Glyphosate does not work by soaking the ground.It has to be sprayed onto the leaves, which will then absorb it and translocate it to the roots. When the plants have all died you can replant straight away. So it is probably most effective when the plants are growing strongly.

Agree with Steragram - glyphosate is not a pre emergent weedkiller, and will only kill 'through the green' so to speak - best sprayed when weeds are growing strongly and not in drought, so it works well in spring and early summer. Otherwise, there are no pre emergent weedkillers for amateur gardeners available in the UK. If you want to plant quickly, then dig out what's there, removing the roots.

First read all of the directions on the bottle to fully inform yourself on what it does and how to apply it. That's the best way to go with any garden chemical.

Different soil conditions and weather will cause quite a bit of variability in the timing, but I have seen badly contaminated soil affecting growth and health up to two years after application. My recommendation: if you haven't done it yet, DON'T DO IT!! Glyphosphate works best, as others have said, when sprayed lightly on the leaves of weeds or grass, just enough to wet the foliage.