We have reached that tentative point of the season where the finish line is in coming in to view, but the race is not quite over yet.  Summer typically provides the most difficult environmental test for our favored cool-season grasses. The months of July and August historically have the lowest precipitation rates on average when compared across the calendar.  Couple this with frequent hot and breezy conditions that intensify moisture loss to the atmosphere via evapotranspiration (ET), and you have the perfect recipe for turf (and turf manager) stress.

Where rainfall amounts and evaporative demand are concerned, this season has not brought extraordinary challenges, but some locations are nevertheless dry.  In the Connecticut area the pattern seems to be drier to the east, with gradual improvement east to west, and some fortunate pockets that have received inordinate benefit from frequent scattered storms.  The biggest aberration this year came about shortly following snow melt in the spring, when April and May set up alarmingly dry until relief arrived in June, just in time to provide a boost before the arrival of summer.  Remember that the primary period for cool-season turfgrass root development occurs in the spring.  If any drought related issues at this time seem more pronounced than they should be, less than stellar root growth this spring could be the main culprit, and these problems are likely to continue over the next few weeks if the rainfall pattern does not pick up before temperatures moderate.

The collective means by which plants cope with moisture stress is often termed drought resistance.  At the outset it is helpful to understand that within drought resistance there are two principal, distinct components:

Drought avoidance mechanisms promote the maintenance of a water supply sufficient to prevent plant tissue injury, despite supply limitations.  In other words, attempting to defer the effects of drought entirely. Centers on factors such as water absorption (rooting), and water loss (ET).

Drought tolerance mechanisms promote the continuation of crucial biological processes, despite a water supply inadequate for normal functioning and likely tissue injury.  In other words, attempting to endure the effects once drought sets in. Includes physiological responses such as increasing solute concentration to help conserve water within plant tissues.

A wide range of plant species use these mechanisms to varying degrees in response to reduced moisture levels. Avoidance mechanisms are most active during short-term water deficits, such as between irrigation events. Avoidance is a more important aspect for most turf managers, because avoidance supports the prevention of turgor loss and any associated tissue injury and performance reductions that follow. Maintenance of turf appearance and function is vital in systems for which performance is a primary consideration, such as golf and sports surfaces and high-value ornamental lawns.

Tolerance mechanisms are more about outright survival as opposed to performance preservation, thus tolerance becomes increasingly important as the duration and severity of drought progresses. A good understanding of tolerance mechanisms is especially useful for the management of lower maintenance systems such as roadsides, parks or cemeteries which are not irrigated and for which drought dormancy is commonly allowed in the summer.

The sensitivity and effectiveness of either avoidance or tolerance mechanisms can vary considerably based on many factors including turfgrass species and cultivars present, management practices, specific environmental conditions, etc. We definitely don’t have much control over temperatures or precipitation rates, but we can influence the drought resistance of a turf stand with approaches such as careful turfgrass selection and focused management. For example, turfgrass species and cultivars with propensity for deep and extensive root systems or leaf morphology that reduces ET loss will be better ‘avoiders’, while those with notable tendency for osmotic adjustment or synthesis of protective proteins will be better ‘tolerators’. Management practices that promote greater rooting and lower ET (regular mowing at an appropriate height, judicious fertility, etc) will encourage avoidance capability.  Research has shown that deep, infrequent irrigation that allows for mild moisture stress between irrigation events enhances both rooting and the capacity for osmotic adjustment, leading to better drought resistance overall.

Summer moisture stress is a troublesome annual reality in the cool-season environment. Careful turfgrass selection and goal-oriented management with a mind to drought resistance, however, can promote water conservation, help in the maintenance of a desirable level of turf function and appearance, and enable better persistence of turf cover from season to season.

Planting time is right around the corner!