This Spring after researching safe alternatives to chemical weed killers, I sprayed white vinegar (mixed with a little water and a drop of dish soap) on some dandelions in our yard. It killed them almost right away, but also killed the grass right around the weeds (where the spray also hit).

I poured straight vinegar in the cracks in our driveway where weeds were growing up and it worked like a dream. If your weeds are just surrounded by gravel, this method should work well as a spot treatment as long as you can keep from pouring/spraying directly on the good stuff that you want to keep. I don't think it will seep into the roots of the trees and bushes and do any harm, but I am not an expert.

I have also read that pouring boiling water directly onto the weeds will kill them too, but have not been able to figure out how to keep the water hot enought if you are working with lots of weeds.

Bellagoss

Roundup will not affect the non-target plant roots of trees and shrubs in underlying soil since the herbicidal activity of Roundup is anactivated once it hits soil. Roundup only needs to be applied to foliage and other "green parts" of plants. However, if there are large amounts of roots in pure sand or pure gravel they might be affected if drenched with Roundup.
And at best, vinegar based herbicides will only top kill most weeds, they will not affect the root systems of weeds. Unless they are very small, grass and other perennial weeds will be back in a few weeks and you will need to reapply.

i use Ortho Ground Clear, but i don't have anything close that i want to stay alive! it will sterilize the sprayed area for up to a year according to the label. we have a tower site with a fenced in area of roughly a quarter acre. due to the layout of icebridges and tower legs, you cannot get in there with anything large, so forever we would jsut use sling blades and weedeaters, followed by RU. within a few weeks you could not see the gravel again. then a couple years ago i bought the OGC and now all i have to do is spot spray with roundup once every couple months. it works wonders!

only problem is each bottle only covers 300 sq ft, so it can get expensive if you have a large area.

next time you have to redo the gravel, put landscape fabric down then spread the gravel over the top. you won't hardly have any weeds come up then.

Has anyone been successful with citric acid as a weed eradicator? If so, what strength?

Nothing that kills a plant is "really safe", although some things may be less harmful to the environment than these popular "weed killers". Anything that kills something else is harmfull, in varying degrees granted, but the popular "weed killers" are starting to show adverse affects on our environment because small quantities accumulate in the soil microbes that are eaten by higher order beasties that are then eaten by higher order animals accumulating in these the same as was found with DDT many years ago.

JAYK did not give the date that the cited "safety Evaluation" was published. Some problems such as cancer have a relatively long period between cause (exposure) and effect.
The link below gives the abstract of a recent reviewed scientific research paper:

http://www3.interscience.wiley.com/journal/120748798/abstract?CRETRY=1&SRETRY=0

The following is a related commentary:

http://www.beyondpesticides.org/dailynewsblog/?p=747

Here is a link that might be useful: recent research

I tried a fatty acid type weed killer once and it worked o.k. (supposed to be "all naturual"). In a paved or gravel area where fire is not a serious danger you can give weeds a blast from a propane torch. There is one designed just for that called Dragon something or other, I never tried it. I think someone made or is making a weeder that blasts the weed with hot u.v. light. I think a weeder that blasts the weeds with hot steam also exists but I don't know if it is sold for household use. I succumb to using RoundUp once in a while.