ATA TURF TIMES: Paul Bartley, Ph.D., Assistant Professor, Department of Horticulture, Auburn University

Contributing: Jeff Atkinson, Ph.D., Harrell’s and Alex Hedgepath, Auburn University

Since discovering and understanding plant hormones, plant growth regulators (PGRs), both naturally and synthetically derived, have been used to suppress or promote plant growth. Their applications have varied from industry to industry to address varying critical issues. In the 1950s, early adopters like the grape and apple industries primarily used PGRs for blossom and fruit thinning to improve marketable yields. The floriculture industry has used PGRs extensively for half a century to meet the plant specifications of their picky clientele (I’m looking in the mirror here). In the 1970s, utility arborists began applying PGRs commercially to combat the age-old problem of trimming trees away from utility lines. But for the professional landscape operator, PGRs haven’t become mainstream. Instead, their utilization has been more niche.

Take Auburn University’s campus for example. With over 700 acres of landscape to maintain, the use of PGRs is primarily limited to maintenance hazards. “We’d like to start using more PGRs on campus, but right now, we are only using them around fences and barriers that are difficult to mow around,” said Wes Miller from Auburn University Landscape Services. So, why aren’t PGRs more widely used in landscape management? The research convincingly favors their applications regarding enhanced blooming, improved plant health, and less frequent pruning. Like many changes to common practice, market penetration and adoption rates are primarily driven by economics. That is exactly what the research could not demonstrate convincingly. At least, not 10-20 years ago when most of the work was conducted. Things have changed. Today, the value-added proposition is less about economics and more about labor conservation.

With an evaporating labor pool and demand for services at an all-time high, landscapers like Toby Hughes with Auburn Grounds Inc. have begun incorporating PGR products into their management practices. For them, it wasn’t as much about economics as it was about managing labor reallocation and boosting morale. “Our guys hate pruning. It was easy to convince them to spray PGRs instead of picking up the hedge trimmers.” Others, like Dennis Pritchett from JubileeScapes think it’s a “win-win, a game-changer. Our customers receive a healthier plant, and we save three or four crews a summer of pruning. The manufacturers are doing a better job educating and that’s helping us communicate the advantages to our clients.” The manufacturers are noting the change as well. “I’ve had about twice as many requests to present on landscape PGRs this year compared to the last three or four years prior,” Dr. Kyle Briscoe, Senior Technical Development Manager with SePRO, remarked. He added, “I think the labor market is driving a lot of first-time use.”

For those looking for an extra hand, it’s essential to understand the various PGR formulations available in the market and how they might be best utilized.

Granular

It’s challenging to beat the simplicity of a granular application. Measure the application area, weigh the granules, and evenly distribute them across the treatment area. Perhaps this simplicity and our familiarity with granules often lead to misapplications and varying results. You understand if you’ve ever striped a yard following a granular fertilizer application. Timing is critical with this formulation. Depending on several factors (species, plant size, and environmental conditions), granular applied PGRs need time for release, uptake, and translocation to the meristematic regions of the plant. As a general suggestion, these products are best used immediately after pruning.

Product Tradename
Active Ingredient
Applications

Cutless 0.33G
Flurprimidol
Herbaceous ornamentals, groundcovers, and woody shrubs

Cutless Shrub and Tree Sculptor
Flurprimidol & Paclobutrazol
Shrubs and trees 4-12’ in height

 

Foliar Sprays

Does calibration keep you up at night? Rest well knowing that concentration with liquid foliar applications of PGRs is most important. Relative to granular products, foliar applications are fast acting. For these reasons, foliar applications have quickly become the preferred method of PGR application in the landscape. Shortly after a flush of new growth, mix the appropriate concentration and apply evenly until drip occurs. Leaf cuticle thickness, plant vigor, and environmental factors may influence the longevity of regulation. For the savvy landscaper, a combination of granular and foliar spray products may extend the effectiveness of PGRs. As always, follow the label to avoid exceeding application thresholds.

Product Tradename
Active Ingredient
Applications

Atrimmec
Dikegulac-sodium
Shrubs and groundcovers, flower and fruit suppression

Cutless QuickStop
Flurprimidol
Herbaceous perennials, groundcovers, and shrubs

Bonzi
Paclobutrazol
Herbaceous bedding plants and shrubs

Anuew
Prohexadione calcium
Warm and cool season grasses

Trimmit 2SC
Paclobutrazol
Warm and cool season grasses

Primo MAXX
Trinexapac-ethyl
Warm and cool season grasses

Edgeless
Flurprimidol

Trinexapac-ethyl
Banded applications to warm and cool season grasses

 

Soil Injection

Trees are often the most significant investment and liability in the landscape. Whether they are century-old oaks, newly planted ‘Princeton’ elms, or established sugar maples in the middle of a construction zone, arborists and urban forestry specialists utilize PGRs to minimize tree stress. As it’s often impractical to apply a foliar spray, the preferred application method for trees and large shrubs is direct soil injection or drenching around the trunk’s basal flare. Like granular applications, these products will take time to translocate to the meristematic regions of the stems. As always, read the label carefully. The dosage requirements for trees vary by species sensitivity and size.

Product Tradename
Active Ingredient
Applications

Shortstop 2SC
Paclobutrazol
Soil injection and basal drench of trees and shrubs

Cambistat
Paclobutrazol
Soil injection and basal drench of trees and shrubs

Profile 2SC
Paclobutrazol
Soil injection and basal drench of trees and shrubs

I was told Burton Sperber, founder and “Head Gardener” of ValleyCrest Companies, once said, “The economy can slow, but the grass always grows.” If the grass…or woody ornamentals…are growing a little too quickly for you, consider the new value proposition PGRs have to offer.

*The products mentioned in this article are only examples of PGR products, not an exhaustive list or an endorsement of any product*

Author: Paul Bartley, Ph.D. has been an Assistant Professor, Department of Horticulture at Auburn University since 2019. His appointment is 60% Research and 40% Teaching. He received his B.S. and Master of Science at Auburn University and his Doctor of Philosophy in Horticulture Science from North Carolina State University. He can be reached by email at paul.bartley@auburn.edu

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