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MORPHOLOGICAL COMPARISONS OF WINTER OVERSEEDING

METHODOLOGIES FOR HIGH-DENSITY DWARF HYBRID BERMUDAGRASS

1997-1998

International Sports Turf Institute, Inc.

S.I. Sifers and J.B Beard

ABSTRACT

Champion dwarf bermudagrass, a new, ultra high-density, vertical dwarf bermudagrass, became commercially available for use on golf putting greens in 1995 and was one of the first super dwarfs commercially available. There is some concern about,

(1) the ability to germinate and establish a winter overseeding of cool-season turfgrasses on these greens due to the very high shoot density of this grass,

(2) whether or not it is necessary to overseed these newer dwarf grasses in certain warmer locations,

(3) what would be the best timing for overseeding,

(4) what is the optimum grass mixture or monostand, and

(5) what is the optimum seeding rate.

Comparisons were made in three different warm- season environments including: hot-dry desert; hot-humid inland; and warm-humid coastal areas.

Alternate methods assessed included:

(a) cultural and/or chemical preplant seedbed preparation methods;

(b) selection of the cultivars and/or mixtures for winter overseeding;

(c) seeding rates; and

(d) determining the optimum preplant timing to insure

maximum growth potential.

The three experimental sites were maintained at a 1/8 inch ( 3.2 mm ) height of cut, with three replications for each of the 12 seeding treatments, two different preplant methods, and three seeding dates. Monthly assessments made following winter overseeding were a visual estimate of the seed germination percentage, a visual estimate of the percentage of total plot surface covered with green vegetation, and a visual estimate of turfgrass quality. Shoot densities of the overseeded turfgrasses were counted and the mat and root depths of the high-density dwarf bermudagrass were measured. Results of this study indicate:

( a ) it is not necessary to overseed dwarf bermudagrass in certain locations;

( b ) the best timing for overseeding, whenever possible, should be determined by soil temperature rather than a calendar overseeding date;

( c ) the optimum cultivar mixtures and seeding rates for putting greens in this study were: a mixture with 8 lbs. ( 4 kg ) of rough bluegrass and 2 lbs. ( 1 kg ) of creeping bentgrass; followed in 30 days by 2 lbs. ( 1 kg ) of rough bluegrass, a mixture with 10 lbs. ( 5 kg ) of rough bluegrass and 2 lbs. ( 1 kg ) of creeping bentgrass; and rough bluegrass at 8 lbs. ( 4 kg ); and

( d ) spring transition was successfully completed using timely cultural methods.

INTRODUCTION

When Champion dwarf bermudagrass, a new ultra high-density, vertical dwarf bermudagrass,

became commercially available for use on golf putting greens there was some concern expressed

about the ability to germinate and establish a winter overseeding of cool-season turfgrasses on

these greens.

There is a need to compare the alternate methods available for winter overseeding of the new high-density, vertical-dwarf, warm-season bermudagrasses, such as Champion, Floradwarf, MS Supreme, and TifEagle with cool-season turfgrasses. The comparisons should include: (a) cultural and/or chemical preplant seedbed preparation methods; (b) selection of the cultivars and/or mixtures for winter overseeding; (c) seeding rates; and (d) determination of the optimum preplant timing to insure maximum growth potential. If possible, the comparisons should be made in different warm-season environments including; hot dry desert; hot-humid inland; warm-humid coastal areas. To attempt to provide solutions to these problems, an extensive study was conducted during the winter and spring of 1997/1998. Champion hybrid dwarf was selected as the turfgrass for this study as it was the only one of the dwarf cultivars that had scientific research data available and that had been established in all of the desired climatic areas.

MATERIALS and METHODS

The experimental sites had been established in 1996 on a high-sand root zone at turf nurseries at the Indian Ridge Golf and Country Club in Palm Desert, California; at the Pointe Hilton at Tapatio Cliff, Phoenix, Arizona; and at the Bentwater C. C., Montgomery, Texas. The turfgrass was maintained using the same cultural methods as the greens with a 1/8-inch ( 3.2 mm ) height of cut. Vertical cutting, and topdressing were used to control the mat/thatch biomass.

