Over the years I have had an ongoing battle with onion weed in my small front garden. I have poisoned and I have carefully dug out every bulb by hand (took forever), but to no avail. I tried using a weed mat, which worked for a while, but eventually the weeds made it through. I have a piece of old carpet which would cover the area, and I thought that if I put the carpet down, and then put gravel on top, that might work, but someone told me that it would make the soil go sour. I also thought about that black thick plastic you can get in Bunnings. Can you tell me if carpet or plastic would be a problem?