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Kill weeds for free with sunlight and trash bags

12 Aug

Untreated-vs-treated“Green.” “Biodegradable.” “Sustainable.” These terms sum up a lot about how I try to live my life, and yet they carry hidden baggage beneath their friendly exteriors:the ecologically-friendly, non-toxic, all-natural alternatives are more expensive and yet less effective than their more-toxic cousins.

Take oven cleaner, for example. Conventional oven cleaning spray is nasty stuff, but it works — it makes a tough job much easier. Cleaning an oven “organically” takes laborious scrubbing and a lot more time. I wish this weren’t the case, but it’s true. After all, if the non-toxic way was cheaper and easier, wouldn’t we all be doing it already?

Therefore, I feel I must celebrate any “green” option that actually saves both money and effort. Today’s example: Black plastic bags can kill grass and weeds on your patio at no cost to you. I have reclaimed an overgrown patio without pulling a single weed or spraying a single chemical by using a few trash bags and the power of the sun. The only cost to you is patience.

Method: Wait for sunny weather. Cover the area of vegetation that you wish to kill with any kind of black plastic. Put rocks or other weights on the plastic so it stays in place. Leave the plastic there for at least a week, if possible.

What will happen: Any vegetation underneath the plastic will wilt and die. Given enough time, the roots will die as well.

Patio setup for weed killing experimentScientific explanation: The color of an object is determined by what kind of visible light it reflects best. White materials look white because they reflect all colors of visible light, which look white when combined together. Black materials look black because they absorb most visible light rather than reflecting much of it, making them dark in appearance. The absorbed light energy then radiates out as heat (a.k.a. infrared light, which we cannot see). Sunlight absorption is what makes the black plastic effective against plants. First, the plastic prevents the plants underneath from receiving any sunlight, which stops their growth. Second, the absorbed sunlight also heats the plants far beyond their comfort zone. The plastic keeps fresh air from circulating across the ground surface, and the high temperatures beneath the plastic during a sunny day will roast the plants to death. Given enough time, this will kill the roots under the ground and discourage the plants from growing back. This is also why trash cans will create a patch of dead grass right under each can.

My evidence: I treated different patches of my brick patio with black plastic for at least a week at a time. During this time, I did not mow, weed, or poison anything on the patio. I left parts of it untreated to serve as the control group (vital for measuring the results of the experiment). I took photos to record how well the plastic killed the plants and how quickly the plants came back. The photos speak for themselves: the treated patches remain almost entirely clear of plants, while the untreated areas are so overgrown that the bricks are barely visible. The only plants to survive the treatment were located under the edge of the plastic where light, air, and moisture could pass more freely. Moisture trapped underneath the bags did lead to some temporary green mold growth on the bricks, but that faded away after a bit of direct sun exposure when the bags were removed.

Cost to me: under $1 (the bags can be reused repeatedly, which will make their initial cost even more negligible over time, or you can use them as trash bags when you’re done). Of course, this will cost more if you want to treat an entire patio in one shot and need lots of coverage.

Time spent: 5 minutes placing and removing the plastic.

Detailed 'after' shotAdvice: Use the thickest, largest, darkest material you have. A grid made of individually-weighted garbage bags will work (as I demonstrated here), but some weeds may survive at the edges of the bags where air and light leaks in. A dark tarp should work, but beware: some tarps include a reflective liner to reduce heat buildup, which defeats the entire exercise. Plants can still invade when you’re done, so you should expect to repeat the process at least once a year (or more frequently if the plants come back faster).

Overall value: High. You have to plan in advance (and accept the unsightly plastic for a week or so while it does its job), but you don’t have to pull weeds manually, spray toxic herbicides, or spend more than a dollar on the project.

 

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  1. McCaul

    05/30/2013 at 8:53 am

    Brilliancy! Love it!