Every fall the delivery boxes pile up in the garage, and every fall they go out with the recycling. A growing number of gardeners have started keeping them instead. Around the end of September they flatten them, carry them out to the yard, and lay them straight over the lawn, the weedy corner by the fence, or last summer’s vegetable bed. The neighbors think it looks like a mess. By April, those same gardeners are planting into the best soil on the street, and they never dug once.
The technique goes by a few names, sheet mulching or lasagna gardening among them, and it has been a staple of no-till growers for years. What has changed is that home gardeners have caught on to why fall is the moment to do it.
What the cardboard is actually doing
Laid flat and weighed down, cardboard blocks the light. Whatever is underneath, grass, weeds or last season’s leftovers, can’t photosynthesize and slowly dies off over the winter. According to the gardening guides of the Old Farmer’s Almanac, it doesn’t matter how grassy or weedy the ground is when you start.
Meanwhile the cardboard itself breaks down. It keeps the soil beneath it moist, and earthworms move in to feed on the softening fibers and the dead plant matter, working the ground as they go. Gardeners who try it often notice far more worms under the cardboard than in the bare soil a few feet away. In spring the cardboard has molded itself to the ground and is well on its way to disappearing.
Why fall and not spring
Timing is the whole trick. Put down in spring, cardboard is still a tough, intact sheet when you want to plant, and you end up cutting holes through it. Laid in late September or October, it gets a full winter of rain, frost and worm activity to soften and decompose, and it protects bare soil from erosion and nutrient loss through the cold months at the same time.
It is also the easiest way to start a new bed in the middle of the lawn. Instead of cutting and lifting sod, you smother it where it is and let the grass become compost.
How to lay it so it works
- Use plain brown cardboard. Remove all tape, shipping labels and staples, and skip anything glossy or waxed.
- Soak the boxes first. Tape peels off easily once the cardboard is wet, and wet cardboard lies flat and stays put.
- Overlap the edges by a few inches so no light gets through the gaps. Two or three overlapping layers make a much better weed barrier than one.
- Cover it with a thick layer of organic material: chopped leaves, grass clippings, compost, or wood chips. That layer weighs it down, keeps it moist and feeds the soil.
Fall leaves are the ideal topping, and this is the season they are free.
Where it doesn’t work
Cardboard is not a cure for every weed. Bindweed and other creeping perennials can travel sideways underneath and pop up at the edges, so dig those out first. In very wet areas, slugs may shelter under the sheets. And in dry climates, cardboard that is allowed to dry out can attract termites, so keep it covered and moist, and don’t lay it right against the foundation of the house.
For the weeds that sprout between the pavers rather than in the beds, the fix is even simpler: the pot you drain after dinner.
A stack of boxes, a pile of leaves and one afternoon this fall, and the digging is done before spring even starts. That’s the trick.
