Water shortages occurring in the northeast, southeast and the continuing water shortages in the arid west have fueled turfgrass managers interest in developing strategies to reduce turfgrass water needs and increase turfgrass drought avoidance. Turfgrass industry and scientific communities have also been stimulated to work together in an effort to develop improved approaches for turfgrass drought stress management.

To develop strategies for turfgrass drought resistance management, it is important to understand that drought resistance involves several components. Turfgrass drought resistance is an encompassing term that includes drought escape, drought avoidance, and drought tolerance. These components may act alone or in combination to give plants the ability to withstand drought stress.

Drought Escape. Annual biotypes of annual bluegrass (Poa annua) are capable of resisting drought through the escape mechanism. Annual bluegrass germinates, grows, flowers and sets viable seed before soil moisture is depleted. Poa annua has a high water use rate when actively growing, but it completes its life cycle quickly and resists drought stress through the escape mechanism. Escape is not a desirable mechanism for maintenance of turfgrass quality and function. Voids that disrupt turfgrass quality and use occur as a result of the annual plants completing their life cycle and dying.

Drought Avoidance. Turfgrasses can resist drought stress by the avoidance mechanisms. Drought avoidant turfgrasses are capable of continuing growth under drought stress conditions. Drought avoidance is achieved by mechanisms such as reduced evapotranspiration (ET), deep root systems, high root/shoot ratios, ability to redistribute roots, reduced radiation absorption, and xeromorphic structures. These mechanisms enable the plant to avoid wilt and continue growth. Radiation absorption is reduced by a steep leaf blade angle, lighter green color and presence of leaf hairs. Xeromorphic structures include leaf rolling, wax layers, leaf hairs, and stomatal characteristics.

Drought Tolerance. Turfgrass plants with drought tolerance characteristics are capable of withstanding low tissue water content for extended periods. Plants may exhibit tolerance by maintaining turgor at low water potentials or by surviving severe cell protoplasm desiccation. Drought tolerance is a long-term exposure and survival mechanism. In many turfgrass situations tolerance may play a less critical role than drought avoidance, since tolerance doesn’t coincide with continued growth and turfgrass function may be impeded.

Turfgrasses are often in a position to receive some water through irrigation or natural precipitation. In these cases drought avoidance mechanisms allow the plants to continue growth between inputs of water Drought tolerance is not needed in these situations, since the drought stress exposure is not extended. If drought stress is extended (i.e. more than 30 days), drought tolerance mechanisms may be required simply to maintain turf survival regardless of its function. The most ideal situation would be to use plants and cultural practices that enhance drought avoidance and tolerance.

Strategies For Reduced Water Use

REDUCED EVAPOTRANSPIRATION. Turfgrass water use includes water lost from the plant by transpiration and evaporation, and from soil by evaporation with that required for growth. Only about 1% to 2% of the water used is required for turfgrass plant growth. Water use is essentially equal to the turfgrass evapotranspiration (ET). ET varies with turfgrass species and cultivar (Tables 1 and 2). Kentucky bluegrass consistently had higher ET rates than the fine-leaved fescues or perennial ryegrass in Nebraska studies. Within Kentucky bluegrass there was a wide variation in ET among cultivars. This variation was as wide as that observed between species. Variation among cultivars exists in tall fescues, perennial ryegrasses and creeping bentgrass. Within tall fescues the turf-type cultivars have lower ET rates than the forage-types. The opportunity exists for selecting and developing reduced ET cultivars of these species.

Turfgrasses with low ET rates tend to be dense, low growing types with slow vertical extension rates. Those with high ET rates form open stands with upright growth and rapid vertical extension rates. These ET differences are attributed to canopy resistance mechanisms which form natural barriers to water vapor movement in and around the turfgrass canopy. A dense, low growing stand with reduced vertical extension rate can be manipulated by turfgrass managers through species selection and proper cultural practices, such as manipulation of mowing height and frequency. ET rates are reduced with lower mowing heights and increased mowing frequency under well-watered conditions.

