Exploring the Ecology Behind Springtime Turfgrass Transition

Impact of annual and perennial ryegrass examined in process

In order to provide prime playing conditions throughout the winter, golf courses and sports fields in the southern United States frequently overseed warm-season turf with cool-season species such as annual and perennial ryegrass.

During late spring, the ryegrass is expected to transition out while the warm-season turf greens up (Horgan and Yelverton 2001). However, newer cultivars of ryegrass are more heat, drought and traffic tolerant. As a result, newer ryegrass cultivars compete with warm-season turf longer into summer than previous selections. The competition for light and nutrients during late spring and early summer can inhibit warm-season turf regrowth. For many Southern turf managers, the inhibition of warm-season turf growth has increased the management difficulties involved during spring transition (Richardson 2004).

Cultural practices that disrupt the playing surface such as scalping and vertical mowing have traditionally been used to speed up spring transition, but these practices can be labor intensive and often cause additional harm to the already stressed warm-season turf (Mazur and Wagner 1987). Superintendents have also resorted to utilizing transitional-aid herbicides to hasten the removal of ryegrass in the spring. However, tight budgets and the move to lower-input, sustainable turfgrass systems has reduced the viability of using transitional-aid herbicides over large turf acreages in some cases.

To combat poor spring transition of cool-season turfgrasses, seed companies have developed overseeding mixes, which utilize both perennial ryegrass and improved turf-type cultivars of annual ryegrass (S.O.S. Program, Barenbrug USA, 2008). Annual ryegrasses have been used for Southern overseeding in the past, but were generally too coarse and upright for high-quality turf.

In addition, annual ryegrasses were prone to transitioning before the warm-season turf had sufficient time to regrow. Improved annual ryegrasses are more prostrate and persist longer into the spring season than older annual ryegrass selections. Despite the lengthened lifespan, improved annual ryegrasses continue to transition out sooner than the new perennial ryegrasses.

The improved annual ryegrass cultivars are more heat, drought, and traffic tolerant than older cultivars. But due to the nature of an annual lifecycle, they're destined to transition by the end of the cool season. Mixing both improved annual and perennial ryegrasses together is thought to provide the benefits of rapid germination and establishment (from annual ryegrass) while still allowing for superior quality and spring persistence (from perennial ryegrass). The mixes are designed to allow for optimum spring transition that can be manipulated according to the climatic zone.

In theory, as annual ryegrass begins to transition out during spring, small voids are created in the overseeded turf canopy which allow increased light penetration and space for warm-season turfgrass regrowth. During this same time period, perennial ryegrass may provide a continuous green turfgrass canopy until the warm-season turf can resume as the dominant canopy species.

By varying seed ratios, a turf manager may be able to customize the speed of transition while minimizing additional inputs. Little research is available evaluating improved turf-type annual ryegrasses for winter overseeding and spring transition into warm-season turfgrasses.

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