The plot size was 6 x 10 foot ( 1.8 x 3.0 m ). Three replications for each of the 12 seeding treatments in the study were used at the Pointe Hilton, Bentwater, and Indian Ridge sites. There was space for one study at Indian Ridge and Bentwater, and for two replicate studies at Pointe Hilton. The turfs were maintained at a height of cut of 1/8 inch ( 3.2 mm ) after establishment of the winter overseeding grass stand. Non-limiting water was applied as needed to prevent visual wilt and standard fertility was used. Core cultivation was completed 30 days prior to planting.

Treatments:

Preplant: Two different methods were assessed:

Method A. This involved using only mechanical preparation. Four days prior to winter overseeding, routine mowing ceased and the turf canopy was lightly vertically cut in five directions at a zero setting, with the debris removed. Only light vertical cutting was required as mat/thatch biomass control was an on-going practice. On the day of planting, gravity drop spreader cans were used to distribute the seed as evenly as possible, followed by a topdressing with 1/8 inch ( 3.2 mm ) of the same material as the root zone mix. The seedbed was kept moist until seedling emergence. Mowing was resumed initially at a bench set height of cut of 3/16 inch ( 4.8 mm ), and gradually lowered to 1/8 inch ( 3.2 mm ) over 21 days. Method A was used at the Indian Ridge site and for study 2 at the Pointe Hilton site.

Method B. This involved both the mechanical preparation used in Method A and a chemical treatment with trinexapac ethyl ( Primo® WSP ), a Class A, type 1 plant growth regulator. Trinexapac ethyl was applied at a 0.03 oz.\1000 ft.2 rate two days before the vertical cutting and six days prior to seeding. This method was used for study 1 at Pointe Hilton and the Bentwater site.

Winter Overseeding Species and Rates: The turfgrass species and seeding rate treatments were weighed and bagged for each treatment plot. There were a total of 12 seeding treatments as shown in Figure 1.

Figure 1. Seeding treatments, rates, and the approximate number of seeds applied per 16 square inches ( dm-2 ) for the high-density, dwarf bermudagrass winter overseeding study.

Seeding Dates: Results of previous research by Batten, Beard, and Johns indicate that the optimum time for winter overseeding cool-season turfgrass into warm-season turfgrass is not a fixed calendar date, rather, that a more precise, biologically sound method of predicting the optimum planting time is to use the soil temperature at the 4-inch ( 100 mm ) depth, with the optimum temperature being between 72 and 78 F ( 22 and 26 C ). However, calendar date continues to be used by many. Therefore, three different seeding dates were used in this study.

1. The seeding date at the Indian Ridge site was 2 October, 1997, which is the approximate calendar date traditionally used in the desert for overseeding. The soil temperature was 85 F ( 29 C ) at a 4-inch ( 100 mm ) depth. This temperature was above the optimum range of 72 to 78 F ( 22 to 26 C ).

2. Two dates were used at the Pointe Hilton site.

Study 1 was seeded 10 October, 1997, based upon the traditional calendar date. The soil temperature was 82 F ( 28 C ) at a 4-inch ( 100 mm ) depth.

Study 2 was to be seeded based upon soil temperature, however on the 12 November, 1997 seeding date the soil temperature was 68 F ( 20 C ) at a 4-inch ( 100 mm ) depth. This temperature was below the optimum range of 72 to 78 F ( 22 to 26 C ).

3. The seeding date at Bentwater was determined by soil temperature. Overseeding was on 5 November, 1997 at soil temperature at 4-inch of 78 F ( 26 C ).

