Why did this happen?


Old 11-20-13, 06:05 PM
Thread Starter
Join Date: Nov 2013
Location: USA
Posts: 3
My Landscape Mulching Project

I am a ninth grader doing a science fair project. I decided to test some landscape coverings to see which ones blocked out the most weeds. I planted 16 weeds in each section and then put the landscape coverings over them to see which one would have the most weeds left over at the end. These are the results.
Control: 28 weeds
Clear Mulch (cut up gatorade bottles): 19 weeds
Rubber Mulch: 4
Pine Needles: 3
Black Mulch: 1
Granite Gravel: 0
Sand: 2
Weed Fabric: 2

I am a little confused on a couple of things.

What caused the black mulch to be so effective? It retains a lot of moisture, so would presume it steals some of the weeds' water, but it also keeps them cool.

Sand blocks out all sunlight, so why would there be any growth there? It is basically the same as gravel except for the particle size, so it should do much better than gravel.

I know that plants sense sunlight and grow towards it. Is this why the weeds in the weed fabric skirted around the fabric and came up out of the side?

Keep in mind that I planted 16 weeds in a 2x8 formation in a 11.5"x28" space. There were about 2" thickness of the materials, except for weed fabric (we put three layers of that). This was done in South Florida (Zone 10B).
Thank you!

Last edited by Dino_Rabbit; 11-20-13 at 06:26 PM.
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Old 11-22-13, 09:44 AM
Join Date: Sep 2003
Location: somewhere down in texas
Posts: 506
Not sure how to answer your question but keep in mind that most weeds germinate on top of most "weed" barriers in the silt that accumulates on them over time. Remember that weeds are very "opportunistic"!
Old 11-22-13, 04:07 PM
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Join Date: Mar 2003
Location: NC, USA
Posts: 18,605
What a cool experiment.

Since you are in sunny Florida do you think temperature might have played a role. The black mulch may have gotten hot enough to sterilize or kill many seeds.

Was the granite gravel a clean gravel or did it contain stone dust that may have formed an impenetrable layer?
Old 11-24-13, 12:38 PM
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Join Date: Nov 2013
Location: USA
Posts: 3
The gravel is actually a granite gravel, so it does not have any stone dust in the middle. Each piece is about 1" to 1.5" in diameter. Here is a picture of what I did in my experiment. I divided a field into eight 29" by 11.5" sections. In each section I planted 16 plants, then I layered the landscape coverings. You can see the gravel is in the lower left corner of the image. Black mulch, sand, and gravel did the best job. The gravel and black mulch surprised me. I thought that they wouldn't do as well as the sand, but all three coverings killed all the plants. I thought the gravel would leave enough void for the weeds to grow, and the black mulch, being a natural material, would not kill all the plants. I guess my current assumption is that the black mulch might have absorbed a lot of heat, and then the temperature killed the plants. The same thing might have happened with the gravel since it is a dark color. I was also surprised that the pine needles worked so well, especially since it was loose and lightweight.
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