If Your Lawn is Cool, Stop Fertilizing it NOW!

Published: May 1, 2011

Common lawn grass species used in Missouri are tall fescue, Kentucky bluegrass, and zoysiagrass. Of these, tall fescue or tall fescue/Kentucky bluegrass mixtures are most common due to the lower cost and ease of establishment (seed vs. zoysia sodding or sprigging), and relative pest and drought tolerance compared to other species. Both of these grasses are cool-season C3 grasses, meaning their form of photosynthesis allows best growth in the moderate temperatures of spring and fall. Because of this, recommendations center on fertilizing during these two seasons to match flushes of growth. However, fertilizing too late or too heavily in the spring can have dire consequences for your lawn when the summer heat rolls around.

Nitrogen is frequently of limited availability in the soil, and generally is the nutrient that most restricts plant growth and vigor. Nitrogen is therefore the most important nutrient out of the big 3 (nitrogen, phosphorous, and potassium) commonly found in a bag of fertilizer, and annual fertilizer recommendations for lawns are based around supplemental nitrogen applications. Suggested nitrogen fertilizer rates are from 2-4 lbs N/1000 sq ft per year for cool-season lawns, and from 1-2 lbs N/1000 sq ft per year for warm-season lawns.

Nitrogen applications can have a profound effect on the severity of turfgrass diseases (see figure). Too little nitrogen can promote diseases such as dollar spot and anthracnose, which are most common on golf courses and sports fields. Conversely too much nitrogen creates succulent tissues, which greatly promote the two most common and devastating diseases that occur during the summer on home lawns: brown patch and Pythium. In lawn situations, you can kill your grass more readily with nitrogen kindness in the spring by encouraging these two diseases.

Brown patch is favored by warm (highs in the mid 80°Fs), humid conditions that start in late spring and occur throughout the summer. As the name implies, the disease occurs in straw colored, bleached patches that can get to 2-4 feet in diameter. On the outer margins of patches, individual leaves will show characteristic lesions that have straw-colored interiors and dark brown outer margins. Brown patch is more severe on tall fescue than Kentucky bluegrass.

Note: Interestingly, brown patch has a "kissing cousin" called large patch that can severely affect zoysiagrass lawns in the fall and spring… again most severely when nitrogen has been applied at the wrong time during these periods. If you have zoysiagrass in your lawn, disregard most of the theme of this article, and start fertilizing soon.

Pythium is commonly referred to as a "water mold" and has spores that swim in films of water with motile flagella. Pythium needs a bit more moisture and a bit more heat (highs in the 90°Fs) than brown patch to affect lawns in Missouri. Pythium may appear patch-like but will have a greasy or matted appearance, and mycelium is often readily visible in the early mornings. The disease is more severe on Kentucky bluegrass than tall fescue, which may also be a reason it is not as common as brown patch on lawns in MO. Along with its high moisture requirement, Pythium is more prevalent in shaded or over-watered areas.

In the summer of 2010, extended heat and frequent rains provided a perfect storm for both of these diseases. In many cases, fungicide applications were necessary to curtail significant damage in cool-season lawns, and many required subsequent renovation and reseeding in the fall. The one common thread I found in the most severely impacted lawns was a single mistimed nitrogen application in mid-May. This application was the bump, the heat and moisture were the set, and the brown patch and Pythium solidly spiked home lawns into submission last year. Avoid the mistake again this year, and hold off fertilizing your fescues or bluegrasses until the fall.