Spring Transition: Earlier research by Beard and DiPaolo, 1987 and by DiPaolo, Beard, and Brawand, 1982 found that although the aboveground stems and shoots of bermudagrasses lose their color and become dormant as temperatures lower to 55 F ( 13 C ) and below, the root system remains white and functional. Also, if winter temperatures do not go below 15 F ( -9 C ) for long periods, there should be little damage to the stolons, rhizomes, and root system. Sifers, Beard, and Kim, 1985 noted that regrowth will begin from lateral buds at the nodes of the stolons and rhizomes and greenup of the new shoots will occur when the soil temperature at the 4 inch depth reaches 64 F ( 18 C ). The lateral stems and roots supply the carbohydrates for this new growth until energy is available through photosynthesis in the new shoot. If there is a gradual warming in the spring without a rapid increase in temperature, the lateral stems will meet the energy needs and will reinitiate root growth from the tips of old roots in about two weeks. If the temperature increase is too rapid, the new shoot growth energy demand on the root system rapidly depletes the carbohydrate reserve, and the roots die back to the crown area, phenomena called Spring Root Decline (SRD). If this happens, extreme care must be taken with management of mowing, fertilization, pesticide application, and irrigation practices until the roots have regrown. This time period can be as long as six weeks. Unfortunately this also is the time when growth of the overseeded cool-season grass roots and shoots are at a peak, thus providing severe competition to the emerging bermudagrass plant for the sunlight it requires for photosynthetic-based growth. Until temperatures change to favor the warm-season grass, a daytime temperature above 75 F ( 23 C ), the cool-season grass growth must be gradually suppressed and thinned out through cultural practices that reduce the cool-season turf, but promote growth of the bermudagrass. If no attempt is made to create this timely gradual transition to bermudagrass, the bermudagrass stand may be lost and when the higher temperatures of summer eliminate the overseeded cool-season turfgrass there may be no grass cover.

Spring transition cultural treatments at each site began when the soil temperature at a 4-inch depth reached 62 F ( 17 C ) and continued until transition was complete. This consisted of a the following: (a) the mowing height was lowered to or maintained at 1/8 inch ( 3.2 mm ); (b) the nitrogen fertilization rate was doubled during the transition period; (c) each plot was core cultivated one time with cores removed; (d ) weekly vertical cutting and light topdressing were completed; and (e) non-stress soil moisture levels were maintained.

Assessments: Assessments made following winter overseeding were:

(a) A visual estimate of the seed germination percentage and a monthly visual estimate of the percentage of total plot surface covered with green vegetation.

(b) A visual estimate of turfgrass quality at 30 to 45-day intervals.

(c) The shoot densities of the overseeded turfgrasses were counted and the mat and root depths of the high-density dwarf bermudagrass were measured for all treatments in January at Indian Ridge, in February at Pointe Hilton, and in March at Bentwater. The study was conducted for a 180-day period during the autumn through spring of 1997-1998.

Analysis of variances for all turf parameters assessed in the studies were prepared.

RESULTS

Germination percentages :

The seed germination percent using preplant Method A varied from 63 to100 %, with a mean of 74.2 %, at Indian Ridge, to 0 to 67 %, with a 28.8 % mean, for Study 2 at Pointe Hilton. Germination using preplant Method B varied from 23 to 97 %, with the mean of 61.1 %, at Bentwater, to 63 to 100 %, and a mean of 80 %, for Study 1 at Pointe Hilton (Table 1.). Generally, except for the Method A treatments with perennial ryegrass alone at Indian Ridge and Point Hilton, the smaller seed size rough bluegrass and creeping bentgrass had a higher germination percentage than mixtures with the larger seed size perennial ryegrass.

Study 2 at Pointe Hilton had much lower germination percentages in all treatments which was probably due more to the low planting temperature of 68F ( 20C ) at the 4-inch ( 100mm ) depth, rather than the planting method used. Where the planting temperatures were at or above the biologically recommended temperature, germination was more successful. This partially answers the timing question. Whenever possible the overseeding date should be determined by soil temperature rather than calendar date and in this year a temperature above optimum was better than one below the biological optimum.

The preplant method used did not indicate a clear choice between the preplant methods A and B. This question requires additional research. However, it appears that the more critical factor for seed germination was temperature.

Stand density: Density comparisons between the numbers of seed sown per plot and the resultant stand density number are shown in Table 1a. Also shown is the percentage of seed sown that resulted in a turfgrass plant. This varied from 0 to 79 % depending on the cultivar and location. Overall the highest percentage means were the rough bluegrasses at the 4.0, 2.0, and 8.0 lbs. ( 2.0, 1.0, and 4.0 kg ) seeding rates, followed by creeping bentgrass at 1.0 lb. ( 0.5 kg ). The lowest germination rate was perennial ryegrass at 25 lbs. ( 12.5 kg ).