Turfgrass ET increases with nutrition rates that exceed the nutritional needs of the plant. This situation is particularly true with nitrogen nutrition. Potassium is an exception, since ET is higher in potassium deficient turfs than those receiving adequate potassium. In studies at Nebraska, wilting tendency of Fylking Kentucky bluegrass declined as potassium nutrition increased from 0 to 8 lbs. actual potassium/1000 sq. ft./growing season. The decline in wilting tendency occurred even though soil tests indicated potassium levels were high (i.e. greater than 300 lbs. per acre).

Rooting Responses. Reduced ET may not be sufficient for a turfgrass to avoid drought stress. The ability to effectively obtain soil moisture is also important. Turfgrasses with viable and deep, extensive root systems can draw upon a large volume of soil for water and nutrients. Tall fescue and Bermuda grass are excellent examples of grasses with deep, extensive root systems. Tall fescue ranks very high in depth and extent of rooting is a very drought avoidant species, ranking high among warm and cool season species. Turfgrass cultivars differ in depth and extent of rooting (Tables 2 and 3) Some Kentucky bluegrass cultivars have the capability to redistribute roots deep in the soil as surface moisture is depleted. Others tend to concentrate their root development near the soil surface, and lack the root redistribution characteristics. Touchdown Kentucky bluegrass had 27% of its total root growth located in the subsoil moisture, while Aspen Kentucky bluegrass had only 10% in a drought resistance study conducted in Nebraska. Touchdown has been identified as a drought avoidant Kentucky bluegrass cultivar. Turfgrasses with deep, extensive root systems, the ability to redistribute root growth, and low ET rates are more suited to avoid drought stress than those species with shallow root systems concentrated near the soil surface, and high ET rates.

Cultural practices influence turfgrass rooting depth, extent and viability. Rooting depth decreases with reduced mowing height. Excessive nitrogen nutrition decreases root growth and enhances top growth. Mowing and nutrition are interactive. Low mowing and excessive nitrogen decrease depth and extent of rooting more than either practice alone. Turfs receiving light, frequent irrigation develop shallower root systems than those receiving infrequent irrigation. Potassium nutrition increases rooting depth. In a study conducted with Seaside creeping bentgrass, rooting depth and distribution was greater in turfs receiving high potassium and infrequent irrigation than those receiving high potassium and frequent irrigation. The former turfs had better drought avoidance than the latter when exposed to drought stress, because the deeper root system and reduced ET associated with the high potassium fertilization rate and the infrequent irrigation helped the plants avoid wilt symptoms.

Drought Management Strategies

Turfgrass managers need to be aware of potential interactions among cultural practices and turfgrass selection to best manage turfs for drought resistance. Since most turfs receive some irrigation or are grown in areas where rainfall supplements their water use requirements, management strategies oriented to drought avoidance enhancement makes sense as an area of concentration to reduce turfgrass water consumption and maintain desired quality and function.

Turfgrass managers should be coupled with the ability to develop a deep, extensive root system capable of redistribution during periods of soil moisture depletion. Turfgrass managers need to manipulate cultural practices to enhance the drought avoidance characteristics of the turfgrass species and cultivars selected. For example, mowing high and frequently maintains a deeper root system and a tighter canopy than simply r mowing high and infrequently. Turfs with deep root systems and dense turf canopies are generally more drought avoidant than those lacking these traits.

Our understanding of turfgrass drought resistance is growing. More researchers are emphasizing this subject in their programs. The United States Golf Association and the Golf Course Superintendents Association of America are jointly sponsoring research projects oriented toward reduction of water and energy consumption by turfs. Their support has been chiefly responsible for the Nebraska research information shared in this article. This research emphasis and support is generating more information for recommendations relating to turfgrass management during drought stress and potential strategies for reduced water use.