Plant growth regulator trials

By Joyce Latimer and John Freeborn, Virginia Tech, Department of Horticulture

Growth control of herbaceous perennials during container production enhances the marketability of the plants by keeping the plant size in balance with the container. Although the popularity of herbaceous perennials is still very high, many of these crops have not been evaluated for response to chemical plant growth regulators (PGRs).

Previous research has identified effective height control recommendations for specific cultivars in many different families of herbaceous perennials, but there was no consistent response to a given PGR within a family. In addition, most of the previous research on herbaceous perennials has been conducted with foliar spray applications. There is increasing grower interest in drench applications of PGRs. Therefore, our objective was to test a variety of PGRs and multiple rates of spray and drench applications of these PGRs to evaluate growth response of vigorous perennials.

Rooted liners (size 72) of Agastache foeniculum 'Purple Haze' and Hypericum calycinum were planted in mid May 2010. Plants were maintained in an unheated plastic covered hoop house under natural day length conditions. Treatments were applied 2 weeks after plugs were planted in trade gallon containers filled with a peat-pine bark medium. Foliar sprays were applied using a hand-held CO2 pressurized sprayer to apply the treatments at the label recommended volume of one gallon per 200 sq. ft. Drenches were applied evenly over the surface of moist medium at 10 fl. oz. per pot. Dazide was reapplied two weeks later. The PGRs treatments included:
(1)  Concise applied at two spray rates (30 or 60 ppm) or two drench rates (1 or 2 ppm)
(2)  Piccolo 10 XC applied at two spray rates (80 or 160 ppm) or two drench rates (4 or 8 ppm)
(3)  Dazide applied as a spray at 5000 ppm at 0 and 2 WAT
(4)  Dazide + Citadel Tank Mix applied as a spray at 5000 ppm Dazide + 1500 ppm Citadel at 0 WAT
(5)  Untreated control

Agastache was responsive to all Concise treatments with up to 31% reductions in vegetative plant height at 4 weeks after treatment (WAT) with similar reductions persisting through 6 WAT with all treatments except the 1 ppm drench. Plant width was less affected with up to 22% reductions in plants treated with the 2 ppm drench or the 30 ppm spray at 2 WAT. These reductions in width were less than 10% at subsequent measurement dates. Plant appearance was excellent (Fig. 1).

Only the higher drench rate, 8 ppm, of Piccolo 10 XC significantly reduced Agastache vegetative plant height, by 18% at 4 WAT and 20% at 6 WAT. Plant width was not affected.

Agastache was also responsive to Dazide but less so to the tank mix of Dazide and Citadel. Two applications of Dazide reduced plant height 22% at 4 WAT and 20% at 6 WAT. The tank mix did not affect plant height but reduced plant width. The Dazide-treated plants appeared "thin" at 4 WAT (Fig. 2). Only Dazide significantly reduced the number of flowers at 6 WAT (control 27.3 vs. Dazide 15.2). The days to flower was not significantly affected by PGR treatment.

Fig. 1. Effects of Concise on Agastache 'Purple Haze' at 4 weeks after treatment (WAT); left to right: untreated control, drench 1 ppm, drench 2 ppm, spray 30 ppm, spray 60 ppm.

Fig. 2. Effects of Dazide or the tank mix of Dazide and Citadel on Agastache 'Purple Haze' at 4 weeks after treatment (WAT); left to right: untreated control, Dazide 5000 ppm spray applied twice, tank mix 5000 ppm Dazide + 1500 ppm Citadel spray applied once.

Hypericum calycinum is a groundcover so plant width was a better indicator of growth regulation than plant height. Hypericum was very responsive to the PGRs and rates applied except Dazide. Plant width was reduced with most treatments at 2 WAT with excellent appearance at 4 WAT (Fig. 3). However, reductions persisted through 6 WAT with spray treatments and through 8 WAT with the drench treatments of Concise and Piccolo 10 XC. Concise drench and spray treatments reduced plant width up to 21% at 4 WAT, but persistence of the drench effect was excessive with up to 38% reductions in average plant width at 8 WAT. Even the higher spray rate, 60 ppm Concise, only provided moderate control, ~20% reduction in plant width at 6 WAT.

Piccolo drenches also caused excessive reductions in plant width with 49% reductions in growth at 4 WAT and over 60% at 6 and 8 WAT with the 8 ppm drench (Fig. 3). However, the higher spray rate, 160 ppm Piccolo 10 XC, was required to give moderate control of Hypericum.

Dazide had no effect on plant width and the tank mix of Dazide and Citadel had little effect on plant width with 16% and 10% reductions at 4 and 6 WAT.

Fig. 3. Effects of PGRs on Hypericum calycinum at 4 WAT; top: Piccolo 10 XC (left to right): untreated control, drench 4 ppm, drench 8 ppm, spray 80 ppm, spray 160 ppm; and bottom: Concise (left to right): untreated control, drench 1 ppm, drench 2 ppm, spray 30 ppm, spray 60 ppm.

In summary, Agastache was responsive to all of the Concise and Dazide treatments with reductions in vegetative plant height, but less responsive to Piccolo 10 XC and the Dazide/ Citadel tank mix. Recommended PGR applications include: Concise as a 1 to 2 ppm drench or a 30 ppm spray or Piccolo 10 XC as an 8 to 12 ppm drench. All treatments except Dazide reduced Hypericum plant width. Recommended PGR applications include: Concise as less than 1 ppm drench or 60 ppm spray or Piccolo 10 XC as less than 4 ppm drench or a 160 ppm spray.