This is the third article in a three-part series on hydrangea production that addresses effective use of plant growth regulators. Part one (August 2017) covered mineral nutrition. Part two (September 2017) focused on flowering and bluing of H. macrophylla.
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Hydrangeas are the No. 2 selling deciduous flowering shrubs in the U.S. In addition to having them in bloom with the intended floral color for your market, as discussed in the previous two articles in this series, what can make this crop even more profitable is growing it faster. For many shrubs, gone are the days of having a 3-gallon crop in production for more than a season. With larger liners, and refined fertilization and irrigation practices, we are turning crops faster than ever. To keep pace with your competition, consider that plant growth regulators (PGRs) can help increase the number and size of blooms, accelerate bloom time compared to pruning, and can help develop a finished crop while controlling size and quality to aid with requirements for both rack shipping and retail display – all key to shortening your production cycle while maintaining the quality and blooms your customers demand. Pruning can’t do that.

What are PGRs?

PGRs are compounds that can be sprayed, drenched or sprenched on plants to ellicit a desired growth effect. They include synthetic compounds that mimic naturally produced plant growth hormones, as well as chemical compounds that do not normally occur in nature. Most PGRs used on woody ornamentals to induce branching can be divided into two groups: branch inducers or chemical pinchers. Branch inducers include benzyladenine, a synthetically derived version of the plant growth hormone cytokinin, which stimulates branching, and, along with auxin, helps plants balance shoot and root growth. Chemical pinchers include those products that chemically “remove” (damage) the branch tips, eliciting a response similar to pruning. Other PGRs used on H. macrophylla during production, e.g., gibberellic acid inhibitors, function as growth retardants by inhibiting stem elongation.

While the industry commonly relies on pruning as a way to stimulate branching and control growth, stem removal creates several complications in the production of hydrangeas, as well as other flowering shrubs. For example, pruning can remove tissue containing flower buds and therefore delay flowering, while increasing the time needed after pruning for plants to regain a natural appearance. An additional complication is that pruning does not always stimulate branching. Sometimes a branch is pruned and a single branch replaces it – there is no increase in branch number. Numerous studies with hydrangea at the University of Tennessee, Knoxville have shown a pruning response similar to water sprayed “controls,” nontreated plants. In comparison, some PGRs cause a two- to three-times increase in branching and flowering on hydrangea. In a production setting, pruning can be used to control height and often does stimulate branching. But if stimulating branching can be done in a way that preserves flowering tissue and eliminates delays in flowering and recovery time, that is an improvement and a big step toward greater efficiency.

Success with PGRs

Using PGRs is not without its own challenges. Preparing and applying PGRs is not a task for new or otherwise inexperienced employees. It is critical to measure PGRs accurately. Some PGRs are highly concentrated and ¼ teaspoon too much or too little can make a difference in plant response. Needle-less syringes create the ability to accurately measure to the milliliter and are far superior to kitchen measuring spoons.

In addition to accurately measuring and mixing the product, growers must apply PGRs accurately and consistently. Knowing whether the PGR or combination of PGRs you have tank mixed have root, shoot, or leaf activity is critical. This affects the dose.

Some experts, including perennial flower PGR guru, Dr. Joyce Latimer at Virginia Tech, advocate making a couple, lower-dose applications versus a single more concentrated application. This allows for mistakes in mixing and reapplication, if conditions warrant it, whereas once a strongly mixed or heavy volume application is made, a crop may never recover or may not recover within a marketable window.

Generally, overall growth is slower following PGR application, so time applications when growth is about to occur, and definitely before plants are larger than desired for the application to have the maximum benefit. PGRs can’t make a plant smaller than it already is.

Plant species, crop stage, intended outcome, and time to sale dictate the type of PGR and the appropriate rate, while mixing and application (accurately diluting, uniformly spraying each plant in a crop), as well as environmental conditions during and following application influence PGR activity. Conditions that allow the leaf to remain wet with the product for a long duration following application, as opposed to those that cause it to evaporate quickly, will maximize absorption. Irrigation may need to be delayed following application of certain PGRs to protect absorption and activity. Likewise, if your nursery has locations in multiple areas, a rate that works for your more northern location may need to be applied more than once a season at your more southern location. In 2015, a relatively mild growing season, there were about five flowers per 3-gallon H. paniculata ‘Phantom’ when 800 ppm of dikegulac sodium was applied. In 2016, an abnormally hot season, double the volume was needed to achieve five flowers per plant at that same rate. It is important to experiment first with a small group of plants and compare to your standard pruning practices to determine where PGRs can add efficiency within your production system and to determine if phytotoxicity occurs, including when the crop regains a normal appearance. Do not assume that all cultivars of a species will respond similarly. Likewise, new cultivars of hydrangea may be complex hybrids with several species. Be sure to conduct your own trials on the selections that you produce.

