I've seen people just spray round up into the seams periodically. Of course it's not the most organic method, but it works.

I resisted using a non-selective herbicide like Roundup for a long time; hand digging and prying between patio bricks and driveway seams instead. Roundup is inexpensive and effective and readily available - just read and follow the label directions.

fill the expansion gaps with a flexible substance like a polyurea gel.

I use a gallon of vinegar mixed with 2 cups of salt and apply it monthly to the cracks. You may have to do some scraping and pulling of some weeds, but it works well.

Guess i shoulda been more detailed...I used Round-up for the lasyt couple years, but i worry about it running off into the ponds downhill, plus there're a lot of amphibious creatures around...i thought there might be some physical barrier they've come up with, salt would be an option...

Thanks all!

Salt getting into the ponds downhill could be potentially damaging, too. Plus I wouldn't be putting salt on a concrete driveway - though it takes time, it has its adverse effects.

Some people claim boiling water works - if you're of a mind to lug it from the stove to the driveway.

Distinguish between preventing and killing. Prevention requires a barrier or a pre-emergent herbicide or potion--you could sprinkle pre-emerge granules into the cracks. Can kill by pulling, or with flame, or herbicide, or various potions as above.

The least damaging to the environment method when using an herbicide it to paint it on with a paint brush, touching the weed tops lightly. No need to spray or pour.

When faced with down on-the-knees weeding, use a linoleum knife as the weeding tool. It easily slips into tight spaces and greatly helps to loosen stubborn weed roots.