Authors: Joyce G. Latimer, Extension Specialist, Greenhouse Crops; Holly L. Scoggins, Assistant Professor of Horticulture and Thomas J. Banko, Associate Professor of Horticulture, Hampton Roads AREC; Virginia Tech
Publication Number 430-103, December 2001
Production of Herbaceous Perennials
|
Availability of chemical plant growth regulators (PGRs) for perennials is not a problem. Due to recent label expansions, all of the primary floriculture growth retardants are labeled for use on perennials. However, lack of knowledge about rates and the diversity of plant responses to these PGRs are problems. Many of the herbaceous perennials in the market have never been evaluated for response to any of these chemicals.
A summary of the results of research trials using PGRs on over 100 perennial species/cultivars is presented in Table 1. This summary includes rates found effective in reducing plant height in published articles, as well as many of our own research results. Foliar spray applications are tested more often than drenches, presumably due to the higher labor costs involved in applying drenches in large scale production areas.
Return to Table of Contents
Treatment of large or dense plants may require higher volumes to treat the interior portions of the plant. However, remember that deviations from the recommended volumes applied will result in a different dosage of PGR applied. Always consider the rates presented in Table 1, or from any other resource, to be a guideline to assist you in developing your own rates based on your growing conditions and application methods.
Plant growth regulators should be applied to new growth (1.5 to 2" long) before it enters the rapid elongation phase. Generally, PGRs should be applied before flower initiation to minimize flowering delays. Remember that reductions in plant height are not always the growth control desired during production. For gaura (Gaura lindheimeri), where excessive height was not a production issue, B-Nine or a B-Nine/Cycocel Tank Mix was very effective in reducing plant width (Figure 2). In cases like this, the results may not be indicated in Table 1.
Return to Table of Contents
Cycocel (chlormequat chloride, Olympic Horticultural Products) is another PGR with a long history in floriculture. Cycocel is generally applied as a foliar spray at 1500 to 3000 ppm. Although rates above 1500 ppm often cause chlorosis on treated leaves of other floricultural crops, we have seen few examples of phytotoxicity on perennials. However, Cycocel alone has not been tested on a wide variety of perennials. It was very effective on Campanula carpatica and purple coneflower (Echinacea purpurea) where multiple applications of 1500 ppm resulted in excessive reductions in growth (Table 1). Rose mallow (Hibiscus moscheutos) also is sensitive to Cycocel, responding well to multiple applications of 1000 ppm. Cycocel also promotes earlier flowering and greater flower numbers on Hibiscus. First application should be made when the laterals are 0.5 to 1 inch long.
A B-Nine/Cycocel Tank Mix has more PGR activity than either B-Nine or Cycocel alone and has been tested on a wide variety of perennials. Three-lobed coneflower (Rudbeckia triloba) was very responsive to B-Nine applied twice at 5000 ppm, but not responsive to Cycocel at rates up to 4000 ppm (Figure 3a). A tank mix of 5000 ppm B-Nine with increasing rates of Cycocel resulted in similar height control with a single application at four weeks prior to the photograph (Figure 3b). The "high activity" rate generally used for the tank mix is 5000 ppm B-Nine plus 1500 ppm Cycocel. Although the rate of B-Nine is usually adjusted to increase or decrease activity, changing the Cycocel rate also affects activity. Shoot height of 31 of the perennials listed in Table 1 was effectively controlled by this tank mix including blanket flower (Gaillardia grandiflora) and Russian sage (Perovskia atriplicifolia) (Figure 4). As described with speedwell, single applications of the tank mix may be more effective than multiple applications of B-Nine alone. In other crops in Table 1 where the tank mix is listed as non-responsive (NR) with one application, the effects may have been too short term for the research evaluation. Multiple applications of the tank mix may provide control on these species.
