Cardboard mulch is a quick way to recover or create a garden out of a weedy patch.
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 cm (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.
- But please consider when planting food, that cardboard is made of/partly made of recycled paper which is contaminated with oil-based ink. Also, depending on what was packaged in the cardboard, it could have been sprayed with pesticide, or contaminated with fire resistant or other chemicals.