For H. paniculata ‘Limelight,’ the treatment is listed followed by the number of branches in parenthesis (from left to right): pruned (30.4), dikegulac sodium at 800 ppm and pruned (55.1), water control (33.6), dikegulac sodium at 800 ppm (46.1). Pruning in addition to dikegulac sodium application didn’t significantly increase branch number compared to just applying dikegulac sodium and, like pruning alone, prevented flowering.
Photo by Diana Cochran

Considerations

Use of PGRs, like pesticides, involves some risk. For example, daminozide is considered a potential human carcinogen by the U.S. Environmental Protection Agency. Experienced, well trained employees must follow all label reccomendations, including wearing the specified personal protective equipment and respect the re-entry interval for each product.

Labels and availability of plant growth regulators change periodically based on patents and volume of use by growers. For example, cyclanilide and dikegulac sodium, two of the most active PGRs for hydrangeas in nursery production, may not be available at the time of press. However, Jim Goodrich of BPI Gordon indicated they plan to have nursery application for dikegulac sodium on the Atrimmec label soon. The pursuit of these and other PGRs for nursery production is possible if companies recognize the potential of the market and support their use for ornamentals.

Table 1. Effect of plant growth regulator application, rate, or pruning on growth and flowering characteristics of select hydrangea.
H. paniculata ‘Phantom’ (from left to right): water control, pruned, dikegulac sodium at 800 ppm, two benzyladenine applications 4 weeks apart (900 ppm), and flurprimidol (200 ppm). Dikegulac sodium treated plants had fewer but smaller flowers and 3x more branches than any other PGR treatment. Branch number of dikegulac sodium treated plants was equivalent to pruned plants but with no reduction in flower number.
Photo by Quinn Cypher

PGR recommendations for container-grown hydrangeas

Branch number of H. paniculata ‘Limelight’ or Little Lime in 3-gal containers was similar whether plants were pruned or not, and pruned plants generally had fewer and smaller flowers than PGR-treated plants or simply doing nothing (Table 1). This highlights a need for an alternative to pruning to stimulate branch development for this species. In fact, pruning Little Lime reduced flower number by at least 78% compared with PGR treatments: benzyladenine (300 or 600 ppm), ethephon (500 or 1,000 ppm), or dikegulac sodium (800 and 1,600 ppm). Dikegulac sodium (either rate) was the only treatment that increased branch number and plant quality without reducing the floral display of Little Lime. Time to first flower was not recorded for ‘Limelight’ or Little Lime. In limited trials on H. paniculata ‘Phantom,’ time to first flower for 3-gallon plants was slightly accelerated by dikegulac sodium application, however pruned plants required five to six weeks to achieve the same level of flowering as those treated with dikegulac sodium, benzyladenine, flurprimidol, or nontreated plants.

H. quercifolia can be lanky and asymmetrical during container production, and has been reported as susceptible to dieback after shearing in Oklahoma, especially during wet conditions. Therefore, using PGRs might control plant size while avoiding dieback. In experiments testing market potential, two foliar applications of cyclanilide 4 weeks apart at 100 ppm increased branch number compared to nonpruned plants. Unfortunately, ancymidol (25, 50 or 100 ppm foliar application and 1, 2, or 4 ppm drench application) or uniconazole (12.5, 25, or 50 ppm foliar application and 1, 2, or 4 ppm drench application) applied to foliage of ‘Pee Wee’ or ‘Alice’ oakleaf hydrangea or substrate did not reduce overall size compared to pruned plants at 12 to 16 weeks after treatment. Cyclanilide was not brought to market as a product for controlling growth in ornamentals, but these results indicate its affect on reducing labor and height without pruning.

To control growth of H. arborescens Invincibelle Spirit, Spring Meadow Nursery recommends daminozide at 5,000-7,000 ppm during production. Their recommendations include a minimum of three foliar applications approximately 10-20 days apart be applied beginning when the plants are small. Drenches are not recommended. Plants that are already at the market-dictated height are not good candidates for PGRs. Pruning is not advised before application. The following visual indicators can be used to assess activity and application timing: when the leaves in the apical meristem begin to point upward, the plants are growing out of the application. When the leaves lay flat, the PGR is active.

Care should be taken when using PGRs as residual effects, called habituation, can occur on florist and landscape hydrangeas the following season. Doug Bailey and Bernadette Clark, while at North Carolina State University, found that summer PGR applications the year before spring forcing had reduced flower size and plant height of H. macrophylla ‘Böttstein.’ However, a residual effect can be a value added asset. H. paniculata ‘Phantom’ treated with heavy applications of dikegulac sodium at the labeled rate of 800 ppm had more flowers (approximately double) the season following application.

