Weeds are the most susceptible to gly when they are young, and actively growing. When weeds get tall, they also tend to protect each other and it is hard to get a nice even application. So, the question becomes timing. My preference is to mow first but then you need to wait for about 2 weeks for the weeds to put up new growth from the root system. This also lets the weeds you mowed desiccate and for the new growth to make it though for a more even application.