Regulating growth

Many vigorous herbaceous perennials need growth regulation early in the production cycle. Treatment at the liner stage improves early growth control after planting, or it maintains plant size in the liner tray.

There are a lot of advantages to using liner dips over other types of plant growth regulator (PGR) applications. Liner dips offer an excellent opportunity to get early control of vigorous plants. Another advantage is the ability to dip the plants one day and plant them at a more convenient date, like after restricted entry interval for the treatment has passed. You also can use liner dips to treat vigorous plants used in mixed containers without affecting the surrounding plants. Liner dips are generally very effective, economical and safe to apply, and the application can be automated in larger greenhouse operations.

Uniconazole PGR
In various talks, we have presented the results of some of our earlier work with liner dips with crops that are more difficult to control with foliar PGR sprays. In ornamental grasses, for example, Miscanthus and Calamagrostis are not very responsive to foliar sprays of uniconazole (Concise, Fine Americas Inc., or Sumagic, Valent USA).

Liner dips of 1, 2, or 3 ppm Concise applied for two minutes to “dry” (ready for irrigation) liners gave excellent control of Miscanthus ‘Gracillimus’ height, however (Fig. 1). Growth of plants treated with higher rates, 4 or 5 ppm, was essentially shut down about four weeks after treatment (WAT), and plants did not grow out of the treatment over the 12-week test. The rate response of Calamagrostis ‘Karl Foerster’ was saturated at about 2 ppm; higher dip rates did not result in a significant amount of additional growth control (Fig. 2).

In the liner trays, Concise gave significant growth control through 12 WAT in both grasses. Sensitivity to control varied with the grass species. Uniconazole rates of 1 to 2 ppm are recommended, depending on the crop species and how long growth control is desired.

Rudbeckia ‘Goldsturm’ is also very responsive to uniconazole. Plant height was reduced with increasing rates of the Concise liner dip by 4 WAT (Fig. 3). Concise at 1 ppm provided significant control of plant height without excessive reductions in plant width or a delay in flowering. Higher dip rates caused excessive growth reduction and delayed flowering.
 

Paclobutrazol
More recently, we have been evaluating plant response to liner dips of Piccolo 10 XC (4 percent paclobutrazol, Fine Americas Inc.). Again the liner dips were applied to actively growing plant material when the root ball was ready for irrigation.

Gaura ‘Pink Fountain’ height was reduced 33 to 44 percent with rates at or above 2 ppm (Fig. 4). Only an 8 ppm liner dip resulted in plants that appeared stunted at six WAT. Plant width was also decreased with the liner dips. In addition, the plant growth habit was more upright than in control plants, which was more attractive for marketing the plants. Piccolo 10 XC liner dips did not affect the percentage of plants flowering or the number of days required for first flower.

Other crops in this trial included Monarda ‘Raspberry Wine’ and Penstemon ‘Laura,’ which were much less responsive to paclobutrazol liner dips, with significant but short-term growth control only with the 16 ppm Piccolo 10 XC rate. The appearance of these plants was excellent, and the liner dip provided the baseline control that we desired.

Holding these crops longer than about four WAT, however, would require a higher initial liner dip rate or a subsequent PGR spray application.

Veronica ‘Pink Panther’ was also responsive to liner dips of Piccolo 10 XC with moderate height control through eight WAT with 4 or 8 ppm. These non-vernalized Monarda and Veronica plants did not flower during the trial, but there was no treatment effect on flowering of Penstemon ‘Laura.’
 

Summary
Liner dips with soil-active PGRs are very effective for baseline control of the early growth of vigorous herbaceous perennials. They are economical, so you can treat a large number of plants at one time by dipping an entire liner tray.

Although we test the method by actually dipping the tray in the PGR solution, University of Florida research has shown that applying the PGR as a drench to saturate the root ball of the liners is equally effective.

As with all PGR treatments, establish your own consistent application method and determine the best rate for your crops under that method.

Liner dips are effective on a wide variety of crops with any soil-active PGR. Very low concentrations of paclobutrazol or uniconazole can be used in the liner tray prior to transplanting. We would recommend testing uniconazole rates of 0.5 to 1 ppm or paclobutrazol rates of 4 to 8 ppm on most herbaceous perennials unless you have experience that supports using a higher rate. Increasing the growth retardant rates will provide not only greater growth regulation but also a longer control period, so use caution when increasing the rates.

Again, we recommend that you treat this application technique as a baseline control method. For many crops, this may be all of the growth regulation required, but err on the side of caution. You can always apply another PGR treatment later in production if necessary.
 

Joyce Latimer is a professor and extension specialist at Virginia Tech in Blacksburg, Va. John Freeborn is a research assistant in the department of horticulture at Virginia Tech. For more information, contact Joyce at (540) 231-7906 or jlatime@vt.edu.