Turning Up The Heat: Summer Turf
Posted: June 12, 2012
Growth and Development of Cool Season Turfgrass
The growth and development of cool season turfgrass is temperature dependent. During extended periods of heat and moisture stress, cool season grasses stop growing, and the leaves turn brown and die, a condition known as summer dormancy. Summer dormancy is reversible, so when cooler temperatures and moisture return, the grass will re-grow from the crowns. Dormancy symptoms can be confusing and patchy, especially at the onset. Dormancy itself can be incorrectly diagnosed as pest damage and may camouflage the activities of actual pests.
Heat Tolerant Grasses
Grass species and cultivars differ in their genetic ability to tolerate heat. Of the cool season grasses, tall fescue has good heat hardiness, Kentucky bluegrass has medium heat hardiness, while fine fescues and perennial ryegrass have fair heat hardiness. Studies have shown that with good culture, we can reduce some of the effects of heat stress. For example, a little water stress increases heat hardiness; excessive nitrogen levels or overly acidic soils decrease heat hardiness; and a balance of P and K in relation to nitrogen helps attain maximum heat hardiness. We can’t control the heat, but we can control some of the other stress factors that make summer damage more of a problem. It’s a good idea to be aware of the grasses present in the lawns you maintain, and to adjust your culture depending on the site and the grasses present there.
Summer marks an active period for various turf diseases, although the maladies we see on turf change with the weather patterns.
Red thread or pink patch is easily recognized on lawns when temperatures are 50-75 degrees and rain is abundant. At that time the pink coloration of red thread mycelium, or pink clumps of pink patch are easily seen. After conditions favoring red thread fade, namely warmer temperatures and drier weather, the pink fades and you are left with circles of straw-brown turf. Although red thread can be prolific on perennial ryegrass and fine fescues, the damage is normally cosmetic and not deadly. It’s a good idea to collect the clippings since that is how the disease is spread.
Dollar spot is another common summer disease. It creates small, brown patches of dead grass and peaks when temperatures are in the 80’s with high humidity. White mycelium covers infected leaves early in the morning when humidity is high. Leaves are marked by yellow-green blotches that turn into straw colored lesions bordered by a reddish-brown margin. It is worsened by poor fertility, improper watering, and is spread by moving infected clippings by wind or water, on shoes or equipment. It is also a good idea to collect clippings in areas where dollar spot is active.
We often say that balanced fertility is a valuable cultural tool, and it is especially important for disease management. There are many examples where fertility practices can affect disease development. Over-application of quick release nitrogen during late spring can worsen the development of leaf spot diseases. Brown patch is worsened by applications of water soluble nitrogen alone, and is less severe when complete fertilizers with slow release nitrogen are used. Starving turf is more likely to succumb to red thread, dollar spot, or rust disease. High pH creates an environment ideal for summer patch disease. So as we enter the summer it is important to have a plan in place that uses soil testing as the basis for fertility programs, and to use care and moderation when applying nitrogen. Become familiar with the disease problems common on a certain sites, and modify your cultural practices, including fertility, accordingly.
When turf is stressed from high temperatures it will show wear more rapidly. Foot traffic and mowing are two common examples of wear that can contribute to turf damage during the summer. The problem is that it doesn’t always show up immediately. Trampled turf can’t regenerate new leaf tissue as readily during the summer. We see mowing damage frequently during the summer, when tires heat up during mid-day mowing or when exhaust and blade rotation dries the grass to a critical point. Brown streaks and tire marks are the result.
Low mowing heights increase overall stress, especially during the summer. Low cut turf is prone to diseases such as leaf spot and summer patch; the shallow roots under low cut turf are prone to heat and drought damage and less able to supply water to the plant. A common practice is to raise the mowing height for the summer to help shade the soil surface and reduce stress on the turf. But waiting until early summer to raise the mowing height probably does more harm than good. Not surprisingly, root growth on cool season grasses is most active when soil temperatures are cool (during the spring) and root growth slows and stops as the soil temperatures heat up. Raising the mowing height too late in the season doesn’t help since the already-shortened roots cannot grow to support the tops. It’s better instead to keep the mowing height higher - in the 3-3.5 inch range - in the spring and leave it there all season. And of course mowing height and frequency can also influence weed development.
As the cool season grasses slow down, competition from warm-loving weeds can become severe. Weed management begins with attention to fertility and mowing practices. Once those are covered, identify and understand the weeds present to develop a management strategy.
Crabgrass is evident in early summer and can be treated with postemergence herbicides. Herbicide selection depends on the growth stage of the crabgrass, which will vary depending on the site.
Nutsedge will also poke through during the early summer, and can be treated when it’s visible. There seems to be an increase in nutsedge populations over the last few years. The tubers can be transported by flooding rains, with soil or mulches. Whatever the reason, be on the lookout for light green, triangular stems that grow faster than the surrounding turf between mowings. Postemergence turf herbicides containing the active ingredients halosulfuron or sulfentrazone are effective but require quick response to nutsedge sightings and repeated applications are necessary for control.
Broadleaf weeds can also be treated through the summer, and there are products or combinations effective for just about any broadleaf weed you’ll find. For the best results, accurate identification of problem weeds is critical, so you know which cultural practices and herbicides will provide the most effective control.
Weed Management Tools
A couple of helpful tools for weed management are available from Penn State. The website http://cropsoil.psu.edu/turf/extension/factsheets/weed-management/weed-id includes guidelines for identification and management options for specific weeds in turf. Another great tool is at http://cropsoil.psu.edu/turf/weed-id where you’ll find photos and management tips for most weed problems in turf, some with videos of the key identification characteristics.
Any kind of weed control can be tricky during the summer. Herbicide labels provide instructions about coordinating mowing schedules, irrigation, temperature extremes, and actively growing target weeds. Properly calibrated spot treatments can help limit the excess stress. Remember that phenoxy herbicides (like the 2,4-D, MCPP or dicamba found in many broadleaf herbicides) can make lawn grasses more susceptible to disease (like leaf spot), and that’s another reason to approach summer weed control with caution.
Turf that looks extra drought-stressed, or doesn’t rebound from summer dormancy may be suffering from insect damage. In the beginning of the summer, be on the lookout for chinch bugs, sod webworms, and billbugs. These are all insects that are more common on lawns with excessive thatch. Carefully inspect the turf and thatch around brown patches to look for clues.
You’ll find chinch bugs buried in the thatch or scurrying along the surface of dried thatch. They move quickly and their feeding usually causes turf to turn yellow or reddish before it browns.
Look for feeding damage (holes) made by webworms and adult billbugs on the grass blades. A soap solution applied to the surface helps bring webworms and cutworms to the surface where you can see them
Check areas where there are many dead stems for billbugs by doing the “tug-test”. If the stems pull right out at the ground and have chewed ends packed with sawdust-like frass, it is likely that you’ll find billbug adults at the base of the plant and small legless grubs along the roots.
When it comes to turf management, summer is anything but lazy. There are plenty of things to watch out for, while keeping in mind that heat plays a major role in the decline of cool season grasses during the summer. Everything we do on and to our yards can add to the stress of summer.
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Owen, M.C. and Lanier, J.D. Lawn and Landscape Best Management Practices for Turf, A Handbook for the Lawn and Landscape Industry in Massachusetts. 103 pages. University of Massachusetts.
Lawn Diseases. North Dakota State University.
University of Nebraska-Lincoln. TurfInfo.