Best time to use a lawn weedkiller

The best time to use lawn weedkiller is in May, when plants are growing at their fastest rate, particularly if you are using Verdone or a dry phosphate. All plants have a chemical growth clock which will stop them from outgrowing themselves. Verdone, for example, contains 2-4-D, which alters the growth clock so that the plant will outgrow itself at such a rate that it will collapse (a bit like an overdose of steroids). It will shoot up and look very healthy, but will die after about three weeks.
You need to be assured of six hours dry weather so that it gets absorbed into the leaf and not washed away by rain. It is best to apply it from a can with a very fine rose (such as a Hawes watering can which have extremely fine roses), which will allow the droplets to stay on the leaves for quite a while. This is better than using a spray which could drift and kill plants that are not weeds.