Cool Season Turf Care - September 28, 2011
Jeff Schalau, Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County

Cool season turf grasses are those that stay green through the winter months. Fall is the time to fertilize and do other maintenance tasks to cool season turf grasses. Ultimately, proper fall maintenance will help cool season grasses cope with summer heat and minimize disease problems.

Our most common cool season turf types are pure tall fescue, pure Kentucky bluegrass (KBG) or a mixture of KBG, fine fescue, and perennial rye. If you don’t know your particular species, you can pluck a leaf blade and look closely. If the leaf tip has a "boat-shape” rather than a flat blade, then it is KBG. Tall fescue can be easily identified by firmly holding the tip of a leaf blade, pulling it so the leaf edges pass lightly between your thumb and forefinger. If it feels a little like a saw blade in this direction and is much smoother when pulled in the opposite direction, then it is tall fescue. Perennial ryegrass has parallel leaf margins, the back of the leaf is shiny, and the tips of the leaf blades are tapered.

Fall nitrogen fertilization of all cool season grasses in the Verde Valley should be one pound of actual nitrogen per 1,000 square feet (about 5 lbs ammonium sulfate 21-0-0) in both October and November. Most fertilizer bags will also tell you how much of their product to use. KBG and KBG mixtures can require dethatching. Compacted soils and areas with poor drainage can also benefit from aerification which should be done in early to mid-October.

Spring fertilization is also recommended for cool season grasses. Half as much (1/2 pound of actual N per 1,000 square feet) should be applied in both early April and mid-May. Fertilizer should not be applied to cool season lawn in the heat of summer. Summer is a resting time for cool season turf.

Thatch is formed from the dead shoots, roots, rhizomes, and stolons that accumulate just above the soil surface. These plant parts are more resistant to decomposition than grass clippings (leaf clippings rarely contribute to thatch). KBG produces more thatch than other cool season grasses because it spreads by rhizomes (underground shoots). To test your turf for thatch, slice a pie-shaped wedge of grass from your lawn and measure the depth of dead material accumulation. Thatch at depths of ˝ inch or less is not detrimental to turf grasses. Thatch depths greater than ˝ inch can cause many turf grass problems including: poor rooting; disease problems, insect infestations, dry spots, and increased potential for scalping when mowing.

Dethatching is an injurious process and should only be done at times when the turf is most likely to recover from the treatment. The proper time is in the fall. Dethatching equipment (verticutters) can be rented from equipment rental yards. Make sure the equipment is set to treat at a depth to remove the thatch buildup and not the entire grass plant.

Aerification is not a substitute for dethatching. Aeration is usually achieved through the use of a core aerator. This machine takes out small plugs of soil to allow better water penetration, reduce compaction, and promote deeper roots. This is most important in situations where traffic is high and water penetration is poor. The resulting cores can be simply broken up and redistributed on the soil surface. More information on cool season turf maintenance is available in the on-line Arizona Master Gardener Manual at:

In the Phoenix area, bermudagrass is often overseeded in the fall with annual or perennial ryegrass to cover the dormant, brown bermuda for the winter months. While it works well in the low desert areas, this practice is not recommended for the higher elevation areas (including the Verde Valley). In colder climates, ryegrass competes with and shades out the bermudagrass inhibiting regrowth of bermudagrass in the spring and summer. It is also thought that ryegrass produces compounds in the roots which are mildly toxic to the bermudagrass. The result is a patchy stand of bermudagrass with small patches of ryegrass the following summer.

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: November 21, 2011
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