what weed killer do i put under garden gravel membrane?

Glyphosate based weed killers will do the job and you can get these in powder form from most garden centres, also I strongly recommend paying extra for better quality membrane as the cheaper stuff wont hold back the push of established Dandelion and Dock plants spouting up. These things can push up through hard-core and tarmac so a flimsy membrane won't hold back the growth. Ray Ok. Unfortunately I can't remember any brand names but we definitely used to use powdered weed killer to sprinkle under membrane underneath patio's, drives, decking or gravel gardens. I do have to say that since I have been a sole trader working for myself, I don't use chemicals any longer. I'm no hippy but I believe they damage the mini eco system of each and every garden. Sorry I can't be of any help

How long has the old membrane been down? Are there weeds growing through it? If not I fear you may be asking how to kill weeds which, if there at all, long since perished. If there are, modern weedkillers work through being absorbed through the leaves of growing plants so you need to wait spring is well under way, when they have young growing leaves. Then you spray them. Whether the source of the spray is powder mixed with water or liquid diluted with water. Weeds are plants. Once plant seeds germinate they go looking for light. If they fail to find light they die. If there are seeds beneath your existing membrane they either need some stronger stimulus to germination than exists there at the moment or they have perished prior to germination. If there are plants in your gravel bed are you sure they have not arrived by air, on the feet of birds or animals (including humans), and exploited the build up of earth, rotten vegetable matter and other plant nutrients which have collected there from a variety of sources? I strongly suspect that applying weed killer or any other pesticide beneath replacement membrane is a waste of time, effort and money.