I have been anti-pesticide for a long time, but we have had so much rain that to many weeds are growing between my patio stones so I bought one that's 'green' by Eco-Sense. I called and asked the same question as you and they said once the weed has dried up you can re-plant. I think roundup works the same way. Taking off the tops may help because it has less plant tissue to get through to kill the entire weed. The process should take about a week.