Together these data answer the concern expressed about the ability to overseed the high-density dwarf bermudagrasses. If a cool-season turfgrass with a small seed is selected, there will be no problem overseeding using either method. However, large seeded turfgrasses, such as the ryegrasses, will be more difficult to establish and will require a more vigorous disruptive preparation to open the turf canopy to accept the larger seed.

Visual turfgrass quality.

Overall. This rating was based on a scale of 1 to 9, with 9 being best. The rough bluegrass at 8 lbs. ( 4 kg ) with 2 lbs.( 1 kg ) of creeping bentgrass, followed in 30 days by 2 lbs. ( 1 kg ) of rough bluegrass, had the best visual turfgrass quality at each location and the next highest was the rough bluegrass at 10 lbs. with 2 lbs. of creeping bentgrass, which was numerically different but statistically the same (Table 2 ). Perennial ryegrass and mixtures containing perennial ryegrass were the lowest in visual turfgrass quality.

Specific locations. Tables 2a, 2b, and 2c list the monthly visual turfgrass quality ratings for Indian Ridge, Bentwater, and for Study 1 at Pointe Hilton. Also listed in these tables is the air temperature and soil temperature at a 4-inch (100-mm ) depth measured on the observation day.

There were individual differences between locations during each month. However, the rough bluegrasses generally received the best rating and the ryegrasses the lowest.

The non-overseeded Champion rated at 6.0 or above each month at the Bentwater site and the Pointe Hilton site, except in November, and for 2 of the 5 months at the Indian Ridge site. With environmental conditions similar to this specific year acceptable visual turfgrass quality can be maintained and overseeding would not be required at the Bentwater location and necessary for only one month at the Pointe Hilton site.

For Study 2 at Pointe Hilton, assessments for the first 60 days after planting show a very low percent germination, low visual quality, and poor plot coverage probably due to the lower than optimum soil temperature at planting. For this reason this study was terminated.

Coverage percentage.

The percent of the plot covered by the winter overseeded turfgrass is a function of seeding density, germination percentage, and the resultant plant density, but also is an indication of uniformity of the stand. The overall data in Table 3 are very similar to Tables 1 and 1a. However, the monthly data listed in the individual location tables are different (Table 3a, 3b, and 3c ). The data reflect that the overseeded cool-season turfgrasses coverage for most treatments decreased in March. However the rough bluegrass persisted longer in the spring than the other species. Also, those treatments with a higher plant density persisted longer, possibly due to increased competition.

Morphological data.

Shoot density. The overseeded turfgrasses shoot density data are presented in Table 4. Generally a high seeding density formed high shoot density. At the Indian Ridge location the 10 lbs. rough bluegrass polystands with creeping bentgrass had the best density, followed by rough bluegrass at the 18 and 8 lbs. ( 9 or 4 kg ) rate. The 18 lbs. (9-kg ) rate of rough bluegrass was best at Bentwater and Pointe Hilton. Perennial ryegrass was lowest at each site.

Root length. There were no roots from any of the overseeded turfgrasses found below the mat of the Champion dwarf. This will be of benefit during spring transition when it is desirable to remove the overseeded turfgrass, but may not be desirable when winter stresses occur. The use of creeping bentgrasses for overseeding of turfgrass with shoot/mat densities similar to Champion, as part of a mixture or alone, was possible as the roots of the bentgrass remained in the mat.

Table 5 shows the data for root length at each location for the Champion dwarf bermudagrass. Length of roots at Bentwater and Pointe Hilton were similar with little separation between treatments, except for the shorter roots with the 18 lbs. ( 9 kg ) rate of rough bluegrass at all locations and the 4 lbs. ( 2 kg ) rate of creeping bentgrass at Indian Ridge and Bentwater, which were significantly different. The high seeding density and resultant high shoot density for these two treatments have resulted in significantly reduced bermudagrass root length indicating that this density of seeding should be avoided.

