Can someone tell me how these stupid weeds keep growing through my rocks, when there is thick plastic underneath? If this has happened to you, how did you stop it?

Not of much help Honi.When the Plastic Sheeting was put down What was under this? Bare Earth?
Where I've seen it laid at least a 2"-3" thick layer of sand between the ground and the plastic keeps the weeds away .
If this is missing there is a good chance certain weeds will take root
Ps Any Plastic which is colored yellow/white or clear is also inadvisible to use as it acts like a Greenhouse,if there is any air pockets !

Dust, debris and fallen leaves fall between the rocks and compost. Along with weed seeds which will take advantage of the tiniest amount of soil to grow in. This composted material is rich in nutrients and will make weeds thrive even on the tiniest amounts of compost.

Some weeds can and will grow through the smallest of holes in plastic ground covers.

Things you can try:

1. Restrict watering on the rocks. This may work well in the summer to keep things from growing, however come the first rains of fall and you have sudden growth. I think in the land of Lincoln you also get summer rain - Strange concept for us here in California. So restricting water may be a problem.

2. Add more rock, to a depth of 4 inches ( 4 inches isn't that deep, its about the width of your palm) or more. This will cause the weed to have to grow fairly tall to get sunlight early enough to mature. Some weeds can still grow in this kind of condition. Blow off leaves and grass clippings as they accumulate to prevent material filling in and turning into rich soil.

If this was a new rock laying project I would suggest 3 inches of the cheapest gravel followed by an inch to an inch and a half of your more expensive decorative rock on top to cover the unsightly cheap gravel. I would use half again the size of cheap rock compared to the more expensive stuff. That is if you are using 1 1/2 inch pretty gravel use 2 inch cheap stone - this will allow more drainage and will let debris fall deeper to where it is more difficult for weeds to get a foot hold.

One of the mistakes people make is spreading out their pretty stone too thin. The use of gravel to prevent stuff from growing requires a fairly deep "mulch" of gravel. The same principle works with using bark, hay and other forms of "mulch".

People think that the use of plastic will prevent weeds - in theory it will prevent the weeds from getting started in the soil below, but it does not address the issue of accumulated dust and debris. However the act of spreading gravel will cause holes to develop in the plastic.

3. Use roundup or other direct spray weed killer.

David

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Originally posted by DvdGStwrt:
3. Use roundup or other direct spray weed killer.

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Roundup? That's quite good Kills roots off as well most others just kill what you can see(leaving the root)

Thanks so much for the replies. I had thought about adding more rocks but I figured weeds would still grow though. I never thought of sand or gravel, but I think the easiest thing would be the roundup. I just pulled the weeds about a week ago and I see more growing. Thanks so much. The roundup, it is.