What to use to kill weed and little trees between Fences ?

https://drericz.com/natural-weed-killer/

If there is nothing else that you want to keep cut them back as far as possible and us Orthos Total Brush killer

I've been using Roundup around my pool fence, & walkways/driveway for years. Use on a calm day only.

Vegetation kill is what I use. The brush kill doesn't seem to work as well anymore. Ortho makes one I've used with success. Good luck.

applecider vinneger

Round up

Mix up plain white vinegar store brand 5% (comes in a gallon container, make sure it is 5%, it will state so on the label. ) I put it in a spray bottle, with a few table spoons of table salt, and a squirt of dishwashing liquid. Spray on a hot morning, with no wind. It should kill most if not all of it. As Round up is so detrimental to the eco system as well as your own body, I would reserve it for only the most stubborn weeds etc. Other wise it is not needed. And the White vinegar is really, really inexpensive.

We use Weed-B-Gone and just spray the areas where we want the weeds killed. Then we sprinkle the areas we want left clear with a soil sterilizer. Both products can be purchased at Walmart, Home Depot, etc. We do this each spring so we don't have to use a weed-eater to keep the fence lines nice and neat; it helps cut down on the amount of work we have to do and that's most important for us as hubby is 77 and I'm 72.