While PGRs are routinely applied in the production of florist hydrangeas, results can vary and, yet, pruning may not be a better solution, especially where flower development is affected. Applications of dikegulac sodium caused phytotoxicity initially but increased the number of branches and flowers of container grown H. macrophylla ‘Merritt’s Supreme.’ Dikegulac sodium plus pinching increased branching at 400 ppm in one location of the study while 800 ppm plus pruning increased the number of flowers compared with unpinched controls at both locations. Pruning plants was only successful at increasing branch number compared with untreated controls in one location of the study. Dikegulac sodium (800 and 1,600 ppm) did not significantly increase branching compared to untreated H. macrophylla ‘Nikko Blue.’ Benzyladenine (300 ppm and 600 ppm) and ethephon (500 and 1,000 ppm) were not effective at inducing branching on either ‘Merritt’s Supreme’ or ‘Nikko Blue.’

PGRS can help increase the number and size of blooms, accelerate bloom time compared to pruning, and can help develop a finished crop while controlling size and quality.
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Irrigation: the original PGR

Available water in the substrate and subsequent plant water stress influence plant growth; thus, growers can control plant growth by managing water effectively. Too little water and plants are stunted, undersized, and unsaleable. Too much water and plants are lanky, require pruning and may flop during shipping. By managing irrigation to approach water stress but not allow the substrate to get too dry, growers can produce a compact plant that finishes on schedule. Nurseries using this strategy have reported no negative effects on flowering. This is accomplished with greatest success in controlled environments, such as a greenhouse, when producing or forcing hydrangeas; however, this management tool can also be successfully accomplished in outdoor production. Producing compact plants with less pruning or no PGRs can be accomplished at your nursery by irrigation scheduling, including use of new technologies such as substrate moisture sensors to “grow dry.”

Dr. Marc van Iersel of the University of Georgia, achieved growth control of H. macrophylla ‘Lady in Red’ by growing compact, saleable plants at 14 percent substrate water content (a relatively low moisture level which may not be suitable for other crops or other substrates). More recently, Amy Fulcher of the University of Tennessee, in collaboration with Holden Nursery, produced H. quercifolia ‘Snow Queen’ oakleaf hydrangea using an automated leachate-based irrigation system in which water leaching from the bottom of the container was monitored and used to dictate irrigation runtime, targeting a 15 percent leaching fraction. This method ensures plants receive the appropriate water volume each day without overwatering. Plant growth was controlled and plant quality met or exceeded the nursery standard without the use of PGRs. Dr. Fulcher’s lab also grew ‘Alice’ oakleaf hydrangea using either a daily water replacement irrigation schedule (replacing water lost from evapotranspiration each day) or an on-demand irrigation schedule (irrigating when the container dried to a predetermined water content monitored by sensors). Both systems adequately controlled growth and reduced water use ~60 percent without the use of PGRs. Researchers led by Tom Fernandez at Michigan State University used partial daily water replacement by replacing 100 percent of daily water used on day one, but only 75 percent on days two and three to produce H. paniculata ‘Limelight’ without use of PGRs. When compared to conventional production practices, these instances used less water and labor while improving resource use efficiency. That is smart growing. Jim Owen and former graduate student Jeb Fields of Virginia Tech, found that incorporating sphagnum peat moss or coir into pine bark increased the available water found at low water contents (the water buffering capacity) compared to 100 percent pine bark, providing insurance the plant will not undergo detrimental water stress when “growing dry.” These substrate amendments make available water easier to obtain for roots during the driest period between irrigation events, thus growers can grow plants drier without the crop experiencing a severe water stress. They produced marketable, more compact plants of H. arborescens ‘Annabelle’ grown in substrates containing peat or coir compared to plants in unamended pine bark grown at the same low water content.

Hydrangeas are beautiful landscape plants that hold lasting memories across many generations. Use this three-part series to provide correct nutrients in a timely fashion during production; create many large flowers in the desired colors; and manage growth using either PGRs, irrigation, or both to produce marketable plants at the time of sale.

Dr. Anthony LeBude (avlebude@ncsu.edu) is an Associate Professor of Horticultural Science at NC State University located at the Mountain Horticultural Crops Research and Extension Center, Mills River, NC. Dr. Jim Owen (jsowen@vt.edu) is an Associate Professor of Horticulture at Virginia Tech located at the Hampton Roads Agricultural Research and Extension Center in Virginia Beach, VA. Dr. Amy Fulcher (afulcher@utk.edu) is an Associate Professor in the Department of Plant Sciences at the University of Tennessee, Knoxville, TN. Acknowledgements: The IR-4 Projects, http://ir4.rutgers.edu, funded much of the PGR work described in this article through their former PGR program.