Asteraceae Response to PGRs
As we complete more research on plant response to plant growth regulators (PGRs), it may be helpful to start looking for any commonalities in the responses by plants of the same genus or family. Using information from our own research or other published sources, this month’s article will look at the currently known responses of perennial plants from the family Asteraceae (Compositae) to PGRs.
Asteraceae is a huge family, containing about 950 genera, but only about 190 of these are grown commercially. These cultivated genera include many of our floricultural annuals and potted crops and more than 100 commercially important herbaceous perennial crop species. Some of the more popular perennials include achillea, aster, artemisia, coreopsis, echinacea, gaillardia, leucanthemum, rudbeckia and solidago.
For PGRs applied for height control, it is important to remember that the goal is regulation of plant growth, which is what determines the final plant height. In general, the applications should begin about two weeks after transplanting spring plugs, which allows time for the roots to establish and for the plant to resume active growth. For overwintered material, the PGR should be applied when the new growth is 1-3 inches long, depending on the crop. Overwintered material is likely to be more vigorous than newly planted material and may require slightly higher PGR rates or earlier treatment if the growing temperatures are high enough for growth.
Another caution to heed is that PGR rates used in the South are generally higher than rates used in the North. In addition, these lower rates may be applied at wider treatment intervals in the North. Most of the information in this article is from research done in the South. Northern growers should evaluate rates at about half those presented in this article.
Responses to B-Nine
B-Nine is a short-term growth retardant that has been quite effective on a wide variety of perennials. In the South, we generally use 5,000 ppm at 10- to 14-day intervals for control of plant height and form. Of the 21 genera in the GPN PGR database at www.onhort.com that have been tested for response to B-Nine, 15 were responsive. Height control was generally moderate (20- to 30-percent reductions) and required multiple applications at 5,000 ppm to obtain 4-6 weeks of height control. Many of these plant species were transplanted into the landscape, and we found no landscape persistence with B-Nine treatments.
Only six Asteraceae genera were not responsive to multiple applications of B-Nine. These genera included several cultivars of Phlox paniculata (except ‘David’, which was responsive), Chelone glabra, Eupatorium coelestinum, Gaillardia grandiflora ‘Goblin’ (‘Burgundy’ was responsive), Leucanthemum x superbum ‘Alaska’ and ‘Becky’, and Rudbeckia fulgida var. sullivantii ‘Goldsturm’.
On the other end of the spectrum, Heliopsis helianthoides ‘Summer Sun’ was very sensitive to B-Nine with a greater-than-50-percent reduction in plant height at four weeks after treatment with two applications of 5,000 ppm B-Nine. This reduction persisted through the 12 weeks of evaluation in the nursery with no additional treatments and during subsequent landscape evaluation. Overdoses are possible with B-Nine, but this is the first time we have seen an overdose on perennials with this PGR. For your operation, test lower rates of B-Nine, 2,500-3,000 ppm, on Summer Sun with critical evaluation of growth prior to additional applications.
In summary, B-Nine proved effective on a wide variety of genera in the Asteraceae family. Multiple applications are likely to be required, with most species in the South requiring 5,000 ppm. Rates for Northern growers may be slightly less to one-half this rate, again with multiple applications required.
Responses to B-Nine/Cycocel Tank Mix
The tank mix of B-Nine and Cycocel provides a very active PGR that is more forgiving of non-uniform application than are some PGRs. However, relatively few of the Asteraceae genera have been tested for response to this tank mix. Of the 12 genera in the database tested, 10 were responsive to a single application of the tank mix, usually at 5,000 ppm B-Nine plus 1,500 ppm Cycocel. These included Achillea ‘Paprika’, Agastache x ‘Blue Fortune’, Coreopsis grandiflora ‘Sunray’ and ‘Baby Sun’, Coreopsis verticillata ‘Zagreb’, gaillardia Burgundy, Rudbeckia triloba, and Stokesia laevis ‘Purple Parasols’ and ‘Klaus Jelitto’. In most cases, a single application of the tank mix provides control equivalent to two or more applications of B-Nine alone. However, in some cases, such as with gaillardia Burgundy, multiple applications of the tank mix may be required to provide adequate height control through a 6-week production period.
Of the non-responsive entries, phlox David was responsive to B-Nine alone, while leucanthemum Becky was not. As with the B-Nine alone, a single application of the tank mix resulted in an overdose effect on heliopsis Summer Sun with height reductions similar to B-Nine alone that also persisted throughout the 12-week test.
Although there have not been enough genera tested to make a generalization for the Asteraceae family, the tank mix of B-Nine and Cycocel is worth trying on your perennial plants. Evaluate the growth response at 3-4 weeks after treatment to determine if a second application will be required to maintain your plants for your market window.
