Cardboard and egg carton kill mulches
My favorite material for
blocking sunlight and weeds at the bottom of a kill
mulch is cardboard, so I was thrilled when a friend told me he was
getting rid of a lot of cardboard boxes. I've been carrying in
about ten from the parking area each morning when I walk Lucy, but I
used up a whole weeks' worth in about half an hour Friday. That
sent me hunting for other kill mulch bases.
Egg cartons seem to have
a lot of potential in a certain niche. If you open them out, the
bottom half of one carton overlaps the top half of another carton quite
well, although the 
seam between rows is less
secure. I wouldn't want to use egg cartons as a kill mulch if
there were really ornery weeds underneath since the plants would
certainly find that gap.
That said, I have a
feeling that the cups of the egg carton might capture and hold water,
helping it infiltrate the soil rather than running off. My gut
says that fungi will also like the extra air space left behind when I
top the egg cartons off with mulch. As with my bark kill
mulch, I'll be
waiting to see how the experimental weed blocker works as the growing
season progresses.
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About us: Anna Hess and Mark Hamilton spent over a decade living self-sufficiently in the mountains of Virginia before moving north to start over from scratch in the foothills of Ohio. They've experimented with permaculture, no-till gardening, trailersteading, home-based microbusinesses and much more, writing about their adventures in both blogs and books.
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There are lots of different permutations, depending on whether you're using seed-free compost (can go on top of the cardboard) or seedy biomass (has to go under the cardboard.) And whether you're building a whole new bed or just getting lazy so you don't have to weed. I was doing the latter and don't actually plan to plant there at all since it's directly under the canopy of the tree.
In general, you leave the cardboard in place no matter what. You can cut holes in it if the cardboard covers seedy biomass and you have something to transplant there. But if you do it now, you might not even have to worry about that --- cardboard rots very quickly when in direct contact with moist ground. It should be gone within a few months.