I don't know for 100% sure but consider that once it gets wet and swells up, there is plenty of room between the fibers for water to move. If anything is dripping through your piece of cardboard, that's probably faster than an inch of clay. Keep in mind that water percolates through the soil above down to the cardboard layer, at a limited pace.

Besides none of this will matter after a few months because the cardboard will have degraded and been munched up by worms.

I mulch between my fruit bushes with cardboard to suppress weeds. Over the cardboard I spread 4 - 6 inches of leaves. The card slowly decomposes and needs replacing annually. Even in my rainy climate there is no problem at all with water pooling or running off. The leaves absorb a lot and it reaches the card slowly. Then the card gets moist very quickly. Once damp, water passes through it easily. Shredded or balled up paper would be ineffective at weed suppression. The coverage has to exclude light, so the bigger the sheets the better.

Perhaps I misunderstood the original question. I was thinking more about placing it on top of grass or weeds to build a raised bed on top, rather than as a mulch in an already-existing garden. I have not used cardboard as a mulch but I have used newspaper under shredded wood mulch trying to smother Bermuda Grass. I've also put several layers of newspaper to keep down grass under a new raised bed and never had any problem with it.

Well, that's a fair point. If you're trying to suppress growth, then putting down layered newspapers that will last a while might be the right strategy.

I laid down some serious cardboard to snuff out the grass in the tree lawn and then mulched on top of it. It seems to soak up the water just fine. It's early in the process but I think I'm going to like it. Just pull off the tape and as much of the labels as possible.

"If you pick up a piece of cardboard with a puddle of water in it, it only slowly drips through. It seems to percolate about as fast as it would through heavy clay."

That is true, and is the reason many of us wet the soil that will be under the newspaper or cardboard befoe laying them down, and is the reason given by those that wet the newspaper or cardboard before laying it down. If the soil that will be covered is dry placing layers of paper on that soil may well keep rain, or water from other sources, from reaching the soil for a while and can be one of the benefits of using newspaper or cardboard. However, if you are laying that paper around growing plants wetting the soil before laying the paper would be a very good idea.

Keep in mind that cardboard is made of multiple layers of paper.

kimmq is kimmsr

Take any kind of spike or sharp stick and punch some holes through the cardboard - as many or as few as needed to let water flow through and keep puddles from forming.

I'll just repeat what I said above. I mulch with cardboard - the thick kind from packaging boxes, not cereal boxes- covered in leaves between my fruit bushes. I've never made holes and never had problems with water either pooling on top or failing to penetrate. of course the soil under the card is always damp and there is plentiful rain. Hairmetal is anticipating problems but has not actually tried it yet. I'd give it a go on a small area and see what happens. There may be nothing to worry about.

Tox, I agree with you; but if someone were worried about it or found it to be a problem , the solution is easy, low tech and would take no time at all.