How do I stop weeds from growing back in my flower bed (8 x18 ft)?

You could try PREEN. It comes organic also. Put it down and water in after you have removed all the weeds and dug the ground lightly between the plants. Follow directions. Lasts 6 weeks at least.

There are several here on HT:

https://www.hometalk.com/search/posts?filter=weed%20killer

I just soak the area and pull the next day...I need the exercise! LOL

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This is not an exact science – approximate measures are totally fine 1 gallon white vinegar and cup Epsom salts and ¼ cup liquid dish soap Combine ingredients in spray bottle and treat weeds at the hottest time of day for best results.

Lay down some landscape fabric leaving cut outs for plants. Two to three inches of mulch. Will choke out weeds.

I double layered the landscape fabric, first I sprayed to kill everything. Then mulched this very large area the weeds an grass has all grown back.