Knowing when to fertilize a lawn can be challenging—it's not always as easy as following the instructions on a bag of fertilizer. The natural growth cycle of the lawn dictates specific times to fertilize. If fertilizer is applied at the wrong time, it can weaken or damage the plant, the exact opposite of the desired effect.
Contrary to popular belief, the lawn does not need to be fertilized at the first sign of nice weather in the spring. If a solid lawn care regimen is in place, there will be enough carbohydrate reserves in the grass—from the slow-release fertilizer you applied in previous fall—to last through most of the spring. The carbohydrate reserves in the roots begin to decline in late spring, which is when you should first apply fertilizer for the growing season ahead.
If you're looking for more specific advice, fertilize the grass when the soil reaches around 55 degrees Fahrenheit—right around the time that flowers start to blossom and the lawn is showing signs of growth. This is typically around mid-April for most parts of the United States.
A lawn will use its late-spring feeding to restore its carbohydrate reserves, so it can get through the ups and downs of summer. Drought, heat, frequent mowing, and other stresses can damage a lawn. Therefore, by late summer, the grass will be ready for another application of fertilizer. The late-summer application is considered a "bridge feeding" to get the lawn through the fall.
By late fall, it's time to start thinking about the next season. A substantial, late-season fertilizer application will help maintain some green color in the winter and create a quick green-up in the spring. However, keep your expectations in check: The goal of this late-fall feeding is to nourish the lawn for its winter hibernation, not to make it nice and green over the cold months.
Although not fertilizer, lime is often applied seasonally to adjust pH levels in the soil. A soil test is the best way to know whether your lawn needs lime and how much it needs. Although there is not a specific time that is best for applying lime, it is most commonly done in fall before the ground freezes. Fall is also the best time to aerate a lawn, and applying lime after aerating helps the treatment get into the soil.
Get Local Advice
Growth cycles of turf vary from climate to climate, as does the behavior of various grass types. If you're just following the recommendations on your package of fertilizer, you're getting generic advice.
A better source for tips on lawn care is a local extension service. Most universities have an extension that offers free advice online or through a phone-in hotline staff by experts who understand how to care for plants in your climate, something a national producer of fertilizer cannot do.