A smooth, even slope, adequate to sheet off water would be needed to prevent puddling. And sticks and rocks possibly able to poke through the plastic some day must be cleared away (laying it on a bed of sand might preclude this).

Rocks placed above plastic must be deep enough to prevent sun from photodegrading the plastic. Also important to keep spilled/washing soil, organic debris from accumulating among rocks and forming a substrate for weeds on top of plastic.

A large, broad area of plastic placed over roots of existing trees/shrubs might cause them some grief.

Over time, debris blows in atop decorative rock and weeds grow. Some landscape fabrics are supposed to contain copper and have herbicidal activity but their long-term performance is a question mark. Ordinary plastic sheeting might well cause drainage problems.

If you use plastic, it is to be hoped that if you ever sell your home, the disclosure of defects would include the poor quality, compacted and slimy soil beneath the decorative rocks and plastic.

I have been on several properties where both plastic and landscape fabric have been used under either rock or mulch, and both end up looking ratty. The plastic mulched area often has mold/slime growing in and among the rocks or mulch because there is no drainage. All ends up poking through the mulch material in tattered ends, and the weeds are perfectly happy to germinate on top of these materials.

I have found that a great alternative is thick layers of newspapers, laid down with the edges overlapping, and covered with bark mulch. If the initial layer of mulch is four or five inches thick, a layer of two inches applied each year in the spring keeps weed seeds from germinating pretty effectively. There will always be the occasional weed to pull, because plants will find a wayÂ

Hey rkent
I notice you live in zone 10. Fabric is totally out of the question since you live in such a warm climate. Weeds will definitely grow up through or roots of weeds down through that stuff. Someone mentioned earlier that if you intend on leaving your plastic down for a while, cover it completely and thickly due to UV degradation. Also, do not allow any build-up of material to accumulate under the rocks otherwise, weeds will grow in it. Yes, it will get moldy under there. Also, some good advice above. Build this set-up on as a severe slope as you can to help avoid build up and, help promote drainage.

Plastic stops water. It stops decayed small debris from leaching down.

Fabric is better and does not look bad if the gravel or rock is deep enough.

If there will be active kids or dogs and the gravel is finer, you need to dig "V" shaped slit trenches (very narrow) to tuck the edges down into - then fill with a bit of sand or gravel to hold. That will keep the sides of the fabric sheets from getting raised. It's the pits when that happens and rock gets under the fabric.