A-Rest (ancymidol, SePRO Corporation) is a more active compound than B-Nine or Cycocel. It is labeled for use on containerized plants in greenhouses and nurseries. A-Rest is active as a spray or a drench so application volume affects plant response. In addition, A-Rest is one of the few PGRs labeled for distribution through the irrigation system via flood, sprinkler or drip systems. Although it has not been evaluated for effectiveness on a large number of perennial species, A-Rest is specifically labeled for foliar sprays on bleeding heart (Dicentra) and columbine (Aquilegia) at 65 to 132 ppm, gayfeather (Liatris) (25 to132 ppm) and hybrid bee delphinium (Delphinium) (35 to 132 ppm). A-Rest also can be applied as a drench at 2 to 4 ppm for these crops. For best results, treatments should be applied to well-rooted plants in active growth, prior to the initiation of flowering. In research reports, tickseed (Coreopsis) was not responsive to A-Rest, but Campanula, hardy ageratum (Eupatorium), purple coneflower (Echinacea), lavender (Lavandula), cardinal flower (Lobelia) and speedwell (Veronica) were very responsive (Table 1). The higher rates necessary for foliar applications to some crops may not be economically feasible. The use of A-Rest as a drench or as a treatment of plants in the plug stage is more economical than foliar application on finished plants.
Bonzi (paclobutrazol, Uniroyal Chemical Company) and Sumagic (uniconazole, Valent USA) are members of the triazole class of PGRs and are much more active than the previous compounds. Sumagic is more potent than Bonzi. For perennials, Bonzi is generally applied at rates of 30 to 100 ppm and Sumagic at 15 to 45 ppm. These PGRs are rapidly absorbed by plant stems and petioles or through the roots. Excess spray dripping off treated plants acts as a drench to the substrate, increasing the activity of the treatment. For foliar sprays, uniform application of a consistent volume per unit area is critical to uniform and consistent crop responses to the triazoles. Both compounds are labeled for application to the media surface prior to planting plugs. In this case the PGR is applied as a spray (at rates one-third to one-half the recommended foliar spray rate) to the surface of the medium in filled pots. The PGR moves into the medium with subsequent irrigations and effectively behaves as a drench which is the reason for the lower recommended rate. Effectiveness also is reduced by bark in the medium as it is with drenches.
Neither PGR has exhibited any specific phyotoxicity symptoms on perennials, but care must be taken with application of excessive rates on sensitive plants. In some cases, excessive stunting can be very persistent, e.g., goldenrod (Solidago sphacelata) treated with higher rates of Bonzi (240 ppm) or Sumagic (60 ppm) did not recover normal size at five months after planting into the landscape. Growth of velvet sage (Salvia leucantha) was excessively reduced by 45 or 60 ppm Sumagic in the greenhouse (Figure 5a). Furthermore, 60 ppm Sumagic caused a significant delay in landscape growth (Figure 5b). These compounds must be used carefully and appropriately. Especially when working with the triazoles, thoroughly test your application methods and rates on a small number of plants before treating your entire crop.
Avoid late applications of the triazoles. They should be applied prior to flower initiation when possible. The persistence of these compounds in plant stems and petioles can have significant effects on the flower display as well. As with many of the growth retardants, the triazoles inhibit gibberellin synthesis. Generally, the most rapidly elongating tissues have the highest production of gibberellins and, therefore, are most affected by reductions in gibberellin production. For example, elongation of the flower inflorescences of gaura (Gaura lindheimeri) was much more sensitive to growth inhibition than was the elongation of the stem tissues (Figure 6). Usually this is acceptable because it keeps the flower height in better proportion to the plant height. However, differences in response vary, and in some yarrow (Achillea) cultivars, flower height has been excessively reduced at moderate application rates.
Bonzi has a broad label for ornamentals that includes use on greenhouse or outdoor grown containerized crops. Bonzi also is labeled for application through the irrigation system or by subirrigation, including ebb/flow or flooded floor systems. Bonzi has been tested on a wide variety of perennials with species ranging from extremely sensitive to low rates to non-responsive to very high rates. To establish rates for plants not listed in Table 1 or on the product label, treat a small number of plants with 30 to 100 ppm. In many cases, multiple treatments with lower rates have been more effective, with less chance of overstunting, than a single application at a higher rate.
