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Paclobutrazol and Cutless treatments have been evaluated on perennial biotype Poa annua [Poa annua (L.) var. repens] control in creeping bentgrass green from 1991 to 1993 at Griffin, GA. Treatments given in Table 1 were applied to the same plots during 1991 and 1992 (Experiment I) and to different plots during 1992 and 1993 (Experiment II). The response from paclobutrazol and Cutless applied in 1 to 4 applications annually in our studies have been inconsistent in controlling the perennial Poa. These results are similar to those obtained from several Superintendents on a similar program. Therefore, we wanted to determine in a new experiment (III) in 1993, what are the rates required of either paclobutrazol or Cutless to control perennial Poa and influence of the treatments on bentgrass tolerance? Treatments initiated in spring of 1993 were applied at various rates in three applications during the spring and three applications during the fall (Table 2) to equal a total annual rate of 1.5 to 2.26 lb ai/A for paclobutrazol and Cutless. In contrast, the total annual rate of chemicals applied in Experiment I and II ranged from 0.5 to 1.5 lb ai/A.

Table 1. Effect of rates and frequency of paclobutrazol and Cutless treatments on perennial Poa control in creeping bentgrass greens. Experiment I and II.
 

PGR	RATE/APPLICATION  lb ai/A	TOTAL RATE PER YEAR  lb ai/A	DATE APPLIED
Untreated	-	-	-
Paclobutrazol	0.5	0.5	3/15
	0.5 + 0.5	1.0	3/15 + 10/1
	0.5+ .25	0.75	3/15 + 5/1
	0.25+.25+.25+.25	1.0	3/15+5/1+10/1+11/15
	0.5+.25+.5+.25	1.5	3/15+5/1+10/1+11/15
Cutless	0.5	0.5	3/15
	0.5 + 0.5	1.0	3/15 + 10/1
	0.5 + .25	0.75	3/15 + 5/1
	0.25+.25+.25+.25	1.0	3/15+5/1+10/1+11/15
	0.5+.25+.5+.25	1.5	3/15+5/1+10.1+11/15

 
 

Table 2. Effect of rates and frequency of paclobutrazol and Cutless treatments on perennial Poa control in creeping bentgrass greens. Experiment III.
 

PGR1	RATE  lb ai/A	TOTAL RATE PER YEAR  lb ai/A
Untreated	-	-
Paclobutrazol	0.5+.38+.25+.25+.125+.125	1.63
	0.5+.5+.38+.38+.25+.25	2.26
	0.5+.5+.25+.38+.38+.25	2.26
	0.25+.25+.25+.25+.25+.25	1.50
	0.5+.25+.125+.38+.25+.25	1.63
Cutless	0.5+.38+.25+.25+.125+.125	1.63
	0.5+.5+.38+.38+.25+.25	2.26
	0.5+.5+.25+.38+.38+.25	2.26
	0.25+.25+.25+.25+.25+.25	1.50
	0.5+.25+.125+.38+.25+.125	1.63

¹ Six applications of each PGR were made to the same plots as follows: The first - March 18, second -April 19, third - May 18, fourth - September 29, fifth - November 1, and sixth - December 1, 1993.
 

Perennial Poa control:

Experiments I and II. Paclobutrazol applied in two applications at 0.5 plus 0.25 lb/A (March 15 + May 1) in the spring and two applications at 0.5 plus 0.25 lb/A (Oct. 1 + Nov. 15) in the fall controlled perennial Poa higher than when Cutless was applied at the same rates and dates. In Experiment I, maximum Poa control in December following the four applications for two consecutive years was 60% for paclobutrazol and 41% for Cutless. By the following March, the control was reduced to 34% for paclobutrazol and 12% for Cutless. The results were similar when treatment were repeated in Experiment II. These data indicate that paclobutrazol and Cutless has initial activity on perennial Poa, but rates used and timing of the applications did not prevent recovery of Poa.

Experiment III. Since treatments in Experiment III were initiated March 18, 1993, it is too early to determine what the perennial Poa control will be from six annual paclobutrazol and Cutless applications. However, the control with paclobutrazol was 98% on October 20, 1993 following three applications (0.5 + 0.5 + 0.38 lb/A) in the spring and one application (0.38 lb/A) on September 29. The control was 21% in plots treated with Cutless at the same rates. It should be emphasized that these results are preliminary and additional results are needed.

It was observed that perennial Poa was affected by summer heat stresses more during 1993 than during the previous years. The higher Poa control in paclobutrazol-treated plots may be, in part, to heat-stressed Poa.

Bentgrass injury. Generally, bentgrass was injured higher from paclobutrazol and Cutless treatments applied in the fall than when applied in the spring. The higher injury in the fall was related to poor root growth following summer stresses.

Maximum bentgrass injury following 0.5 lb/A rate of either paclobutrazol or Cutless was about 20% (0 = no injury to 100 = complete kill) in the spring and about 40% in the fall. In some instances, similar levels of injury may occur from repeated PGR treatments. It was observed that while a 0.5 lb/A rate can be safely applied to bentgrass in the spring, a 0.38 lb/A rate was the maximum that could be safely applied to bentgrass in the fall. When bentgrass injury occurred, the turf required 3 to 4 weeks for recovery.

While high PGR rates are needed for perennial Poa control, creeping bentgrass will tolerate only low rates. Therefore, a multi-year program utilizing sequential applications will probably be necessary to eliminate perennial Poa from creeping bentgrass. Additional research to identify dates, rates, and application frequency will be needed to refine this approach.