Responses to Bonzi
Of the 20 Asteraceae genera reported in the database that were evaluated for response to Bonzi, 12 were adequately controlled by foliar spray applications. Plants controlled by Bonzi varied in their responsiveness, from Chrysanthemum parthenium, which is very sensitive to 40 ppm, to rudbeckia Goldsturm, which requires 160 ppm for an effective single application. Multiple applications of lower rates may provide better control than a single application of a higher rate.
Eight genera had species or cultivars that were non-responsive to foliar sprays of Bonzi. However, four of these genera, Aster x frikartii ‘Monch’, Coreopsis rosea, Eupatorium coelestinum and heliopsis Summer Sun, were responsive to drench applications of Bonzi. Others not responsive to Bonzi included gaillardia Goblin, Liatris spicata ‘Floristan Violet’ and leucanthemum Becky. However, Alaska and Leucanthemum maximum ‘Snow Lady’ were responsive to Bonzi at rates around 40 ppm. In addition, gaillardia Burgundy was responsive to drench applications of Bonzi (sprays have not been evaluated on this cultivar).
The effectiveness of the drench applications suggests that using higher-volume application methods, such as drenches or sprenches, will improve the efficacy of Bonzi on perennial crops. However, the differences in cultivar response make it unlikely that we can make meaningful recommendations based on family relationships. For new crops, test rates from 40-60 ppm and plan on multiple applications and develop estimates for single applications based on your test results.
Responses to Sumagic
Sumagic has a higher activity level than Bonzi and has also been very effective in height control of perennials. Of the 16 Asteraceae genera reported in the database that were evaluated for response to Sumagic, nine were responsive to foliar sprays. The final plant height in these tests tended to be fairly linear, i.e., the height was reduced to an increasing extent with increasing rates of Sumagic. Coreopsis, with three species and multiple cultivars tested, was consistently responsive to Sumagic, but rates varied from multiple applications of 15 ppm to a single application at 40 ppm. Achillea, with four cultivars tested, two — ‘Coronation Gold’ and Paprika — were very responsive at a low rate (15 ppm) whereas two — ‘Moonshine’ and ‘Summer Pastels’ — were not responsive to rates up to 60 ppm Sumagic.
Seven genera had species or cultivars that were non-responsive to Sumagic: achillea Moonshine and Summer Pastels, aster Monch, heliopsis Summer Sun, liatris Floristan Violet, and stokesia Purple Parasols and Klaus Jelitto. Gaillardia Goblin was non-responsive (as seen with Bonzi) but Burgundy was responsive to a single application of 60 ppm. Leucanthemum Becky was also not responsive to Sumagic while Alaska was very sensitive to 15 ppm; again, these cultivar differences were similar to those seen with Bonzi.
In summary, for the family, the effective rates of Sumagic varied widely from less than 15 ppm to more than 80 ppm with just over half of the 16 genera tested showing height control with Sumagic. In addition, cultivar differences were found in several genera. Based on the existing information, we can make no rate recommendations for other Asteraceae crops. For untested crops, test rates around 30 ppm, make additional applications as necessary and be alert to excessive growth regulation for sensitive crops.
Responses to Other PGRs
Although there are some data available for perennial plant response to Cycocel, Florel, Atrimmec and Topflor, there have not been enough genera tested to make any suggestions of rates. Check the GPN database at www.onhort.com for crops of particular interest to you or search by product to see on which crops it has been effective on to date.
In summary, either there is little commonality of plant response to PGRs within a family, or we simply haven’t tested enough members of the family to find those commonalities. Given the differences that we find between cultivars of the same species, it is not surprising that we can’t make generalizations across an entire family, especially one as large and diverse as the Asteraceae.
However, much of the information available on the primary crops in this family will provide you with a good starting point for using PGRs in your operation. Be aware that many of the results discussed are from a single experiment and most involve a single application. These are also the results of scientific experiments, which means that the treatments are applied on a pre-determined schedule. A grower would evaluate growth on a frequent basis and apply treatments according to plant growth and growing conditions, which would improve the overall quality of the plant and may actually affect the efficacy of the treatment. In other words, don’t be afraid to try a treatment that we found to be ineffective.
Plant condition, growing environment and your specific application methods will all affect the response of the plants to your treatment. Keep records so you can duplicate your successes and learn from your mistakes. Through science, we can give you suggestions for starting points in the process and guidelines to develop your own PGR program, but successful growth regulation of these wild and wonderful perennials is in your hands.
The authors wish to thank the Fred C. Gloeckner Foundation, Crompton/Uniroyal, Valent USA, SePRO, and Olympic Horticultural Products for their support of this research and Yoder/Green Leaf for donation of plant materials. Much of the work reported herein was conducted at the University of Georgia and the authors appreciate the assistance of Dr. Paul Thomas and Ms. Sherrod Baden.