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Plastic weed mat and cardboard residues?

 
                        
Posts: 34
Location: Big Island, Hawaii
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A topic I've been wndering about a lot.

I've used standard sheet mulching (cardboard/whaever and loads of mulch) on acres of land with good results. I've also used black plastic weedmat on acres with good results, at least visibly for a few years. The plastic I use by placing weedmat over invasive grasses, shrubs, unwanted coppicng trees for 3-12 months, then flipping it over to the next patch and planting viney soil builders (legumes, sweet potato, nasturitium etc) that grow over the newly exposed soil then over the weedmat. The weedmat is then flipped after 3-12 months again and the soil building vines grow intothe next exposed patch. The first weedmatted area can then be planted to trees, crops food forest etc. This is rolling permaculture, I guess, but I call it the "uala (Hawaiian for sweet potato - "ooowallah" tsunami." this eliminates 99% of invasive grasses and many weed seeds (due to heating during sunny days of the top few nches of soil under black plastic?) when properly spaced. Every grass I've run into can be excluded if each flip of the weedmat overlaps 6 feet into the previously weedmatted area. So if I use 12 foot weedmat with each flip I gain 6 feet of mostly weed free area ready to be planted to soil builders or cultvated with Ammendments and crops. Less invasive grasses need only a 1 foot overlap. A very successfully technique I have used recently goes like this. First, I plant a row of plants, crops trees nfts etc. Then sheetmulch a strip 3-6 feet wide Along the row. Then weedmat is placed on top of the mulch and extanding away from the crops on either side. This is directly over whatever weeds grow around. So viewed from above it's like an Oreo cookie- two strips of weedmat sandwiching a narrow exposed strip of sheetmuclh. The weedmat is flipped out as abbove every few months, and the polyculture grows frm a weed free "strip nuclues(sp?)" of pinoneer species. A tall crop with viney crops below works best, ie sugarcane, sweet potato and perrenial peanut or acai palm sweet potato and squash. Sweet potatos rock.

Anyway I see lots of concern about toxic residues from plastic and cardboard, and I'm wondering if the ease and effectiveness of this method is outweighed by the potential residues.

A note about weeds: i am thankful for all the hard work weeds do building soil, providing nectar, and generally being alive. I can recognize plants that are inappropriate in a food forest/garden and do my best to exclude or reduce them where needed.



     
       
 
Troy Rhodes
Posts: 618
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There are certainly examples where some polymers can leach solvents or plasticizers or whatever.  But, having studied a little polymer chemistry, I personally am not concerned with what you are doing.

I have no specific scientific basis for making that judgement about your particular polymer, except that it was designed to be pretty stable in the presence of sunlight and water

I think it is unfair to label all plastics as evil and dangerous because they leach.  Look at it from the other side of the coin for a minute.  One of the problems with most polymers is that they are so danged persistent in the environment.  So I have a little problem with folks who claim on the one hand that plastics are evil because they're not biodegradable and hang around for centuries, and on the other hand, that claim they are leaching all kinds of toxic chemicals into the soil and the groundwater.  You really can't have it both ways.

Further, in healthy soil, there are jillions of bacteria, fungi, protozoa, invertebrates, and on and on and tens of thousands of different species.  It turns out that there are not too many things they can't eat, or surround and neutralize (bio-remediation).

So, I don't go dumping weird chemicals on my soil willy nilly, but I don't lose sleep over whatever minute amount of stuff might be leaching out of your plastic.

Would I be happier with a polymer made from corn starch that biodegraded quickly after 3 years?  Maybe.  And maybe/probably it came from evil GMO FrankenCorn that's screwing up the countryside rapidly, so maybe not.

Certainly, your plastic sheeting that is under mulch/soil will break down chemically very very slowly, and by extension, leach stuff into your soil very very slowly.  Probably insignificantly.

That's my educated gut feeling.

Degree in chemistry and biology, and organic gardener for 30 years, for whatever those qualifications are worth.

Oh yeah, and I'm a doctor and have some formal training in toxicology.

I would bet my last pair of socks that your food from your land, even though it touched a piece of plastic, is better than the food at 99% of the grocery stores in terms of chemical residue.

HTH,

troy
 
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