By Bryan G. Hopkins, PhD, Austin P. Hopkins & Neil C. Hansen, PhD
As populations grow and the climate warms, the demand for water increases. Recent droughts have impacted nearly every region, including those traditionally having high precipitation. These left many turfgrass managers scrambling for solutions to grow grass with less water. The time to fine tune water conservation techniques and to train turfgrass to be drought resistant comes before the drought arrives. We recommend the following Best Management Practices (BMPs) to conserve water. The other good news is that these practices are good for growing grass even when there is ample water.
BMPs for conserving irrigation water
Mowing height. The disadvantages of short mowing are increased weed/disease pressure and shallow rooting (and resultant reduction in plant available water storage)—reducing the lifespan of a field. The main advantage for short mowing is that it can result in increased shoot density and a more tightly knit together playing surface—providing that other factors are optimal. From a sport perspective, the advantage of shortly mowed grass is increased ball roll speed and reliability in trajectory. Despite popular opinion, mowing height does not impact athlete speed, although tall grass can retain more water and result in slippage. Raise mowing height as close to ideal as possible. This height is dependent upon sport and grass species. Generally, heights below 1.25 inch are not advisable for long-term health of upright species (such as Kentucky bluegrass). Prostrate species (such as bentgrass and bermudagrass) can tolerate much lower mowing, but even these will have more problems and reduced lifespan as mowing height decreases.
Mowing frequency. Do not “scalp” the grass by removing more than 40% of shoot height when mowing. Scalped plants have a 10-14 day period of root growth inhibition, which results in shallower and less efficient roots and poor water use efficiency.
Nitrogen fertilization. Nitrogen is the most important nutrient for plant health and water conservation. Excess nitrogen stimulates shoot growth at the expense of roots, leading to higher water demand with less access to water stored deeper in the soil. Nitrogen fertilizer management can be accomplished by: 1) frequent (every 2-4 weeks), low dose foliar applications of traditional, “quick-release” nitrogen fertilizer through the growing season, 2) fertigation through injection of low levels of nitrogen fertilizer into the water each irrigation event, or 3) application of a blend of “quick-release” and “controlled-release” fertilizers twice per year. For example, we have used an application of 1-2 lb N/1000 ft2 of a proven polymer coated urea (PCU) and 0.5-1 lb N/1000 ft2 of ammonium sulfate (if sulfur is also needed) or urea (if sulfur is not needed due to high soil organic matter or high amounts in the irrigation water)].
As a general rule, the total amount of nitrogen needed annually is 0.5 to 1.0 lb N/1000 ft2 for each month of active growth. The high end of the range is needed for: 1) soils that leach readily, 2) soils with moderate to low organic matter content (<3%), and 3) venues where clippings and, thus, nutrients, are removed with each mowing. Tissue analysis can be used to determine if the fertilization scheme is working properly (ideally, the turfgrass shoots have about 2.8-4.1% nitrogen concentration). It is important to avoid spikes in nitrogen availability to plants. In terms of growing healthy deep roots, and thus water conservation impacts, it is essential to have adequate nitrogen in the fall months for the northern hemisphere (spring for southern hemisphere) when plants are hormonally recognizing that it is time to prepare for winter. Also, moderate nitrogen is even needed when plants are suffering from drought stress.
Phosphorus, potassium, and secondary and micronutrient fertilization. These other nutrients are equally important, but they are easier to manage than nitrogen because their soil supply is more buffered and stable in most soil (not true in predominately sand soil). It is essential to base the fertilization of these nutrients on soil testing to the depth of root growth, using good sampling techniques (http://eal.byu.edu/SampleSubmission) and a high quality laboratory with research-based recommendations. Avoid toxicities and nutrient imbalances caused by over-fertilization. Adequate potassium is particularly important for proper water relations, as this nutrient is important to maintain proper cellular turgor. As with nitrogen, plant tissue analysis can be an effective tool to monitor the effectiveness of the fertilization program.
Irrigation system design and installation. An irrigation professional should design the system and the installation should follow the design precisely, with appropriate water pressure at each nozzle, high quality irrigation parts, and appropriately sized piping, heads, regulators, and nozzles.
Achieve great than 75% irrigation uniformity (DU). Conduct regular irrigation uniformity water audits. If uniformity is not within a reasonable limit, correct problems with heads, nozzles, etc. Periodically check each irrigation head for proper upright orientation, water pressure, throw distance, and coverage pattern. Use aerial imagery (drone, etc.) to evaluate plant health uniformity during times of intentional dry down of the landscape. For large operations, a dedicated crew of irrigation uniformity specialists often can cover their wages with savings in water, fertilizer, pesticide, and plant replacements.
