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How grass grows
February 2nd 2018 -

All species of grass have unique growing seasons and dormant months. The U.S. Department of Agriculture publishes a plant hardiness zone map. The 2012 map separates the United States into 13 zones with multiple subzones. Frankly, it far too complex for the question, when will my lawn turn green? The map does show a clear line between regions with cool-season grasses and warm-season grasses. Interstate 70 closely follows the line between these regions.

Dormancy

The ability to go dormant under duress defines a quality grass species. Cool-season grasses will go dormant when temperatures fall below 40 degrees and during dry conditions above 90 degrees. Warm-season grasses typically tolerate higher temperatures. They also tend to be more adaptive than cool-season grasses-halting growth during cold dips and responding when temperatures rise. History displays a pattern for typical growth periods.

Cool-Season Grasses

Nearly every spring, cool season grasses will germinate in very late March and flourish throughout April and May. Regions north of I-70 typically feature cool-season grasses like Kentucky Blue grass, perrenial rye grass and blue fescue. Ground and air temperature trigger the growth of these plants. When the ground temperature reaches 55 to 65 degrees, the roots begin absorbing moisture and ground nutrients. When air temperature maintains 60 to 75 degrees, the grass plant begins to grow. As all of us who live in the cool-season region can attest, once growth begins these lawns are incredibly healthy. 

Warm-Season Grasses

Species like Bermuda grass and St. Augustine grass require much higher temperatures before emerging. Soil temperatures above 70 degrees are needed for most warm season species. When daytime temperatures reach 80 degrees and nighttime temperatures don't dip below 65 degrees, warm-season grasses will thrive. In southern regions of the United States, these grasses will remain hardy from the first of March through September. Warm-season grasses are a bit more agile than cool-season counterparts. Because these species may stunt growth in response to dips and spikes in temperature, I like to add blend warm season species into sunny spots, high traffic areas and along hills even in cooler regions. When temperatures stabilize, warm-season grass will quickly recover and prevent bare patches. 

Spring seed tips

Whether recovering or planting new seed in your lawn, consider when and what to plant. Many weeds grow like warm-season grasses. So cool-season grass seed is ideally planted in the fall and early spring when temperatures are still mild. The new grass seed will germinate before weeds have a chance to grow. Warm-season grass seed won't grow until temperatures rise. If these species are planted too early in the season, the seeds may wash away or simply be pushed out by weeds. Also consider planting the right grass for the needs of your lawn. As I mentioned earlier, I like to blend my lawn with a mix of cool and warm season species.

Learn more from an article I posted last year.