How long does cardboard take to decompose? Find answers to using a cardboard weed barrier in your garden and to build soil with a no-till farming method.
Usually, you have to choose between fast, cheap, and good; using cardboard in the garden achieves all three. It’s fast, it requires no special tools, and you can obtain everything you need to do it free of charge. It’s a no-till, chemical-free way of taking down overgrowth and planting vegetable, perennial, or pollinator gardens. It controls weeds and retains moisture while building soil fertility. Just lay cardboard right where you want to garden, on top of whatever weeds are already growing there, and cover it with wood chips, straw, or mulch.
How Long Does Cardboard Take to Decompose?
The best part is that the cardboard stays on the ground to decompose, adding biomass and nutrients to the soil. The cardboard holds up best for one growing season, so it’s a good fit for annual vegetable gardens that die back and will be replanted every year. But you could also use it around perennials, adding a new layer of cardboard and mulch each year to build up the soil. By the end of the first year, the cardboard will be mostly decomposed, so in future years, you can plant right over where you’ve laid cardboard. If it hasn’t quite turned into soil, simply dig a hole through it to plant, and the cardboard will continue to return to the earth. Another wonderful thing about using cardboard for ground cover is that if you ever want to rearrange your garden layout, you can add a fresh layer of cardboard and mulch and leave your old layer underneath.
When my husband, baby, and I moved onto an old homestead in 2020, it had no established garden plot, even though I’m sure the original family had done a lot of gardening over the past 80 years. Our kindly neighbor, who lives on the original family homestead, came over with his grandfather’s tractor, which had tilled this soil for decades, and plowed up a section of the hayfield for us to turn into a garden. Watching while holding my baby in my arms, both of us excited to see this old machine do such powerful work, is a memory I’ll always cherish.
Before I discovered cardboard gardening, I’d mulched with layers of newspaper, and I’d also tried black non-woven polyester landscaping fabric. Both had drawbacks. Mulching with newspaper took so much time and work to lay out the papers and weigh them down before they blew away. And enough black polyester fabric to cover my 30-by-30-foot garden was expensive. Plus, crabgrass poked its way through the fabric and encroached from all edges, quickly taking over a third of my garden. The fabric became fragile as it aged, and it got tangled in weeds, making it hard to remove. The polyester turned into tiny pieces of plastic in the soil.
So, this spring, I hopped on the cardboard gardening train!
How to Use a Cardboard Weed Barrier
Gather cardboard. You’ll need plenty of it. We had lots of cardboard folded flat in the barn from projects around the farm. If you don’t already have enough, ask your friends and neighbors to save boxes for you. Make a post on your local Buy Nothing group or Craigslist, and people will be more than happy to have you haul away the cardboard cluttering up their garages. I’ve also been known to pull over and take big boxes I see sticking out of recycling bins. The grocery store near me has bales of compacted cardboard out by its dumpsters, so your local grocery store might be a good source too; just check in with the manager. Make sure your collection trips are worth it, and don’t drive across town for one box.
Cardboard of all sizes can be useful. Big boxes fill in large areas of ground, and smaller boxes are great for laying around existing plants you want to keep. Use plain brown cardboard with no plastic lamination. Boxes with a laminated finish won’t let water through to the ground and might leave behind plastic when they decompose.
Cover the ground. Another benefit of cardboard gardening is that you won’t have to till, so you won’t disturb the soil structure, and the cardboard will block the sunlight, leaving weed seeds dormant. Leaving the soil structure intact means fungi and good bacteria will keep your soil healthy and help plants grow.
I laid my cardboard in rows, leaving 1-foot gaps between to plant vegetables. As you lay out the cardboard, overlap the edges by at least 6 inches to keep weeds from easily growing through gaps. I liked the thickness of leaving the cardboard doubled. And in a few spots, I threw some old 100 percent cotton cloths underneath to compost too.
If you’re starting a garden from scratch over an area where plants are already growing, lay down the cardboard and mulch at least six weeks before you want to plant, and water regularly so it flattens to the ground. This will smother the plants underneath, and the cardboard will soften, so you’ll easily be able to dig holes through it to place your plants or seeds. If you have a short growing season, try putting down cardboard in fall so you’ll have the next year’s full growing season to lay a fresh layer and plant.
Cover the cardboard so it doesn’t blow away. I used wood mulch over top because we cut down a tree and had a big pile of wood chips left over. The mulch looked beautiful and did a great job retaining moisture. You can also use straw or just some heavy rocks. If your neighbors are particular about how things look, wood chips or wood mulch are the most aesthetically pleasing. To get free wood mulch, call your local tree-service businesses and offer to take a load of mulch from chipped trees. But be aware, you might get a pile of wood chips as big as a small car!
When deciding how thick your layer of mulch should be, consider how much rainfall you expect. In a rainy area, a mulch layer that’s too thick will just make a fresh layer of organic matter for weeds to sprout and spread through. In dry areas, thicker mulch will retain more moisture. Minnesota, where I live, has been wet in spring and fall but dry in summer months, so it’s a tough balance. Because I had such a big pile of wood chips, I made my mulch layer more than 2 inches thick. But next year, I’m going to try 1 inch and see if it’ll still contribute to moisture retention without providing such an inviting place for quackgrass to spread its roots.
Keep weeds from creeping in from around the edges, which is easiest if you have edging. Sidewalks and other concrete edges are great. In my farm garden, I use old rugs cut into 2-foot widths – they’re easy to move and porous, so they let water through, but they smother weeds and last a long time without rotting. You could also use scraps of plywood or boards – anything unwanted plants will have a hard time getting through. Now, I can’t speak to old rugs and plywood breaking down and leaving microplastics or chemical residue in the soil, but since the black polyester left such tiny pieces that I couldn’t remove them all, I see my new method as an improvement.
Plant and enjoy! Plant your seeds or seedlings as you normally would, following spacing and depth guidelines. Push a little mulch around them once they sprout. I found that weeding once or twice early in the season was enough to keep the weeds down, and after that, I just pulled wayward offending weeds as I was out tending and harvesting.
Cardboard Method = No Till Farming
After just three years of not tilling, the soil is so much richer than when we first plowed. Cardboard gardening retains moisture, adds nutrients, and limits erosion and weeds, and my veggies are growing great! I’m hooked, and from now on I’ll never throw cardboard in the recycling bin again – I’ll garden with it!
To learn about reducing the risk of cardboard-introduced PFAS, or “forever chemicals,” in your plot, see “Avoid ‘Forever Chemicals’ in Your Garden.” – Mother
Mercedes Tuma-Hansen is a proud mother of two young children and a history educator, and she and her husband are working to establish an organic farm in Minnesota.
