I am helping my son to do a science fair project. He decided to test some landscape coverings to see which ones blocked out the most weeds. He 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 think that some sunlight is maybe reflected off of the sand particles, but I am not sure.

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!