getting rid of weed on gravel roads without killing tree roots

We're in a bit of a pickle:
The parking area and driveway are gravel, weed love that and I planted many trees around.
I tried salt and vinegar last year, it was somewhat effective but before I go all out and buy a 55 gal drum of 30% I was brought back to reality that our trees have establish and their roots are probably under the gravel now.
What is another solution to weed control, short of going at it with a flame thrower (tried that too, takes time)

One idea is to look at the kinds of weeds growing there.  Is there anything in the soil they really like or really hate?  For example, we had far too many wild buttercups, but this variety grows in the same conditions clover likes.  On further research, we discovered they only grow where there is a deficit of boron and clover grows where there is enough of this mineral.  So add some to the soil and the weeds went away.  

When we had a gravel driveway as a kid, my parents did something called grading it every year to keep the weeds at bay.  

My gravel yard down by the shop it almost a lawn now. Someone just built a house last year up the road from me. The construction company dug all the top soil off, about 12 inches worth, and got it down to clay subsoil. Then they dumped gravel on it, lots of it. I imagine seeds will sprout eventually after some humus washes down into the gravel giving them a medium to sprout and grow in and at some point, she might be down to two tire tracks where stuff won't grow due to being driven over all the time. That's what our driveway between the road and our house looks like now. I just don't drive down by the shop much.

The actual gravel roads maintained by the county are simply graded on a regular basis for maintenance so that kills off grass and weeds growing in from the edges.

Laurent Voulzy wrote:We're in a bit of a pickle:
The parking area and driveway are gravel, weed love that and I planted many trees around.
I tried salt and vinegar last year, it was somewhat effective but before I go all out and buy a 55 gal drum of 30% I was brought back to reality that our trees have establish and their roots are probably under the gravel now.
What is another solution to weed control, short of going at it with a flame thrower (tried that too, takes time)

Vinegar will evaporate into the air leaving nothing behind. Wait until the top layer of soil is completely dry and there isn't any rain in the forecast for a while. Then spray a light mist of vinegar on the leaves of the weeds, being careful not to saturate the soil. If there's a little bit of vinegar on the top of dry soil that's okay as long as it evaporates before rain washes it down into the soil. Don't spray vinegar on soil that is moist because water and the substances in it can tend to move around in wet soil. The ideal time of day to spray the weeds would be in the late evening, allowing the vinegar to sit on the weeds overnight before it evaporates the next morning or afternoon.