Irrigation rate and frequency. The average turfgrass manager is applying 2-4 times more irrigation water than is needed. Watering daily is rarely needed if most or all of the BMPs discussed herein are followed. Times between irrigations may be every 10-14 days in spring and fall and every 2-5 days in summer. Sandy, low organic matter soils with low water holding capacity will need to be watered relatively frequently compared to loam soils. The rule of irrigating “deeply and infrequently” is best for plant health and for water conservation. Plant roots need oxygen. Watering too frequently can contribute to oxygen depletion. This problem occurs far more often than the average turf manager realizes. Apply enough irrigation water to wet soil to the depth of the roots, but avoiding leaching below the rootzone (water slightly below the roots will wick upwards). Frequently assess the depth of roots because it changes through the year as a function of temperature stresses.
After irrigating, allow the soil to dry down somewhat in order to maintain appropriate oxygen levels. The time between irrigation can be determined through calculation of water holding capacity and measurement of evapotranspiration (ET) rates. Alternatively, this can be determined through the use of soil moisture sensors—calibrating through visual assessments to establish a correlation with sensor values with the limit of dryness a specific grass can tolerate without negative effects. Also, irrigation rate needs to be evaluated against the water infiltration rate of the soil. It is possible, especially on sloped surfaces, that the irrigation system applies water at a rate that exceeds the infiltration rate, resulting in wasted water runoff and development of dry spots. In these cases, the heads can be changed out for low-flow models. Alternatively, the system can be cycled by applying lesser amounts of water, stopping it to allow for some drainage, restarting it a short time later, and repeating until the water has reached the desired depth in the soil.
Aeration. Proper aeration results in roots having adequate oxygen and healthier plants with deep roots. Monitor compaction by measuring soil hardness/density and aerate the soil as often as is needed. Some soils need to be aerated as often as every other week during the playing season.
Syringing. Heat stress can cause cool season turfgrasses to go dormant. This is often mistaken for water stress. In cases of extreme heat it may be helpful to apply a very light rate of irrigation to cool the canopy during the heat of the day when temperatures are greater than 80F.
Intentionally water stress the grass in the spring during times of minimal or non-use. We have found it effective to do this twice annually, taking the grass to the point of visible water stress (note: this also helps calibrate soil moisture sensors). There is often spatial variability that causes problems with managing this practice, with part(s) of the field showing water stress and other sections not. Often, this requires hand watering the areas that dry out first so as to allow the other areas enough time to develop stress before the entire field is irrigated.
Fix dry spots. The typical knee jerk reaction for a single dry spot is to up the rate of irrigation over the whole field. Doing so results in the over irrigation of the rest of the field which results in wasted water, leaching/runoff of fertilizer and pesticides, and possible plant health problems. These dry spots are indicative of localized problems that need attention, such as differences in drainage rates, soil hydrophobicity, irrigation system pattern problems, presence of water guzzling tree roots, differences in species (especially when shallow rooted annual bluegrass is dominant), and disease, insect, or nematode damage to the roots/crowns. Correctly diagnosing and addressing problem areas is better than compensating with more irrigation.
Brown = Dry? Most people see brown spots and assume these areas are dry. However, a probe of the soil often shows ample moisture–suggesting that the problem is related to insect, disease, heat, etc., damage. Buy a shovel or a soil probe and use it to look at soil and roots.
Application of a high quality, research proven surfactant/wetting agent is often necessary on soils with a tendency to become hydrophobic or those that form crusts.
Manage tree roots. Tree roots can drain soil moisture, resulting in dry spots and poor uniformity. Periodically trench deeply enough to prune tree roots along the edges of sports field so as to prevent them from negatively impacting the turf. However, it is important to consider that these trees may be desirable for the overall landscape and the root pruning may negatively impact them. Consult a professional arborist to avoid damaging the trees.
Manage weeds. Excellent weed control is essential for healthy sports turfgrass. This is especially an important issue when fields are infected with non-desirable grass weeds, such as annual bluegrass. Annual bluegrass, in particular, is known to have shallow roots and large patches of this can become dormant when irrigation is managed carefully. These weeds need to be controlled through following Best Management Practices, herbicide use, and, if necessary, renovation of all or part of severely infested fields.
These BMPs help foster healthy plants with deep roots that are “water tough” by being able to thrive with modest levels of irrigation. Many of these BMPs interact with one another. For example, over-irrigating results in the leaching of some nutrients and pesticides, resulting in poor growing conditions and negative environmental impacts. Another example is over-irrigating resulting in problems with disease. The disease can result in unhealthy roots, which then are more prone desiccation. Thus, it is essential to manage all aspects of the turfgrass with scientifically proven practices with measurable results in order to be both a good steward of water and other natural resources, as well as having aesthetically pleasing and functional sports fields.
Bryan G. Hopkins, PhD, is a professor in the Plant & Wildlife Sciences Department at Brigham Young University, Provo, UT. Neil C. Hansen, PhD, is an associate professor in the department. Austin P. Hopkins is a student at BYU.