Don't poison the soil, the water or the people in your neighbourhood with chemicals. Weeds and pests are symptoms of an unhealthy lawn. Allow clover in your lawn for its nitrogen-fixing magic. Hand remove annual weeds before they seed, and remember to remove the roots of perennials.
Hand weeding at any time is better than killing off everything with chemicals.
An old knife or fork will do or dandelion weeders are inexpensive. Diseases can be quite selective. A mixture of grasses will prevent complete destruction of your lawn. TEST YOUR SOIL . Send to the Dept. of Agriculture, Plant Industry Branch, P. O. Box 550, Truro, N. S. B2N 5E3 for a soil test kit.