Looking to put down some landscaping fabric soon, before putting down rock. Would it be best to put down the fabric right over the weeds/grass or should I kill it with something? Not sure if there are any chemicals that will not harm the flowers etc.

Spray the grass with roundup first. Wait until it is fully dead. You can then put your fabric down after that. I think you can plant flowers 3 days after a using roundup but the bottle will tell you for sure.

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I have another thought if you are interested. soak the area well with water. Let the grass dry spray with RU if you want but not necessary, then cover the area with heavy mill clear plastic and seal the edges with soil. Let the area cook for a few weeks.

Peel off the plastic and you are left with sterile soil. What I use with great success instead of landscape fabric is cardboard boxes cut and fit together. Then cover that with whatever you want. If all you want is rock, goferit.

I also use this method of flower beds. Difference is I cover the cardboard up with a few inches of compost-soil mix. Cut a hole to stick a plant in, then cover with mulch. Google Lasagna Gardening and you will find a lot of variations of the method. Works great and a good way to dispose boxes, newspaper, and your paper shredder trash.