Here's my setup and it works great. I used to use a Solo 3 gallon sprayer but it got too heavy to lug around so I picked up a "Roundup" garden type sprayer at a yard sale and switched the tips today. Works fine.
Here's the old Solo sprayer (that I really liked) and new garden type sprayer.
Here's the tip setup I switched to. I took off the old "soaker" type nozzle and switched to a fine spray nozzle. I think it's a #8002 or 8003. Also, I added a screen with a built in check valve to prevent (mostly) drips. Also needed an adapter for the screen to fit into. You need a screen with Roundup and a fine spray tip because Roundup forms a scum. Here are those parts.
Here's the grass I spray around trees. You don't have to soak the weed or grass, I just try to get some on a part of it and if I hit the tree it's only a glancing blow that isn't a direct soaking spray of the tree. I have sprayed hundreds of seedlings this way, never a problem and never killed one. Here are some samples I sprayed tonight.
The following weed/grass is big and stuck under the tree so I just hit an exposed portion of it and only a little on the tree. You don't need to soak the weed/grass but just hit a part of it. The weed/grass will die. Here's that pic
Now...relax and take a deep breath because I also spray fruit tree and ornamental tree "suckers" that grow off the base or even the trunk. Everyone cringes but I have killed this growth for 20 or so years and zero problems. The suckers die and that's the way it is. I sprayed these toninght.
The mix I use is 2 to 2 1/2 ounces of generic Roundup concentrate per gallon of water and I add a little dishwashing detergent to break the surface tension and help the spray stick. It really helps the kill quality. Again and with importance, you do not need to soak the weed/grass. Just a touch will kill what you want. If not, go back in a couple weeks and get what you missed. This has worked for me for years. Happy spraying and wear rubber gloves and glasses.