Both fertilizing at the wrong time and over-fertilizing can hurt your lawn and cause you plenty of heartache. Grass varieties used for lawns at most homes can be divided into two categories: warm-season and cool-season grasses. The names refer to the type of weather in which the grass has adapted to grow. Cool-season grasses, like fescue and bluegrass, grow best when temperatures are mild, like in the early spring and fall. Warm-season grasses, like Bermuda and zoysia, do most of their growing in the heat of summer. It's when your lawn is growing its heart out that it needs fertilizer the most.
But applying fertilizer at the wrong time can hurt a warm-season lawn. Fertilizers of all stripes -- chemical or organic, granular or liquid -- are denoted by a string of numbers; for example, 22-3-14. The numbers represent the percentage of nitrogen, phosphorus and potassium present in the fertilizer by weight [source: ACES]. So, for example, a 100-pound bag of 22-3-14 fertilizer has 22 pounds (9.97 kg) of nitrogen, 3 pounds (1.36 kg) of phosphorus and 14 pounds (6.35 kg) of potassium in it. The rest is filler, or secondary ingredients like iron that help the important components disperse more evenly.
Nitrogen is what harms a warm-season lawn when applied during the wrong time of the year. It's super plant food and promotes rapid growth -- you'll usually see results in a few days after you apply it. Nitrogen promotes quick top growth, or growth of shoots (the actual blades of grass). But other parts of the grass plant need attention too; the phosphorus aids in root growth and development, while potassium keeps the plant healthy and disease-resistant. When too much nitrogen is introduced to the soil, the plant works overtime on top growth, taking energy away from root growth and overall plant health. This leads to weak, thin lawns that are susceptible to pests and disease (as well as unhappy homeowners).
While the fast growth that nitrogen stimulates can be a good thing with warm-season grasses during the summer months, in the fall and winter, these grass varieties go dormant -- essentially hibernating for the cold season. When nitrogen-based fertilizer is applied during cold or cool temperatures, the grass will grow, whether it's supposed to or not. The fertilizer will wake the grass from its slumber, resulting in new growth, which makes grass susceptible to frost and shock [source: LSU]. This exposure can weaken the health of your lawn.
Even during the right time of year, applying fertilizer can hurt a lawn. It's recommended that you fertilize no more than three to four times a year (just three times in the summer for warm-season lawns and two to three in the fall, and once in the spring for cool-season grass). The applications should be a month apart [source: Consumer Reports]. While chemical fertilizers are pretty easy to over-fertilize with, organic fertilizers contain much less nitrogen and don't pose as big a threat to your lawn if overused. If you're crazy about fertilizing your lawn, try leaving your grass clippings in situ, rather than bagging them. Nitrogen, phosphorus and potassium are all present in grass clippings, and provide a good source of food to sustain your lawn between fertilizing applications [source: University of Minnesota].
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