What's a good DIY formula for spot weed killer?

Boiling water is all it would take. If you have a spray bottle with a straight stream option, you can use vinegar too.

Hot boiling water

vinegar or salt

There are many, but I wouldn't recommend most of them because they could affect plants that are close that you want to keep. Best for me is to pull them. I use Preen Weed preventer in the spring when I plant and it keeps almost all weed seeds from germinating. About the only weeds I need to get rid of are creeping Charlie and tree seeds. Doesn't work on root born weeds, but if you clear them out of the garden and go about six inches beyond, you can see them when they are on the edge of the garden for much easier removal.

you can use a mixture 1 part salt 8 parts water in a spray bottle OR vinegar. If you want a stronger type look for one at a garden supply that contains 20% acetic acid. If you go for the heavy duty vinegar be sure not to get on skin or in eyes and it's s good idea not to spray on windy days

boiling water with lots of vinegar in it

Vinegar

Here is a simple recipe that we use:

1 gallon of vinegar, 1 cup of salt, and 1 tablespoon of dishwashing detergent.

It works like a charm!

50/50 vinegar and water.

I use Vinegar and Salt because it isn't injurious to animals.

Boiling hot water work, but you can also try vinegar.

Baking soda. Wet the plant with a bit of sray from your hose; or apply after a rain. Sprinkle on the baking soda. It will kill the plant and the roots.

Salt works well for killing weeds.

This Homemade Weed Killer has only 3 ingredients. Spray it on the weeds and they'll die within a few days.

1/4 cup of salt

1 tsp of doshwashing liquid (Dawn, the blue) is what I use

3 cups of vinegar

Hot water, vinegar or salt water

A gallon of white vinegar, 1 cup of table salt & 1 tablespoo of Dawn dish soap. Mix all the ingredients and fill a spray bottle. Store the rest in the gallon vinegar bottle clearly labeled.

http://www.listotic.com/everyday-products-you-can-easily-make-from-home-homemade-products/20/

After reading so many items, I decided to try vinegar. The weeds think I fertilized them . . . I'm back to Roundup.

vinegar in a spray bottle.

Once again I love vinegar just watch were you spray because it will kill grass

vinagar in a spray bottle just spray on the weed and it will die

http://www.garden-counselor-lawn-care.com/vinegar-weed-killer.html

You can use the horticultural vinegar which is 20% acetic acid, but it will kill all things green.

Im right in the middle of canning season,... When I make my brine for pickling which is 50/50 vinegar and water plus salt... I use the leftovers in my spray bottle!