I Mulched my Veggie Bed With Cardboard and Enjoyed a Bumper Harvest – So Yes, it Does Work

Cardboard mulch is being championed as a sustainable, low-cost method to suppress weeds and lock moisture into soil, but did you know it can boost your harvest, too? A USDA-funded trial found cardboard mulching was not only highly effective at reducing weeds, but increased some vegetable crops by as much as 97%. Yep, you read that right.

A variety of crops were grown over different seasons in the two-year cardboard mulch trial, a collaboration between the University of Wyoming and the USDA’s Western SARE (Sustainable Agriculture Research and Education) program. The growing conditions weren’t easy, either: the trial site of Sweet Hollow Farm in Victor, Idaho, sits in Zone 4 with a super-short growing season. Yet alongside impressive yield increases, results showed weed coverage in cardboard-mulched beds at just 15%, compared to 45% in beds that were tended using traditional tilling methods.

Impressed, I decided to test out the findings for myself and back in the summer I used old cardboard boxes to mulch my vegetable bed and sat back to see what happened. Now it’s October, and I can report that my veg bed is not only 95% free of weeds but I’ve enjoyed my best-ever harvest, I'm happy to vouch for using cardboard as a mulch if you're growing vegetables. I also discovered a few vital details that make a huge difference. So, if you want to try cardboard mulching yourself – and you absolutely should, because now we know a cardboard mulch does work – here’s what you need to know for great results.

1. Use Corrugated Cardboard

While you want to block sunlight to suppress weeds, it’s vital to allow both water and air through for healthy soil and happy roots, and corrugated cardboard makes the layer less dense. In the USDA-funded trial, two layers of corrugated cardboard were used to create a dense barrier to exhaust aggressive perennial weeds such as field bindweed. I only used one layer of corrugated cardboard and it still suppressed weeds well, and the soil beneath it still smells sweet.

I was concerned that water wouldn't penetrate the corrugated cardboard but sliding a long-probe moisture meter like this from Amazon into the soil reassured me that it did. In my very unscientific tests between this mulched bed and the unmulched bed beside it, I was pleased to see the cardboard layer kept the soil moisture level far more consistent, too.

2. Check Cardboard is Uncoated

An uncoated finish also helps soil and root health as the cardboard layer is more permeable and breathable. Another reason to avoid coated cardboard is that glossy or water-repellent finish is typically made up of synthetic polymers – use it as a mulch and you’ll inadvertently be adding microplastics to your soil.

It’s not always easy to spot coated cardboard as some have matt finishes, and you need to watch out for cardboard that’s had water-resistant additives mixed directly into the pulp during production, too. To test, put a drop of any cooking oil onto the cardboard: if it soaks right in, go ahead and use the cardboard; if it sits on the surface, that’s a sure sign the cardboard contains compounds you don’t want in your soil.

3. Only Use Brown Cardboard Printed With Black Ink

While plain cardboard is best, it’s fine to use cardboard that’s been printed with black ink as that’s likely water- or soy-based and will break down safely in soil. You’ll be happy to know that the black ink used on the 20 million standard Amazon packages sent out every day is plant-based. However, highly pigmented colored inks can contain synthetic compounds that you don’t want leaching into your soil.

Similarly, skip white cardboard as this goes through chemical bleaching processes, whereas standard brown cardboard is typically made from recycled paper fibres or raw wood pulp.

4. Remove All Labels and Tape

If you’re recycling boxes that arrived by mail, it’s vital to remove all tape and shipping labels before using. These products are designed to withstand weather and rough handling so don’t think they’re just paper. Standard thermal shipping labels contain synthetic chemicals that soil microbes struggle to break down, and research shows can be taken up by root crops. Some types of paper tape are reinforced with fiberglass, so remove every last bit.

Make it easier to peel off labels and tape, and their adhesive backing, by hosing the cardboard down first and leaving it to sit for five minutes.

You could also take a tip from the USDA-funded trial which re-used cardboard from a local craft brewery down the road, and ask local companies if they have some to recycle. Like the large cardboard slip-sheets used by the brewer to separate layers of beer cans during transit before being recycled in the trial, this packaging is likely to be free of labels or tape.

5. Overlap the Edges Well if You’re Dealing With Persistent Weeds

Running perennials like bindweed can travel horizontally in search of light, so overlap the edges of cardboard sheets by at least 6 inches to ensure they don’t find their way through the barrier.

6. Cover Cardboard With a Layer of Compost

In the USDA-funded trial, a 4-6-inch layer of compost was laid over the top of the cardboard sheets before planting began. A planting hole was then punched through the cardboard, into which a seedling was transplanted. So, while taproots and the lower root system grew into the ground to access moisture, the upper feeder roots grew into the compost layer that was rich in nutrients.

As well as the lack of competition from weeds and the extra moisture locked into the soil by the cardboard layer, trial yields were undoubtedly boosted by the added compost. Standout results included winter squash, where kabocha squash saw a 28% higher yield and spaghetti squash saw a 40% higher yield than the same crops in tilled control beds. Onions and shallots also fared exceptionally well with red onions seeing a 62% higher yield and shallots showing an increased yield of 97%.

Such a thick layer of compost wasn’t practical for my backyard veggie bed or my budget, so I experimented with leaving half of the bed with only the cardboard mulch, and half with a 2-inch layer of compost covering the cardboard layer. While weeds were effectively suppressed in both sides, and all plants cropped exceptionally well, the half with compost-covered cardboard was more productive when it came to crop yield.

I’m all for creating a low-maintenance garden with maximum reward for minimal effort, so I’ll definitely be using this method again. Though it took a little while to lay the cardboard mulch, I easily got that time back several times over because of the reduction in weeding and watering. It's also an easy way to increase the abundance of harvested crops in a small veggie bed like mine.

Cardboard boxes are a great resource for gardeners and can be used for all sorts of tasks from adding cardboard to balance plant material when composting to laying out sheets of cardboard to kill grass in preparation for a new border. Overall, a cardboard mulch made growing backyard veggies much easier, and as I usually mulch with compost once a year anyway, there was no cost but plenty of reward, and I recommend you give it a go.