“WILT-based” irrigation is a strategy golf superintendents can employ
to not only save water, but to help prepare the turfgrass plant to more
effectively cope with stresses, including summer heat stress.
Dr. Jack Fry, a Kansas State University professor of horticulture, endorsed the practice while speaking in Windsor in January at the Ontario Golf Superintendents Association’s conference and trade show.
Maintaining the soil at field capacity promotes shoot development in much the same fashion as applying nitrogen. More clippings will be produced during periods of ample rain, and carbohydrates the plant might use are taken away, he said.
Instead, a “wilt-based” method of irrigation encourages superintendents to allow their soils to dry down from field capacity to a point when wilt is becoming evident. Then, the water can be returned.
“Wilt-basing is one way to go about saving some water,” Fry said.
Studies have shown greater root development is encouraged through wilt-based irrigation practices. Enhanced root hair production is a benefit.
“If we can get something that enhances root hair production, it’s got to be good.”
Reduced soil compaction is another benefit of the strategy because the soil isn’t continually moist on the surface. When the soil is at field capacity, it tends to compact more than at any other time.
Fry said studies also suggest some diseases, including summer patch on Kentucky bluegrass, will be reduced through wilt-based irrigation.
“In general, any time you can reduce the amount of moisture there, it’s going to be a benefit. With a wilt-based irrigation strategy, you’ll find the plant is better set up and prepared for an oncoming stress.”
Adopting the practice when entering a period of heat will have its benefits, Fry said, noting studies suggest that perennial ryegrass exposed to a level of drought stress had become more freezing tolerant prior to the onslaught of freezing temperatures.
In times of severe heat, such as that experienced throughout most of North America in the summer of 2010, a wilt-based irrigation strategy may have to be practised once or twice a day.
Fry said water availability and usage will always be issues in the golf industry, and superintendents are in a highly visible position with the public.
“If you’re irrigating shortly after a rainfall and the public sees that, it reflects upon you and reflects upon the industry. We’re very visible in the golf course industry.”
Water tends to be used more when it’s warmer and evapotranspiration rates increase. When days become longer about the time of the summer solstice, more water is used as wind speeds tend to increase and ET rates are higher.
Also impacting water usage is relative humidity, yet in another manner.
“That’s the only one that’s different. That is, if humidity goes up, ET tends to drop.”
Less water is lost because of the need for the gradient from the leaf to the atmosphere.
“If you don’t have such a gradient, you’re not going to lose as much water.”
ET is expressed as a depth of water lost over time. A typical rate during the growing season is between one and seven millimetres per day. Depending on the extent of the conditions, sometimes those numbers can be more or less than one or seven.
Several golf courses have their own weather stations to record important data and help make more informed irrigation-related decisions. They’re not entirely foolprood, however, Fry said.
Where the weather station is situated on the golf course is a key consideration. It may be located in an open area of the course, yet several greens may be surrounded by protective trees.
“If the weather station is in an open area where there is lots of wind movement, you may get a higher estimated ET from your weather station than you would expect.”
Measuring the amount of moisture remaining in the soil is perhaps a better means of knowing when and how much to water, Fry said. Incorporating soil moisture sensors is akin to using a tire gauge to determine how much air is left in a tire, he said.
“If you’re looking to save water on your site, soil moisture sensors might very well be the way to go.”
It’s important to have an irrigation system which gets the water out efficiently, especially when ET data and soil moisture sensors are used. If the irrigation system is not overlapping properly and water isn’t reaching the right places, efforts to make more efficient use of water will be futile.
More golf courses are making concerted efforts to decrease the amount of turf they want irrigated on their properties. This may include the elimination of water altogether on certain out-of-play areas on the course. As long as the golfers themselves will allow this to happen, water savings will be realized.
Fry said a golf course in his home community in Kansas was forced to take drastic measures last summer as a consequence of the period of severe heat which plagued most of North America. The golf course was mandated to reduce its municipal water supply by 50 per cent, forcing the concentration of water on the more important areas. Ultimately, its driving range ceased to get water and had turned brown by September.
“They had to sacrifice that in order to put that same water back on the golf course.”
Fry said the heat stress from last summer was about as bad as he’s ever seen. For some parts of the United States, it’s typical to experience some degree of heat stress during the summer, but it was a new experience in many other parts of the country.
