Stunted growth

The use of plant growth regulators (PGRs) in suppressing seedheads has grown in popularity. The theory is seedhead suppression creates heartier plants due to the misdirection of energy away from seed production and into plant reserves.

There are other methods besides PGRs for suppressing seedheads, such as verticutting and topdressing. However, PGRs alone can be quite effective. The trick is application timing, which has to be precise.

According to a study conducted by Tim Murphy (University of Georgia), Ted Whitwell (Clemson University), L.B. McCarty (Clemson University) and Fred Yelverton (North Carolina State University) titled, “Plant Growth Regulators Used in Turfgrass Management,” PGRs, when used for seedhead suppression, must be applied before seedhead formation and emergence. Applications made after seedhead emergence will not be effective.

The study goes on to offer recommendations on timing for different turfgrasses. For Bahiagrass, for instance, mowing should occur as seedheads emerge – usually from May to mid-June. Then, PGRs should be applied about 10 to 14 days following mowing or just prior to new seedhead appearance. Additional applications six to eight weeks later may be required if new seedheads begin to emerge.

For creeping bentgrass golf greens, the study recommends applications be made during periods of active root growth – mid-fall and spring in the southern U.S. Applications should not be made during mid-summer and mid-winter months. On warm season turfgrasses, such as Bermudagrass, the appropriate PGR should be applied to actively-growing turfgrasses after full spring green-up and several mowings. Applications can be repeated during summer months if additional growth regulation is needed.

So what is the appropriate PGR to use? According to Todd Bunnell, golf market manager of SePRO, there are two types of PGRs: those that inhibit the overall growth of the plant, and those that are used predominantly for seedhead suppression or the “abortion of seedheads.”

“It comes down to the long-term objectives of the superintendent in certain areas, whether they be fairways or greens,” Bunnell says. “If they want to maintain Poa, they will likely use Primo and then Proxy or Embark to manage the seedheads as they go through the growing season. If they want to reduce the Poa population, then PGRs such as Cutlass are effective to do that over the long-term of one to three years.”

Many superintendents will apply Cutlass in a program over an entire year, Bunnell explains. At the onset of the Cutlass program on fairways, superintendents might have 50 percent Poa annua. If they apply Cutlass throughout the entire year, then they might only have 25 percent Poa the following spring. If they follow the same program the following year, they may go down to 10 percent Poa.

“If a superintendent can go from 50 percent Poa to 10 percent over two years in fairways, once they get to the 10 percent level they may not even need to apply Embark or Proxy. So a more long-term fix to reduce the Poa population eliminates having to use some of the Poa seedhead inhibitors.”

Ryerson was sort of a stranger to seedhead suppression as he had never had to do much about it because it had never been much of an issue. But that all changed during the last two to three years. He first used Primo Maxx, verticutting and topdressing. Last spring, however, he used Embark and, after calculating the GGD (growing degree days), applied 30 ounces per acre on May 12. He then made an additional application three weeks later on June 6 and says it worked pretty well.

“I think you need to verticut and topdress along with applying PGRs as the plants are quite aggressive at that time,” he says.

Gallagher, however, says that PGRs alone seem to be his solution to seedhead suppression.

“I experimented with other PGRs for two years to test new theories and products, but found that success was too dependent on perfect timing and cooperative weather,” says Gallagher, who counts Embark, Trimmit, Primo and Cutlass in his PGR arsenal. “With the right one, however, it could be all you need.”

Gallagher had also cited cost as an issue at one time, but the cost of PGRs is now about half what it was a few years ago due to the active ingredient Trinexapac-ethyl, the main component of Primo Maxx, recently coming off patent.

To get the timing just right, superintendents are encouraged to seek advice from their local product representatives and extension agents to figure out application dates based on degree days, historical data and physical observation.