By far the most important lawn fertilization application is in the fall. Your lawn should be given a good feeding just before winter (after the last cutting) in order for the roots to “fatten up” in much the same way an animal does before hibernating for the winter. Many of us tend to fertilize in the spring as opposed to the fall simply because that’s when we’re in the mood for gardening. This is a mistake. A certain amount of fertilizer in spring is OK but to overstimulate your turf to grow early in the season does more harm than good. It depletes the root system of a lot of stored (food) energy before the summer – when it needs the energy the most.
The second most important feeding is the early summer application that helps carry it through the hot dry season – the most stressful period. Avoid the cheaper fast release fertilizers – especially for this (summer) application. The results are very short-lived and actually promote diseases. I always recommend the slow release fertilizers for lawns as a steady, slow feeding is much healthier, and find them to be much more convenient (cheaper in the long run) especially when labour costs are factored in. There is nothing really wrong with the cheaper, quick release fertilizers (other than nutrient balance – eg. 10-10-10) it’s simply a lot more work to maintain steady, healthy growth. Late summer (early fall) fertilizer is also important to keep weeds under control. A good healthy lawn is your best defence against lawn pests of any kind.