Cardboard mulch is a quick way to recover or create a garden out of a weedy patch. The use of cardboard as a mulch is favoured due to low cost widespread availability of the material, particularly in urban areas where it can frequently be obtained for free. A major advantage of cardboard over other types of mulch is that it is often a waste resource and, by repurposing it in this way, it is possible to alleviate the need to recycle it and allows the generation of value from an otherwise unwanted output.
Any packaging tape or laminated labels must be removed prior to mulching as these will introduce plastics into the environment as the cardboard decomposes. The use of cardboard for planting foodstuffs can also be hazardous if recycled paper has been used in its manufacture: oil-based inks printed onto the paper can contaminate the soil and any crops grown in it. Cardboard may also be contaminated with pesticides, fire retardants or other chemicals that have been introduced by the contents of the packaging. Although low-risk, these concerns should be considered before using cardboard on any project.
Planting into cardboard
You may wish to immediately plant an area covered in weeds. Water the area thoroughly. Cover the weeds with a sheet of cardboard. Cut a small hole in the sheet and insert the seedling you're planting through this hole. The cardboard will kill the weeds by blocking out light. The weeds will then decompose and become nutrients for your plants. The cardboard will decompose too, though it takes a little longer.
You can use it for planting seeds, too. Just clear a small strip of ground from weeds, a few centimeters (a couple of inches) wide. Add compost for best results. Make a slot in the cardboard, narrower than the cleared space, with no weeds showing, and sow the seeds. Follow the same directions you normally would. Keep the ground moist - the mulch will make this easier.
Other uses
Cardboard has also been used to suppress weeds, grasses and other plants when laying pathways. In this manner, cardboard is placed beneath a hardier mulch such as wood- or bark-chips, ensuring that each sheet overlaps the next by several centimetres (at least one inch). The hardier mulch forms the hardstanding for the path and will ensure it is long-lasting, whilst the cardboard prevents weeds or other unwanted plants from growing.
The use of cardboard as a lasagna mulch is also popular. Cardboard is layered with compost, manure or other organic matter to create a raised bed that, over time, will decompose and form a solid structure for planting. Lasagna beds constructed with cardboard can typically be planted in their first year.