My mother had 18 inches between her house and a fence. There were always seedlings and saplings growing where I could not reach and I could not squeeze back there. So, I purchased something called brush and sapling killer. It was in a bottle that would squirt for 12 feet. So I sprayed from the front then from the back to make sure I got everything and it killed everything. I only used it once to get a handle on things. She had a big problem and the foundation of the house was in danger.
If you want a organic weed killer try vinegar. Strong vinegar like pickling kind. Spray full strength when it looks like no rain. This will also lower the PH and keep them from growing back. Use a funnel and make sure you don't get it on anything else.
Also I have heard of vegetable oil but it will stain, if small weeds around a sidewalk you can just use boiling water. My mother also used a 'used oil' to kill very stubborn weeds and Ivy.
A real large area. try a large, thick sheet of clear plastic. The sun will shine through it and cook the weeds. I think 2 weeks will do.
