Why Sheet Mulch?
Sheet mulching is the organic and earth friendly way to remove a sod lawn or to smother weeds without using chemicals. Instead of using plastic or other non-decomposing fabric, the cardobard decomposes over time and enriches the soil. However, it lasts long enough to prevent the sunlight from hitting the weed seeds so that by the time the cardboard is gone, the viability of the weeds is significantly less.
Other advantages are that you do not need to remove the lawn to kill the grass. You can just cut it low, leave the clippings on the ground and cover. This way the organic matter degrades and enriches the soil instead of the grass being taken to the dump.
One exception to this method for lawn removal is pernicious grasses like Bermuda. For such grasses, we recommend that you mechanically remove the grass (sod cut?), water during active growth season to make it root again (because some rhizomes will be left behind), and then remove the new growth before using sheet mulching.
Even if you are not removing a lawn, note that the moment you start working the soil to plant new plants, etc, the weed seeds will be exposed and three inches of mulch will still let the weeds grow. Therefore, the cardboard layer is the only way to stop the sun from reaching those weeds.
Why cardboard?
The reason we do not like the fabric or plastic options is two-fold:
a) Weed seeds fly in and start growing on up of the fabric anyways
b) The soil cannot breath and starts dying under the fabric and reducing the health of your soil long term.
How to Sheet Mulch:
1) Cut down grass. Remove pernicious weeds (as much as possible mechanically)
2) Trench about a 6 inch wide strip that is 3 inches deep around the hardscape so that after step number 6 below, the mulch does not roll over to your hardscape.
3) Apply 1/2" of compost over whole area
4) Apply a layer of cardboard or two layers of butcher's paper. Overlap edges by 4 inches so that no gaps exist.
5) If you walk on this layer, it will punch through, so we recommend that you roll out the cardboard and toss the second layer of 1/2" compost at the same time. The compost layer distributes the weight better and less punching of the cardboard or paper takes place.
6) After the second 1/2" of compost mentioned above, top with 3 inches of mulch. Clean arbor mulch, bark mulch, wood chips are good options.
Other Resources
Lyngso carries the materials for sheet mulching such as 250' long rools of 4 foot wide cardboard, compost, etc and they have their own blog post:
https://www.lyngsogarden.com/community-resources/sheet-mulching/
CA Native Plant Society Notes: