Though the Scrophulariaceae and Verbenaceae family names may not sound too familiar, you've most certainly grown some member species. Here, we will look at the currently known responses of perennial plants from these families to plant growth regulators (PGRs), using information from our own research or other published sources. How do we summarize the response of herbaceous perennials to plant growth regulators? Responses of many of the individual species discussed in this article may be found in our searchable PGR database at GPN's Web site, www.gpnmag.com .
The Scrophulariaceae (Figwort) family is a relatively small family, containing about 210 genera of annual and perennial herbs and shrubs along with a few trees. "Scrophs" usually bear their flower in spikes, and the individual flowers are usually irregular (zygomorphic) and bell-shaped or tubular (think foxglove). Only about 64 of these genera are grown commercially as ornamentals or for medicinal purposes. These cultivated genera include many of our vegetative annuals (angelonia, bacopa, diascia, nemesia, sutera and torenia). This family also includes some of our common annuals, such as snapdragons (Antirrhinum). Some of the more common herbaceous perennials include Chelone, Cymbalaria, Digitalis, Linaria, Mazus, Penstemon, Phygelius, Scrophularia, Synthyris, Verbascum, Veronica and Veronicastrum. Current research results on Scrophulariaceae response to PGRs only covers four genera, which includes eight different species or cultivars, but the major flowering species have been tested.
The Verbenaceae (Vervain or Verbena) family is an even smaller family, containing about 98 genera of herbs, subshrubs, shrubs or trees, which are predominantly tropical or subtropical. Flowers are variable, but usually five-lobed, and are borne in terminal clusters or spikes atop four-sided stems. The opposite leaves are serrated, usually rough, and have a distinct scent when crushed. Only about 21 of these genera are grown commercially. These cultivated genera include many of our ornamental shrubs and trees (Callicarpa, Clerodendrum, Cornutia, Nyctanthes and Vitex). The more common ornamental perennials include Caryopteris, Duranta, Lantana and Verbena. We have PGR response data for three of these genera with five species covered.
Although our summary of the PGR responses for the Scrophulariaceae and Verbenaceae families does not include very many entries, it covers the primary herbaceous perennials that are currently being commercially produced. We have also evaluated two additional perennials from these families, Mazus reptans and Verbena rigida, in branching enhancement studies. Check the database or contact the authors for more information on these crops.
B-Nine. As discussed in previous articles, B-Nine (Crompton/Uniroyal) is a short-term growth retardant that has been quite effective on a wide variety of perennials. However, in the Scrophulariaceae family Chelone glabra, Digitalis purpurea 'Foxy' and Penstemon digitalis 'Huskers Red' were not responsive to multiple applications of 5,000 ppm B-Nine (See Figure 1, page 38). All four cultivars of Veronica were responsive to B-Nine. Height control was generally good (30- to 40-percent reductions) but required multiple applications at 5,000 ppm to obtain 4-6 weeks of height control.
B-Nine/Cycocel Tank Mix. The tank mix of B-Nine and Cycocel (Olympic Horticultural Products) provides a very active PGR that is generally considered easier to apply and manage than triazoles. Digitalis Foxy was not responsive to a single application of the tank mix of 5,000 ppm B-Nine with 1,500 ppm Cycocel, but three Veronica selections were very responsive and growth of Penstemon Huskers Red was moderately controlled with a single application (See Figure 1, page 38). Multiple applications would be required for 4-6 weeks of control of Penstemon. For any of these crops, long production periods may require multiple applications of the tank mix to provide adequate height control. 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.
Bonzi. Bonzi (Syngenta) is one of the very active triazole PGRs and has been very effective on many of the vigorous perennial plants. All eight of the tested Scrophulariaceae species listed in the table were responsive to Bonzi, with many of the Veronica species being very sensitive to moderate rates.
The Chelone data is from Michigan (Paul Pilon's reports), so Southern growers will need to test rates about twice those reported, and multiple applications will be required under most conditions. We also have evaluated Penstemon Huskers Red response to 40 ppm Piccolo (Fine Á Agrochemicals) and found growth reductions similar to those described in the table for Bonzi (as you would expect). Penstemon also will require multiple applications under long production cycles.
For new crops, test rates from 40-60 ppm paclobutrazol in the South (about half that in the North), plan on multiple applications and develop estimates for single applications based on your test results. Many growers find that multiple applications of lower rates may provide better control than a single application of a higher rate. Others prefer making a single application early in the season.
Sumagic. Sumagic (Valent USA) is another triazole PGR that has a higher activity level than Bonzi or Piccolo and has also been very effective in height control of perennials. Of the six Scrophulariaceae species reported in the database that were evaluated for response to Sumagic, all were responsive to foliar sprays. The Penstemon and Veronica selections exhibited excessive growth reductions with 15 ppm Sumagic under Southern growing conditions with excessive landscape persistence in some cases. Test rates closer to 5-10 ppm on the Veronica species.
Again, the Chelone data is from Michigan (Paul Pilon's reports), so Southern growers will need to test rates about twice those reported. For untested crops in Scrophulariaceae, test rates around 10-20 ppm in the South (about half that in the North), and make additional applications as necessary and be alert to excessive growth regulation for sensitive crops. Á
Other PGRs. Although there is some data available for perennial plant response to Cycocel, Florel (Monterey), Atrimmec (PBI Gordon) and Topflor (SePRO), there has not been enough genera tested to make any suggestions of rates. Check the database for crops of particular interest to you or search by the product to see which crops have been tested and found responsive to date.
Verbenaceae Response to PGRs
Lantana. Lantana camara 'Confetti' treated with multiple applications of 5,000 ppm B-Nine under Southern growing conditions exhibited moderate height control with a slight delay in flowering. Lantana Confetti was not responsive to a single spray application of a tank mix of 5,000 ppm B-Nine and 1,500 ppm Cycocel. This cultivar was also responsive to 30 ppm Sumagic, but multiple applications would be required for sufficient growth control. Under Northern conditions (John Erwin, University of Minnesota), vegetative lantana responds to multiple applications of 10-30 ppm Bonzi or 5-15 ppm Sumagic.
Verbena. Under Southern growing conditions, Verbena canadensis 'Homestead Purple' was not responsive to two applications of 5,000 ppm B-Nine but was responsive to a tank mix of 5,000 ppm B-Nine with 1,500 ppm Cycocel. Multiple applications may still be required for adequate growth control. Verbena bonariensis was also responsive to multiple applications B-Nine at 5,000 ppm or to a single application of 120-160 ppm Bonzi under Southern growing conditions. Growth control was sufficient but persistent in the landscape when young plants were treated with a single application of 30 to 45 ppm Sumagic. Rates less than 30 ppm Sumagic should be tested for growth control. Multiple applications of the lower rate applied as necessary should also reduce landscape persistence.
The plants of the Scrophulariaceae and Verbenaceae families that have been evaluated are fairly responsive to PGRs. Simply due to the size of the families and number of species evaluated, we would not suggest any rate recommendations based on family relationships. We are still finding significant differences between the responses of different cultivars of the same species. So, it is not surprising that we can't make generalizations across an entire family.
However, considering the fairly small number of ornamental herbaceous perennials in the Scrophulariaceae and Verbenaceae families, this article provides PGR information on the primary species in this family and 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.
A caution to heed is that PGR rates used in the South are generally higher than rates used in the North. In addition, these higher Southern rates may be applied at more frequent treatment intervals than in the North. Most of the information in the GPN database and presented in this article is from research done in the South. Northern growers should evaluate rates about half those presented in this article.
Application timing and plant size at treatment can significantly affect plant response to PGR applications. Remember that plant growth retardants reduce plant growth so applications must be made prior to excessive stretch. 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. Growth regulation is an art that you must perfect. 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 perennials is in your hands.
The authors wish to thank the Fred C. Gloeckner Foundation, Crompton/Uniroyal, Valent USA, SePRO, Fine Agrochemicals and Olympic Horticultural Products for their financial 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.
Virginia Tech research reveals the PGRs that work best for crops such as penstemon, verbena, chelone and lantana.