Posted: 12/10/2014 at 17:48
The active ingredient in Round up is glyphosate which becomes de-activated on contact with the soil, so you can replant immediately.
However, glyphosate works when applied to actively growing foliage - it won't work if applied to roots at this time of year.
I would leave it until the spring, when the weeds are growing rampantly - apply the Roundup and leave for a few weeks until the weeds are brown and dead. That will show the roots are also dead. You may need to have a couple of goes at it but it will work.
Then you can get on with re-planting the rockery - it will take longer this way, but it will work.