Q. I have a rock garden that is ten years old.  We have fabric under the rocks to prevent weeds, but I seem to have more weeds each year.  Could you recommend any effective pre-emergent herbicides? Weed seedlings A. This will sometimes happen for a couple of reasons. These fabrics do degrade over time and some last longer than others. Those with a tighter mesh usually last longer than those with an open or woven mesh. Landscape fabrics should be overlapped so there are no gaps when it is installed. I am assuming you did all of that.             There are some weeds they just don’t prevent such as Bermudagrass and nutsedges or nutgrass. The key to controlling these is to remove them as soon as you see them when they are young, and don’t let them get older or they just get stronger and more difficult to control.             Over time, dirt and dust accumulate in the rocks on top of the fabric and provide a place for weeds to grow. Some rock mulches, like decomposed granite or sandstone, degrade over time. Sandstone is the worst. As these break down and decompose they leave debris on top of the landscape fabric. This is a place where weed seeds can germinate.             It is always best to get rid of these weeds as early as you can and don’t let them get to the point where they flower and produce seeds. The seeds are spread all over the place. I always look at chemicals as a last resort. If you can spend a few minutes once a week just pulling weeds, 95% of the weeds that you see will pull easily. Pull them just after an irrigation and most will pull easily.             In the same way, 95% of the weeds are most likely annuals. If you can kill the tops before they flower this will reduce the weed population tremendously. There are sprays that will “burn” the tops down and thus prevent flowering and consequently seeds.             When we start talking about weed killers, we have to pair the weed killer with the weeds we want to kill. If we pair the wrong weed killer with a weed, it won’t work and we blame the weed killer. So it is very important to know which weeds you are trying to fight.  Both contain Surflan but you would not know it unless you read the ingredients on Impede             However, there are a few we killers that can kill a wide range of weeds, but again not all of them. Pre-emergent weed killers, aka pre-emergent herbicides, include a variety of products with many different marketing names. Some of the lesser known brands have the same ingredients as more expensive products but are not marketed as well.             The pre-emergent products that you can get from your local nursery or supplier are going to be general, all-purpose pre-emergent herbicides. For instance one product by Monterey is called Impede. It contains Surflan which is a very good pre-emergent herbicide.             I don’t know it by its trade name, Impede, but I do know its active ingredient which is Surflan and that is a very good broad spectrum product. Other ingredients you can look for in the labeling that control a wide range of weeds are Dacthal, Ronstar, Goal and Treflan.             Be sure to read the directions and follow them precisely.