PGR Liner Dips on Herbaceous Perennials

The July 2005 issue of GPN carried an excellent review of the "Factors Affecting PGR Liner Dips" written by two University of Florida graduate students, Rebecca Schnelle and Christopher Cerveny, led by Jim Barrett. This article covered details of application conditions to optimize efficacy of liner dips. They included PGR rate (higher rates are more effective in reducing plant growth), liner moisture level (drier plugs absorb more PGR than wetter plugs), liner dip duration (there was no difference between 30 seconds and 2 minutes if the plugs were wet at the time of dipping, but if dry plugs were dipped, efficacy increased with dipping time), light level (plants grown and treated under higher light levels were more responsive to the PGR, presumably due to greater transpiration rates that increase the uptake of the PGR), rooting quality (younger, less well-rooted cuttings are more responsive than older cuttings), solution depth of the PGR solution (doesn't make a big difference) and time between treatment and planting (up to 10 days later gave the same control as planting one day after treatment). So, to summarize, develop your own protocol — something that you can perform consistently under your operational conditions. The liner dip method is very effective and offers an economical and convenient way to apply PGRs to vigorous crops early in the production cycle.

Perennial Liner Dips

Now, how about perennials? While establishing rates will still require some testing, liner dips are proving to be extremely effective in providing baseline control of vigorous herbaceous perennials. In some of our original work, we attempted to establish liner dip rates relative to spray application rates. Testing 1?10 the spray rate, we treated Russian sage (Perovskia atriplicifolia) and purple coneflower (Echinacea purpurea 'Ruby Star') with 10 ppm Bonzi (paclobutrazol, Syngenta Professional Products), 4.5 ppm Sumagic (uniconazole, Valent USA) or 6 ppm Topflor (flurprimidol, SePRO Corp.). In this test our treatments were applied under cool (54¡ F), humid (97 percent relative humidity) and cloudy conditions with a dip time of ç 15 minutes, and our plugs were "dry" (ready for irrigation). All three PGRs reduced plant height of the Russian sage excessively compared to control plants at four weeks after treatment (WAT), and these effects persisted through eight WAT (see Figure 1, above). The environmental conditions were wet and cool during the spring test. Growth of 'Ruby Star' purple coneflower was slow, but by eight WAT the plants treated with 10 ppm Bonzi were comparable to controls, while those treated with 4.5 ppm Sumagic or 6 ppm Topflor were still significantly smaller than controls (see Figure 2, above). In a followup test, we examined additional rates of PGRs on Russian sage. The treatments were applied in mid-afternoon in the greenhouse under sunny skies at 91¡ F and 50 percent relative humidity. We dipped the "dry" (ready for irrigation) plugs for 15 minutes in Bonzi at 0, 2, 4, 6, 8 or 10 ppm or in Sumagic at 0, 1, 2, 3, 4, 5 or 6 ppm. Perovskia treated with 2 or 4 ppm Bonzi were 26 or 44 percent shorter than control plants at five WAT, but these differences were no longer significant at seven WAT (see Figure 3, page 42). Plants treated with rates higher than 4 ppm were 64 percent shorter than controls at five WAT and did not reach heights comparable to the control plants until nine WAT. Sumagic applied as a liner dip provided excellent baseline control with a 35-percent reduction in plant ç height at five WAT with a 1 ppm solution (see Figure 4, above). Sumagic rates above 2 ppm resulted in severe and persistent stunting of the plants. Plants treated with 5 ppm Sumagic were still 78 percent shorter than the control plants at nine WAT.

We also looked at dipping times using three-lobed coneflower (Rudbeckia triloba) this past spring. Again using "dry" plugs, we dipped plugs in solutions of 2 ppm Paczol (paclobutrazol, Chemtura) or 0.5 ppm Sumagic for 0, 0.5, 1, 2, 4, 6 or 12 minutes. Treatment conditions were warm (79¡ F) with 54 percent relative humidity under partly cloudy skies. Dipping time had no effect on the control of plant height for either chemical. Paczol at 2 ppm reduced plant height about 20 percent relative to controls at ç four WAT regardless of dipping time, and the effects were no longer significant at six WAT (see Figure 5, page 44). Sumagic at 0.5 ppm also reduced plant height by up to 38 percent at four WAT, but again the effects were not significant at six WAT (see Figure 6, above).

Summary

In summary, liner dips offer an excellent opportunity to get early control of vigorous herbaceous perennials. Increasing PGR rates will give longer control, but low rates, 2-4 ppm of Bonzi or Paczol, are good starting rates for short-term control, while 1?2-1 ppm would be a good starting rate for Sumagic. One of the primary advantages of this application method is the ability to dip one day and plant at a later, more convenient date after the REI for the treatment has passed. Other advantages include the ability to dip vigorous plants being used in mixed containers without affecting the surrounding plants.

Like all PGR treatments, the success is in the development of a consistent and repeatable application technique under your own operational conditions. In determining those conditions, pay special attention to the environmental conditions to assure that the plants are actively transpiring — warmth, sun and a moderate relative humidity will enhance PGR uptake. Pay attention to the plug moisture level; a drier plug will absorb more PGR than a wet plug. Be aware that younger plugs (less well developed root systems) will be more strongly affected by your treatment than older plugs, perhaps because the roots of the younger plug will take longer to grow out of the treated media plug. You may need to reduce your rates for younger plugs. Also be aware that rates will vary from different parts of the country just like your spray rates.

Do your own trials, and keep records of the responses and the environmental conditions during and after the treatment. Plant growth regulation is still a combination of art and science.

Joyce Latimer is a professor and extension specialist for greenhouse crops and Velva Groover is the floriculture program assistant in the Department of Horticulture, Virginia Tech, Blacksburg, Va. They can be reached by phone at (540) 231-7906 or E-mail at jlatime@vt.edu.