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I tend to have a lot of corrugated cardboard boxes laying around, due to getting various things shipped to my house. I have heard of using cardboard for a mulching material, but I'm not sure exactly how to do it (maybe I'm overthinking things). Ignoring the option of shredding and composting it:

Should I shred it and treat it like I would straw mulch? Should I rip it into sheets and just lay it down flat on the ground between rows? Should it be covered with anything, or can it just sit out? Is there some other method that I haven't thought of?

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I'd be very wary of corrugated cardboard because of the glue used to stick it together. You don't know whats in that. I wouldn't want that in my veggie garden. Probably fine in flowerbeds... –  wax eagle Jun 14 '11 at 1:58
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@wax - Interesting point on the glue. I've been considering using it in my garden, but now that I'm thinking about it, most of what I've seen for recommendations on using cardboard is as the bottom layer of a deep sheet mulch (where it won't really affect any veggies grown on top). Do you know of any references pointing to harm from the glue? –  bstpierre Jun 14 '11 at 2:20
    
I'm not so sure of that. It might have been true in the past but starch glues are apparently the norm. Even PVA glues should break down quite nicely given enough time. I've just been searching, and wax coatings might be a problem though (as per my comment on plastics and tape) –  winwaed Jun 14 '11 at 2:26
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2 Answers 2

up vote 9 down vote accepted

If you are trying to simply keep the weeds down, then I think some people do just lay it down flat. You will need stakes (eg. Bent metal pins) to keep them down and stop them blowing around. You could also cover the sheets with something more pleasing - wood chip mulch. The card would act as a barrier against weeds, and the whole lot will eventually break down.

Shredding would aid break down and composting. This is what I would do if I was putting the card in a compost bin/barrel (I put a lot of shredded paper in our barrel). it could also go in the bottom of a trench or pit, to aid water retention (actually this is what I'd do - early spring, till my raised beds and put the card in as low down as I can).

When composting paper and card, be sure to remove all the tape and any attached bits of plastic. Shouldn't be a problem with card boxes, but be wary of glossy inks - these can contain oils and metal-based dyes which might be problem (this problem is diminishing as more and more glossy papers and magazines switch to soya based inks).

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+!, You've covered everything I would have mentioned, although would've added that cardboard is biodegradable. –  Ambo100 Jan 18 '12 at 21:06
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I talked about my experience with cardboard as mulch in this answer:

I experimented this past summer by laying down overlapping sheets in the path between two beds in my vegetable garden and weighted it with rocks. It gave very good weed suppression. The only weed problem I had was that some creeping weeds would "hide" under the edges and sneak out the sides; it was harder to hoe these out since the cardboard was in the way. Don't do this if you care about appearance, it's a bit ugly.

Since weeds will poke up through gaps, and a single layer offers excellent weed suppression, I'd recommend against shredding or cutting the cardboard in any way. Just lay down big sheets and overlap the gaps.

Also, I'm going to guess that it's not going to do much for me this coming summer: I think it will be broken down enough that weeds will start to poke through, especially as it tears when I walk on it after everything thaws. I'll probably just put down another layer.

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