Mat depth. Mat depth at each location had been controlled by cultural

methods. There were no differences at Bentwater, and although there were statistical differences at the other two locations these were numerically very small ( Table 6 ), and probably not biologically significant.

Spring transition The cultural technique previously outlined was successfully used at each location. When the 4 inch ( 100 mm ) depth soil temperature exceeded 62 F ( 17 C ) the plots were core cultivated, a 1/8th inch ( 3.2 mm ) height of cut was maintained, weekly vertical cutting was started, and a 2x application of N was applied. All locations experienced a gradual increase in temperature during spring 1998 with transition completed at Bentwater and Pointe Hilton in April. There was a later transition at Indian Ridge, caused by cool springtime temperatures, with full transition occurring in late May. SRD was not evidenced at any location. There was no significant loss of the bermudagrass.

SUMMARY

This study answered the concerns expressed about winter overseeding of the new high-density dwarf bermudagrasses:

( a ) It is not necessary to overseed dwarf bermudagrass in certain locations for years with similar temperature conditions.

( b ) The best timing for overseeding, whenever soil temperature rather than a calendar overseeding date should determine possible.

( c ) The optimum cultivar mixtures and seeding rates for putting greens in this study were:

( 1 ). Mixture with 8 lbs. ( 4 kg ) of rough bluegrass with 2 lbs. ( 1 kg ) of creeping bentgrass followed in 30 days by 2 lbs. ( 1 kg ) of rough bluegrass.

( 2 ). Mixture with 10 lbs. ( 5 kg ) of rough bluegrass and 2 lbs. ( 1 kg ) of creeping bentgrass.

( 3 ). Rough bluegrass at 8.0 lbs. ( 4 kg ).

( d ) Spring transition was successfully completed using specific cultural methods.

The results of this study have not been replicated over multiple years, and therefore should be used with that in mind. However, the study locations were geographically separated

REFERENCES

• Batten S.M., J. B. Beard, and D. Johns. 1981. A new approach to predicting dates for overseeding of perennial ryegrass on bermudagrass through soil temperature monitoring. Texas Agric. Exp. Sta. PR-3841. pp. 52-56

• Beard J. B., and J. DiPaola. 1978. Seasonal rooting characteristics of bermudagrass and St. Augustinegrass. Texas Agric. Exp. Sta. PR-3485. pp.5-11

• Beard J. B., et al. 1980. The influence of spring root dieback on cultural practices. Texas Agric. Exp. Sta. PR-3671. pp.21-26

• DiPaola J., J. B. Beard, and H. Brawand. 1982. Key events in the seasonal root growth of bermudagrass and St. Augustinegrass. HortSci. 17(5): 829-831.

• Sifers S.I., J. B Beard, and K.S.Kim. 1985. Spring root decline ( SRD ). A

research summary. Texas Agric. Exp. Sta. PR-4316. pp.13-17.

• Sifers S. I. et al. 1985. Spring root decline (SRD): discovery, description and

causes. Proceedings of the Fifth International Turfgrass Research Conference. Vol. 5 chapter 73 pp. 777-787.

Assessment Agency: International Sports Turf Institute, Inc. (ISTI).

Investigators: Col. Samuel I. Sifers, ISTI Associate Agronomist.

2204 Bristol

Bryan, Texas 77802

Phone: Home (409)-779-6484 Office (409) 822-1806

Fax: (409) 822-1806

Dr. James B Beard, ISTI President

1812 Shadowood Drive

College Station, Texas 77840

Phone: (409)-693-4066 Fax: (409)-693 4878

Table 1. Assessment of visual turfgrass seed germination as a percentage of the plot covered for the 1997/1998 high-density dwarf bermudagrass winter overseeding study. Means are the average of three replications for each treatment at each location

assessed 30 days following seeding.

*Means followed by the same letter within the same column are not significantly

different at the 5% level, LSD t-test.

Table 1a. Comparative assessment of the number of the winter overseeded turfgrass seeds sown, the resultant turfgrass stand plant density number, the percentage of seeds sown by location, and the overall means for the 1997/1998 high-density dwarf bermudagrass winter overseeding study. Location means are the average of three replications for each treatment assessed.