Standard practice is to apply a dose of lawn fertilizer in the spring, followed by one or two more during the growing season. If you follow this standard practice, though, the most common mistake is to apply the spring fertilizer application too early in spring. In reality, the best time for that first application is in late spring, just as the green grass is beginning to grow eagerly. In early spring, grasses are putting energy into root development, and applying fertilizer at this time will divert the plant's energy into leaf development too soon.
However, the question of whether to fertilize at all and when is a subject of pretty intense debate, as the following discussion will make clear.
Should I Fertilize My Lawn and When?
The question of whether or not a lawn should be fertilized generates a lot of debate, based first of all on where you stand on the organic/low impact-to-chemical spectrum. Organic gardeners who bemoan the use of any and all chemicals would say that fertilizing a lawn really should not be done at all—or if you do it, you should take great pains to apply it sparingly so as to avoid the possibility of any kind of fertilizer run-off into water supplies. And there is good evidence to support this position, as contamination of streams, rivers, and groundwater supplies by the phosphorus and nitrogen found in lawn and agricultural fertilizers is a severe problem.
For most of us, though, the desire for a nice, full green lawn is such that we're willing to apply at least some fertilizer to our lawns.
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For the organic crowd, there are ways to feed a lawn safely. First, you can choose to use a mulching mower that chops up grass into fine particles that then break down on the lawn. Horticultural experts say that over the course of the season, this technique provides a lawn with as much nitrogen as one complete application of lawn fertilizer.
And there are also fertilizers that are truly organic in nature—made from natural materials rather than refined chemicals. These organic fertilizers (they will be labeled as such) will indeed feed your lawn just fine, though they are somewhat less saturated with the key nutrients (nitrogen, phosphorus, and potassium, abbreviated as N-P-K) than the industrially refined fertilizers.
Many people, though, opt for a traditional chemical lawn fertilizer of the type widely available at hardware stores, big box home improvement centers, and garden shops. Manufacturers of these products do a good job of identifying the appropriate use of these products, tailoring their mixes for the time of year. You will, for example, find products aimed at early season feeding, mid-summer fertilizing, and late season "turf-builder" mixes. And you will also find products that combine fertilizer with herbicides or prevention formulations intended to kill weeds or prevent them from germinating. So reading product labels is very important.
Be Wary of Fertilizer Application Rates
Where you should question the fertilizer manufacturer's recommendations, though, is in the quantity and frequency of application recommended on the label.
Fertilizer packages are a little excessive in the amount of fertilizer they recommend. This is understandable, as they want to sell product and have a vested interest in seeing robust applications of fertilizer. So review these recommendations carefully.
A healthy lawn will be a fairly light shade of bright green. If you see a lawn that is a deep, almost blackish green, it is likely because it has been very heavily fertilized. That dark green comes from lots of nitrogen on the lawn, and before you admire it, be aware that almost certainly some of that fertilizer has run off into the streets, storm sewers, and then into streams and rivers.
The best recommendation: Start light, with perhaps half the recommended rate of fertilizer. You can also reapply if you don't like the results. Over a season or two, you'll get a clear sense of how much it really takes to get an acceptable, light green turf lawn.
When to Fertilize
Homeowners on the more organic side of things maintain they can get by with a single "turf-builder" application in the early fall to build root systems, then omit all fertilizers the following spring and summer, except for the nitrogen offered by mulching grass clippings as they go. But more mainstream homeowners may want to apply two or at most three light applications of fertilizer per growing year—one in the spring, one at midsummer in regions where it is necessary, and one "turf-builder" application in the early fall. The actual timing of this will depend on your region, though, and the type of turf grasses you have, so consult someone at a good garden center or your nearest University Extension office for recommendations.
Maximizing the Spring Application
Although spring fertilization is recommended as part of a complete lawn care program, applying it too early can throw off the whole program. When cool season grasses “wake up” in the spring, they enter a natural growth cycle when the root system begins growing and building carbohydrate (energy) reserves. Additionally, if you have fertilized the previous fall, especially late in the season, the slow-release function of the fertilizer will still be lingering, providing extra green up in the spring. Fertilizing in the early spring is often encouraged by fertilizer companies and lawn care services but not by agronomists and turf specialists.
So, rather than fertilizing in the early spring, it is better to wait until the late spring, (late May/early June) just before the heat of summer begins and after the grass is growing robustly. This is preparing the grass for summer when it will begin slowing down carbohydrate production and begins utilizing the reserves. A good feeding of 3/4-1.0 lb of slow-release nitrogen will allow the plant to rebuild its energy (carbohydrate) reserves and ward off the stresses of summer, such as drought, heat, traffic, disease, and insects. An IBDU or polymer-coated slow-release fertilizer can feed the grass for up to 12 weeks.
Post Spring Fertilizing
Warm-season grasses thrive in the heat of the summer and can be fertilized throughout the growing season. But cool season grasses are in a survival mode during the heat of the summer, and the flush of top growth that fertilizing provides should not be encouraged when the lawn is stressed and vulnerable. A cool-season lawn should need no further inputs other than water and Integrated Pest Management until September. Be very wary of applying fertilizer to a lawn in mid summer if you live in a climate where cool-season grasses are the norm in lawn seed mixes.
Recommendations from most lawn-care experts is that a mild dose of a "turf-builder" formulation of fertilizer in the early- to mid-fall, while the turf still has several weeks of active growth before dormancy, is a good idea, as it will build strong root systems going into winter and will help jump-start the lawn in spring. This can be a fairly light application, and should not be done with the aim of returning the lawn to the green of summer. A natural slowdown occurs in fall, and you should expect the lawn to begin to lose its green luster.
Combination Fertilizers and Crabgrass Control Products
Pre-emergent herbicides (which are applied in the early spring) usually contain fertilizer—however, it is only a small amount and not considered a full “feeding.” The fertilizer in a pre-emergent herbicide is added to maintain or slightly boost the growth in the grass, while the herbicide restricts seedling development. Some pre-emergent herbicides have an adverse effect on the grass and the inclusion of fertilizer helps the grass stay strong while the herbicide works.
It's best to coincide any fertilizer application with a rainfall or watering of at least 1/4" to water in the product. Remember that a lawn fertilized with a slow-release fertilizer will not need to be fertilized for up to 12 weeks, and maybe longer. Do not water very heavily or apply fertilizer before a heavy storm is expected, as the chances of the nutrients flowing away into storm drains and streams are likely.
taken from https://www.thespruce.com/when-to-apply-spring-fertilizer-
till next time this is Becky Litterer, Becky's Greenhouse, Dougherty Iowa