One of the more common questions I get on using plant growth regulators (PGRs) is, “Which PGRs and what rates do I use on woody ornamentals?” And, I just don’t have a lot of my own research results to share on this topic. We have conducted PGR trials on what we call the crossover plants – like Buddleia, Caryopteris, hibiscus, ‘Knock Out’ roses and lantana, crops that are often grown in the greenhouse like perennials. And, most of these crops, like Buddleia ‘Lochinch’ are responsive to PGRs (Photo 1), especially to uniconazole (Concise, Fine Americas, Inc.) applied as a drench or with the higher foliar spray rates.
However, the questions I get are about using PGRs on commodity shrubs, like azalea, holly, or camellia. So, can we reduce pruning labor and its potential for disease spread by using PGRs to control growth on woody shrubs grown in the nursery?
Yes, we can!
However, these uses come with all the same considerations – crop, cultivar, plant age and stage of development, application methods and environmental conditions – as we have with using PGRs on other crops. With few universities conducting PGR research trials on woodies, there is little new information on which to base your own trials. However, there has been an amazing amount of research published in this area. So, to give you some starting test PGRs and rates, I have listed some research results from the literature on PGRs and rates that gave moderate growth regulation (Table 1). And, this is by no means an exhaustive list of crops or studies. If you want to conduct your own literature review, start with the Growth Regulation section of the Proceedings of the SNA Research Conference at http://www.sna.org/page-1052564.
Note: Most of these studies were conducted in the South. So adjust your rates accordingly. In addition, in many cases, the rate listed is MY assessment of the published data to give you a TEST rate. These rates are NOT RECOMMENDED for production application. TEST THEM FOR YOURSELF!
Table 1. Literature review of effective spray applications of plant growth retardants on woody ornamentals. (NR = growth not responsive)
|Crop||PGR||Spray rate (ppm)||Notes||University|
|Azalea, forcing||daminozide||1500 x 2||Georgia|
|Azalea, forcing||paclobutrazol||100 to 150 x 1||Auburn|
|Azalea||uniconazole||60 to 80 x 1||May need multiple apps||NC State|
|Azalea, Formosa||paclobutrazol||NR to 250 x 1||3x number of flowers||Florida|
|Azalea, Formosa||uniconazole||60 x 1||NC State|
|Camellia japonica||paclobutrazol||120 to 180 x 1||VaTech|
|Camellia japonica||uniconazole||45 x 1||3.5x number of flowers||VaTech|
|Euonymus||paclobutrazol||1000 x 1||Florida|
|Forsythia||uniconazole||50 x 1||NC State|
|Glossy abelia||uniconazole||100 x 1||NC State|
|Holly, China Girl||flurprimidol||1000 x 1||1500 ppm = 2 seasons of control||Auburn|
|Holly, compacta||paclobutrazol||500 x 1||Florida|
|Holly, dwarf Burford||paclobutrazol||500 x 1||Florida|
|Juniper||paclobutrazol||250 x 1||Florida|
|Pyracantha||uniconazole||100 x 1||May need multiple apps||NC State|
|Rhododendron||paclobutrazol||200 x 1||Drenches increased number of flowers||NC State|
Drench applications in the earlier literature indicated long persistence of growth regulation of containerized plants, and in some cases, in plants transplanted into the landscape. When applied at excessive rates, PGRs like uniconazole and paclobutrazol appear to persist in stem tissues for long periods of time affecting plant growth for more than one season. So, if you conduct your trials with drenches, be sure to keep some of the plants around until you see them grow out of the PGR treatment to assess the persistence of that PGR at the tested rates.
And remember to use good application techniques. If you are unfamiliar with these PGRs, take the time to peruse the previous blogs here on the Fine Americas, Inc. site for details and tips. That way you can make the most of your time investment and get the most reliable results. If you’ve seen me speak, you’ll recognize the photo below on the importance of uniform application volumes (Photo 2). Both of these ‘Forever and Ever’ hydrangeas were treated with 10 ppm uniconazole – with very different results due to poor application uniformity. Pay attention to the details!
Joyce Latimer is an Extension Specialist for Greenhouse Crops at Virginia Tech in Blacksburg, Virginia. Joyce has been evaluating plant growth regulators (PGRs) on herbaceous perennials for over 25 years. Her passion is improving plant quality and ease of production for producers while improving the quality of these plants for the home garden.