banned it at my mother’s organic allotment. However, when buried, I haven’t had this problem.

  4. Jackie Harris

    It will also collect roaches,as they eat it and nest In it, also Palmetto bugs (big roaches) in the south.

  5. I would not have done this, . . . but one of my tasks on Thursday is to ‘dispose of’ waxed cardboard boxes from the food distribution (which is like a mobile food bank) in town. I bring it back here where people I work with cut it into smaller pieces to start fires in their fireplaces or stoves. There is surplus of course, so . . . I used some of it in a few minor portions of the garden where I do not need to step on it much. It is slippery to walk on, which would be a major deterrent to its practicality. I could get plenty of unwaxed cardboard if I wanted it, but I prefer to let it get recycled. Waxed cardboard does not decompose as fast as unwaxed cardboard.

  6. More value of cardboard:
    – you aren’t disturbing the soil and causing dormant seeds of unwanted weeds from sprouting, especially if you also add shredded bark or other mulch on top of it. Hopefully they never see the light of day.
    – If you are getting rid of unwanted deeply rooted growth (briars, tree seedlings, and such) under small trees you don’t have to dig and disturb the trees’ roots.

  7. Thanks, Mathieu. The “lasagna” method is the nearly-perfect way to prepare a piece of ground for new planting. But yes–it takes time. We killed our entire mess-of-a-“lawn” that way, by just letting the cardboard and its topping of 3-4 inches of organic compost sit from Oct-March. By spring it was a lovely “clean slate” ready to be planted. But I’m surprised the experts at your conference recommended it as a regular mulching system around established plants! Thanks for pointing out all the reasons to not do that.

  8. I have a particularly invasive weed that I am battling in my garden spaces (Campanula Rapunculoides — creeping bellflower). Cardboard and newspaper have been my friends in smothering it and keeping it at bay in select areas. I concur that it is a great tactic in preparing new beds. Thanks for the great post, Mathieu. ~ Cindie

  9. I use cardboard delivery boxes all the time in the areas I will eventually add to my gardens. Also, it is the only thing that has kept me civil to my neighbors weed garden adjacent to my property. I load up the cardboard behind my line of ornamental grasses and it prevents 90% of her weed vines from infiltating my gardens, lol.

  10. I’ve used cardboard all around our acre property to help define paths and to prep beds for future planting. It’s also a great way to handle the plethora of boxes from delivery services.

  11. I think that using cardboard is not really a good alternative for weed control on a continuing basis in garden beds because, as you said, of the negative effects on soil and insect life. It does work well though for eliminating grass or sod in an area planned for a new bed. Cardboard put down in fall will last well into the winter and generally can be dug into and mostly incorporated into the new area the following spring. Multiple layers of newspapers also do the job quite well but don’t last as long. The best weed control in a flower bed (not so much in a vegetable plot) is just planting really closely so as to cover the bare soil with vegetation. Nature abhors a vacuum and wants plants (and that includes weeds) there doing their job, controlling erosion with their root systems and breaking down when they die back and adding fertility to the soil. If you plant something else, desirable to you, the weeds won’t have a chance to take up that empty space.

  12. Jt Michaels

    Ditto on the use of cardboard for veggie garden paths when covered with straw, leaves or other organic matter for aesthetics. Very helpful!

  13. i’ve used cardboard successfully when I’ve had a large area of grass to convert to garden or for mulched areas. I recently had to dig up some sod for moving blueberry plants so I can say that cardboard is certainly easier on the back.

  14. I’m a fan of using newspaper it has worked well for our gardens.

  15. We’ve used cardboard to create pathways, covered with the stalks of last year’s decorative grasses, which would otherwise be too loose to prevent weed growth.

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