It’s a technique called Hugelkultur . It’s from a brilliant German permaculture (a way of designing living ecosystems to eliminate the need for external inputs) engineer called Sepp Holzer, although he started doing this well before the permaculture brand emerged.
Mini Toilet Paper Roll Seedling Pots - Recycle your toilet rolls by filling with potting mix to raise seeds. Keep well watered until ready to transplant. Bury the whole 'pot' and the cardboard will decompose with no damage to fragile roots + add carbon to your soil! | The Micro Gardener