It’s all in the name with plant growth regulators—they inhibit a plant hormone called gibberellic acid (GA), which is responsible for plant growth, especially the elongation of leaves, according to Aaron Hathaway, technical services manager, Nufarm.
“When you apply a PGR that inhibits the growth of GA, you tighten up that plant and cause it to be more compact, so it’s not stretching out those cells as much,” Hathaway says.
However, the plant still undergoes mitosis, so cells are still splitting, which means growth is still occurring, Hathway says.
“You're not shutting the plant down, but you actually allow the plant to stop stretching out those cells,” Hathaway says. “So instead of spending its resources and sugars on growth that we’re just going to mow off and discard, you save that energy and put it into other resources like more root mass, deeper roots, more antioxidants and more chlorophyll.”
Matt Giese, technical services representative for the Midwest, agrees.
“They don’t necessarily stop growth; they just regulate it in a different way,” Giese says. “It may prevent upright growth, but the lateral growth continues, so it continues to grow and function as it would normally, except it’s just a shorter version of itself.”
The upsides
While the turfgrass may benefit from PGRs by becoming greener, healthier plants with more root mass, homeowners or mowing crews can also stand to benefit as well.
Because the turf is exerting more energy in other areas and doesn’t grow as tall, it requires less mowing and results in fewer clippings that need to be bagged or cleaned up.
“If you’re a landscaper and you’re only mowing once a week, you have to get out a blower and pick up those clippings, but when you apply a PGR, you can reduce the number of mows, especially in the spring,” Hathaway says. “We have a healthier stand of grass, and we save all that time and energy we put into mowing off the extra growth.”
Giese adds in areas where for homeowners or landscape crews who can’t keep up with the mowing regularly during heavy growth, scalping—mowing the grass too short—and discoloration can occur.
“The PGRs provide an advantage if you can’t keep up with mowing because you won’t have events like scalping that can affect the plant growth as well as the aesthetics of the plant,” Giese says.
However, it’s important to note that it’s often not the LCO benefitting from fewer mows—it’s typically the homeowner or landscape crew performing the mowing.
“There is a disconnect between who would make those applications and whether it benefits those people in particular,” Hathaway says.
Best practices
Roadsides, curb and fence rows are areas that benefit from PGRs to help landscape pros cut down on trimming.
“In my own yard, I use PGRs around landscape beds as a trim and edge type of application—to me, that’s low-hanging fruit in reducing the amount of labor I have to do on a weekly basis,” Giese says.
Giese adds that PGRs may be particularly useful in springtime in Midwest areas with cool-season turf.
“For example, for Kentucky Bluegrass and tall fescue species, spring is conducive for growth, with cool temperatures and cool nights, so these species are growing vigorously,” Giese says.
As far as timing goes, Giese recommends applying PGRs on actively growing turf.
“Do it when you’ve mowed a few times and you’re getting clippings and the turf is coming out of dormancy, rather than during the heat of the summer when growth generally slows down (for cool-season grasses,” Giese says.
Hathway notes that LCOs can use PGRs in different ways.
“Heavy rates could really shut down growth, while lighter rates could reduce how much you have to clean up when you mow and increase the health and green color of the turf,” Hathaway says.
Giese says it’s important that LCOs check the label for application rates because it may vary by species.
“It’s important to know what species you have because that will determine what sorts of rates and maybe how often you're making those applications, whether it’s monthly, every six weeks or whatever the case would be,” Giese says.
While PGRs are relatively forgiving, Giese recommends starting at lower rates and working up from there.
“Start with 50 percent of a given rate and then work your way up over time until you get the effect you’re looking for,” Giese says. “Identify your true objective and determine what your rates are from there.”
It’s also important, Giese says, to properly calibrate equipment before applying PGRs.
“Whether you’re using a ride-on sprayer, a boom sprayer or even a handheld want, make sure you know how much you’re putting out,” Giese says.
What to keep in mind
On the flipside, there are a few items that LCOs should consider when applying PGRs.
For example, because PGRs cut down on vertical growth, if a disease or biotic stress comes along, the plant may not be able to grow out of that, Hathway says.
“I’ve seen instances in the fall with Kentucky Bluegrass with a heavy rate of PGR applied where it’s not able to grow out of that rust as quickly or grow new foliage,” Hathaway says.
Additionally, LCOs should keep in mind that PGRs don’t last forever.
“The warmer it is, the shorter PGRs last because it gets metabolized faster in the plant, so reapplications may be warranted, especially as you are farther South,” Hathaway says.
While LCOs can tout the healthy turf benefits of PGRs, Hathway adds that they should caution customers that the effects are not immediate.
“With a PGR, the effects are not as blatant as, for example, spraying herbicide on weeds,” Hathway says. “Customers may not notice visually the effect of what they just made for.”
Looking ahead
In the future, Giese says he envisions PGRs being used as a tool for water conservation efforts as well.
“In the West and Southwest, rainfall is minimal, and water conservation is a priority,” Giese says. “If you have plants under growth regulation, you tend to have smaller plants, and the smaller the plant, the less moisture loss they have through transpiration and the less moisture they need to grow.”
