Lawn: how grass grows

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Understand cool-season grasses to help them thrive.

Most lawn grasses grown in New York — Kentucky bluegrass, perennial ryegrass and fescues — are cool-season grasses. They grow best in spring and fall.

(The major exception is zoysia grass, a warm-season grass occasionally grown in warmer areas downstate. Most ornamental garden grasses are also warm season grasses.)

The roots of cool-season grasses grow best between 55 F and 65 F. Shoots grow best between 67 F and 75 F. In early spring, even before the grass starts to green up, the roots break dormancy and begin growing.

The combination of long days, cool temperatures and usually adequate moisture produces a flush of growth in the spring. This sometimes makes it challenging just to keep up with mowing. In a normal year, 60 percent of grass growth comes during 6 weeks in spring.

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Lawns grow best in spring & fall when the weather is cool.

Spring is a good time to seed and fertilize bare spots in the lawn from winter damage. But fertilizing healthy lawn at this time just increases topgrowth (and mowing chores) at the expense of root growth. This lush, succulent growth encouraged by spring fertilization makes the plant more susceptible to insects and diseases. Plants with smaller roots are also more vulnerable to drought later in the season.

As temperatures warm during summer, growth slows and lawns require mowing less frequently. Roots can be damaged when temperatures are above 85 F. During this “summer slump,” warm-season weeds such as crabgrass (Digitaria spp.) can thrive because they are more competitive in warm weather.

The combination of warm temperatures and lack of moisture can cause cool-season grasses to go dormant and turn brown during dry summers. In most cases, the grasses haven’t died. They will green up and grow again in fall when cool weather returns and soil moisture is replenished.

Fall is a good time to fertilize lawns because the nutrients primarily support root growth. They help the plants build up reserves to get through the winter and green up healthfully in spring.

An ideal time to fertilize is about 2 weeks after your last mowing. Apply 1 pound of nitrogen per 1,000 square feet. (1 lb. N/1,000 ft.2). Use a fertilizer that is about 70 percent slow-release nitrogen. More about lawn fertilization.