In Kansas, there were 15 days in July last summer which saw temperatures rise above 90 degrees Fahrenheit. There were only five such days the previous year. In August of 2010, there were about 20 days when temperatures were 90 degrees or warmer in Kansas, compared to only seven in 2009. September even saw between five and 10 days with temperatures above 90 degrees, Fry said.
The heat wave’s impact on golf turf loss generated plenty of media exposure, including a Wall Street Journal story which stated: “Golfers themselves deserve part of the blame for insisting the putting surfaces be mowed short and fast, even if weather conditions don’t allow for it.”
Heat stress can be experienced in two ways, Fry said: direct and indirect. Direct heat stress is the least common type, and is a direct exposure to heat which causes rapid death to the plant. He suspected much of the Kentucky bluegrass dieback experienced last year was the result of direct heat stress.
More common is indirect heat stress which occurs during extended periods of high temperatures and high nighttime temperatures. Fry said superintendents will cross their fingers that their creeping bentgrass or annual bluegrass greens will make it through the night unscathed when overnight temperatures remain at 80 degrees Fahrenheit or more. When certain temperatures are reached, photosynthesis is going to drop in bentgrass and poa annua, he said. With cooler temperatures, respiration is lower than photosynthesis which allows the plants to accumulate sugars, “put them in the bank” and store them in the crown for use later as needed.
During periods of extended high temperatures, respiration, or sugar burning, is faster than the rate sugars are manufactured.
“So we’re burning more food than we’re making. It’s good for losing weight as a human being, but it’s not always good.”
Fry likened the situation to a person who is limited to eating one hamburger before he runs a marathon. The runner will make it part of the way, only to discover he doesn’t have enough energy to continue. Creeping bentgrass and annual bluegrass will experience the same thing.
The plants will indicate that by Aug. 1, “‘I just don’t have enough to get through the summer. You haven’t allowed me enough sugars to make it through,’ and that’s particularly true if you’re mowing lower.”
It’s important to note that with heat stress, the roots die first, Fry said.
“So things are going to die on that plant first that you don’t know about before you see it happen on the surface.”
Studies show that if the soil temperature can be modified so that it’s cooler, the air temperature really doesn’t matter, Fry said. Dropping the soil temperature somewhat will benefit the quality of the turf.
There are several means of reducing the effects of heat stress on the plant. Thinning out trees to promote better air flow is one means. Reducing the amount of daily shade cover on greens also enables the plant to produce more sugars to help get it through periods of heat stress.
“You have to have light for the plants to grow. They need light to survive stresses.”
Watering by hand to make up for irrigation deficiencies is a key consideration. Fry said it may be helpful for a superintendent to dedicate a member of his staff to watch the greens and make sure they don’t enter into drought stress for any significant period of time.
Syringing greens during periods of heat stress can be a useful practice as long as there is sufficient air movement, Fry said. Cooling the surface of the leaf by using a fine mist is a different process than soaking the putting surface. He suggested the amount of water used when syringing should just be enough to dampen a sheet of paper towel.
If several trees surround a green, blocking air movement, syringing won’t be much help, especially when humidity is high. With adequate air movement, however, the practice can be helpful because transpiration is being complemented with evaporation from the leaf surface.
Fry said studies show that a significant reduction in canopy temperature can be realized if syringing is done with good air movement.
Raising the height of greens mowers will also help the plant survive heat stress.
“If we’re mowing higher, we’re putting money in the bank.”
A higher cut gives the plant more leaf area, more chlorophyl, more stomates and a greater ability to take in carbon dioxide, allowing sugars to be made and stored.
Studies show that between July and early September, respiration was higher than photosynthesis at mowing heights of 5/32nds of an inch.
“When you get that lower mowing height for an extended period of time, you’re more likely to be running that bank account thin.”
Studies have also showed that using walk-behind mowers to cut greens is less stressful than riding mowers, and rolling greens instead of cutting them on alternate days can also prove helpful.
Drainage should also be maximized, Fry said, noting excess water on a green retains heat.
“The more water you have in the root zone, the warmer the root zone is going to be.”
Last summer’s heat stress was so bad in some parts of the United States that some private clubs shut down for four to six weeks.
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