I used a systemic weedkiller called asteroid. Therecticallly it takes 6hrs to get into the route system, from that point the weed is dead it just takes weeks to visible die upwards
You can blast off the heads of the weeds after 6hrs they will not look dead but they are.
Got to say I tend to spray it down a few day before so the customer can see some evidence of the weedkiller working