Overseeding is necessary for green putting surfaces during southern winters.

Germination rates of rough bluegrass seed vary widely among cultivars and even among seed lots under cool growing conditions, but not under warmer conditions. In an experiment, rough bluegrass had a higher germination rate with day/night temperatures of 77/59 and 68/50 F than with cooler temperatures. Blends of several varieties or seed lots of rough bluegrass are suggested to ensure the success of overseeding during low temperatures.

Overseeding cool-season turfgrasses into bermudagrass greens can provide quality putting surfaces during bermudagrass's winter dormancy. Rough bluegrass (Poa trivialis) has been used for years in overseeding blends with perennial ryegrass (Lolium perenne) (3), but increasingly, superintendents are establishing 100 percent rough bluegrass overseedings.

Compared with perennial ryegrass, rough bluegrass offers easier establishment in the fall without significant disruption of play, more rapid spring transition back to bermudagrass and an excellent putting surface (2).

Temperature has a major influence on germination of an overseeding. Seeding dates and weather fluctuations expose rough bluegrass to different temperature regimes. Low temperatures generally retard germination. However, the effect of temperature on germination rate depends on variety and seed lot, according to the results of our study in growth chambers and on a Tifdwarf bermudagrass putting green at the Pee Dee Research & Education Center, Clemson University, Florence, S.C.

Methods
Chamber study. Twenty-five seeds of 10 rough bluegrass varieties and seed lots were incubated 28 days in four growth chambers at different day/night temperature regimes of 77/59, 68/50, 59/41, and 50/32 F. Fluorescent lamps supplied light during the day. Seed germination was measured seven, 10, 14, 17, 21, 24 and 28 days after seeding. Within each chamber, there were three replications of each seed lot, and the entire study was conducted three separate times.

Field study. Fifty seeds each of the 10 rough bluegrass varieties and seed lots were overseeded on a Tifdwarf bermudagrass green in 4-inch diameter circles on Oct. 23 and Nov. 6, 1998, and Jan. 4, 1999. Seed germination was measured seven, 10, 14, 17, 21, 24 and 28 days after overseeding.

Results and discussion
Both studies showed significant differences in seed germination among the temperature regimes and among the overseeding dates, and significant differences among varieties and seed lots.

Rough bluegrass germinated faster, and more seed germinated, at warmer temperatures in growth chambers and with earlier seeding dates on the bermudagrass green. For instance, the germination percentage exceeded 80 percent on day seven with day/night temperatures of 77/59 F. However, at temperatures of 50/32 F, germination was delayed for about two weeks, and the final germination percentage never reached 70 percent.

On the bermudagrass green, the final germination percentage of the first seeding date (Oct. 23) was about 40 percent at 28 days after seeding. At the last seeding date (Jan. 4), however, the average germination was less than 30 percent. The average maximum and minimum temperatures during the first overseeding period (Oct. 23-Nov. 20, 1998) were 70 and 44 F, whereas those in the third overseeding period (Jan. 4-Feb. 1, 1999) were 61 and 37 F.

Researchers previously reported the base temperature of rough bluegrass germination as 43 F, below which 50 percent germination would not occur (1). The temperatures during the third overseeding period were low; the average minimum temperature (37 F) was lower than the base temperature of 43 F, which may lead to slow germination.

Minimal differences in germination rate occurred among the 10 rough bluegrass varieties and seed lots with warm temperatures, or on the green at early seeding dates.

Big seed lot differences
When temperatures were cold in the growth chamber and at the last seeding date, the germination rate differed widely among the varieties and seed lots. For example, in the growth chamber at 50/32 F, the fastest-germinating rough bluegrass attained 60 percent germination 21 days after seeding, whereas the slowest variety achieved only 35 percent germination at 28 days. The same two varieties had 35 percent and 10 percent germination on the green during the coolest overseeding period.

Rough bluegrass varieties and seed lots varied widely in germination rate, especially at low temperatures. Until a screening procedure is developed to identify cold-sensitive varieties and seed lots, use of blends of several varieties or seed lots is suggested to ensure the establishment of rough bluegrass overseeding. Superintendents may purchase blends of varieties, or purchase separate varieties but blend them for use on the greens.

Early seeding (when temperatures are 50-77 F) and multiple seed applications may help avoid poor establishment of cold-sensitive varieties and seed lots. If superintendents decide to seed in the early fall, however, competition from bermudagrass could disrupt establishment. Potential for disease is also high at this time.

Clearly, successful overseeding with rough bluegrass requires a strategy for overcoming the species' spotty tolerance of cold temperatures during germination. Otherwise, its many advantages as an overseeded turf may be lost.

Literature cited

1. Beard, J.B., and A. Almodares. 1980. Minimum temperature requirements for seed germination of turfgrasses. Texas Turfgrass Research -- 1978-79. Feb., p. 13-15.

2. Edminster, C.W. 1992. The Poa trivialis challenge. Landscape Management 31(2):68, 70..

3. Johnson, B.J. 1994. Fenarimol tolerance of roughstalk bluegrass overseeded in bermudagrass greens. Agronomy Journal 86:519-522.

Chunhua Liu, Ph.D., is a post-doctoral research scientist of turfgrass physiology and management; S. Bruce Martin, Ph.D., is an associate professor of turfgrass pathology and physiology; James J. Camberato, Ph.D., is an associate professor of turf soil and management at Pee Dee Research & Education Center, Clemson University, Florence, S.C.