Sumagic also has a broad label for ornamentals, but its use is limited to containerized plants grown in greenhouses, overwintering structures, shade houses, or lath houses. It is not labeled for outdoor nursery use. At this time, Sumagic is not labeled for application through any irrigation system. Sumagic has been very effective on a large number of perennials. Test rates in the 15 to 45 ppm range. Since it is very potent, pay special attention to uniform application and proper volumes. Use caution in the higher rates or on more sensitive species since Sumagic can be very persistent in the landscape (Figure 5b).
Florel Brand Pistill (Florel) (ethephon, Monterey Chemical) is a compound that breaks down in plant tissue after application to release ethylene, a natural plant hormone. As with ethylene, its effects can vary depending upon the species and the stage of growth at time of application. It has a new broad use label (EPA Reg. No. 54705-8) for increasing lateral branching of floricultural crops. Florel also inhibits internode elongation of many plants; however, research with perennials has been limited (Table 1). Florel has controlled runner elongation of clump verbena (Verbena 'Homestead Purple'), and increased inflorescence numbers of sage (Salvia 'May Night') (Figure 7) and yarrow (Achillea). Florel should be applied to actively growing plants prior to flower development. If flowers are present, they are likely to abort. Florel may delay flowering about one to two weeks, particularly if applied close to time of flower initiation. Florel should not be applied to plants that are heat or drought stressed. The pH of the water used for the spray solution can be important. If the pH is too high, the ethephon will convert to ethylene before it gets to the plant and activity will be reduced. Florel contains sufficient acidifiers and buffers to maintain a pH of 5.0 or lower when mixed with most greenhouse water supplies. In general, water that has sufficient quality for irrigation of greenhouse crops (moderate pH and alkalinity) is suitable for mixing Florel. However, if you are acidifying your water prior to irrigation, use the acidified water for mixing the Florel as well. The solution should be applied within four hours of mixing.
Atrimmec (dikegulac sodium, PBI Gordon) is a compound that interferes with terminal growth by inhibiting deoxyribonucleic acid (DNA) synthesis which is required for new growth. By primarily inhibiting terminals, apical dominance is reduced which enhances the production of lateral branches. This mode of action tends to cause a delay in the resumption of plant growth that may add 2 to 4 weeks to production time. Atrimmec is labeled for use on containerized and landscape woody ornamentals, but also is labeled for use on some hanging basket plants and groundcovers. Of particular interest to perennials growers is its label for lantana (Lantana camara) (750 to 1500 ppm) and butterfly bush (Buddleia) (530 to 1500 ppm). Atrimmec should be applied to actively growing plants with at least two nodes to provide sufficient lateral development. In addition to creating a more full plant, enhancing the number of laterals in a pot generally reduces the overall height of the plant due to the greater distribution of resources. Responses are very species specific so test several rates under your growing conditions. Atrimmec usually causes leaf chlorosis which can be very persistent at higher rates (above 1500 ppm). Other phytotoxic responses, including malformed flowers, have been noted at higher rates on perennials such as gayfeather (Liatris spicata).
Return to Table of Contents
Return to Table of Contents
http://www.ces.ncsu.edu/depts/hort/floriculture/software/pgr.html
This Microsoft Excel spreadsheet allows you to enter your own PGR costs and calculate solutions based on the rate desired and the amount of area to be treated. The spreadsheet includes information on both spray and drench applications. It not only gives you the amount of PGR to mix per gallon or liter of water, but also provides the cost of the application based on the area or number of containers treated.
Updated Research Information. For regularly updated research information on using PGRs on perennials, visit a searchable database of the information in Table 1 on the Scranton-Gillette Communications/ Greenhouse Product News web site:
www.sgcpubs.com/onhort/index.cfm?fuseaction= showpgrsearchform
Return to Table of Contents
Weight
1 ounce (oz) = 28.3 grams (g)
1 pound (lb) = 16 oz = 454 g
Concentration
1% = 10,000 ppm
1 ppm = 1 milligram (mg) per liter
Return to Table of Contents
View this Publication in PDF format