How long it should be rain free after round up concentrate application ?
I have read, two hours.
once it is completely dry, it starts having an effect on the plant. rain too soon will limit this effect and may require a second dose. i always try to spray at least an hour before hand, more if at all possible. heck, i have sprayed in a misting rain before and killed everything i intended to kill.
Current TV ads show rain immediately after not mattering.