CHEMICAL UPDATE: PLANT GROWTH REGULATORS
Plant growth regulators, or PGRs, offer several useful benefits. People often think of them in terms of growth reduction, but this is just one of their functions. Innovative turf and landscape managers are using PGRs both to reduce maintenance and to improve plant and vigor.
PGRs certainly can reduce mowing requirements. However, other needs can be met with PGRs. For example:
PGRs can suppress turf growth during overseeding to give the new seedlings a quick start.
Some PGRs inhibit or suppress unsightly Poa annua seedheads.
PGRs can eliminate tall seedheads in roadside grasses (which, in some cases, might be the only reason to mow).
PGRs can differentially suppress turf growth, giving desirable turfgrass a competitive advantage over Poa annua.
Some PGRs can improve the overall health and vigor of their turf, resulting in better color, improved stress tolerance and perhaps better root development.
Ornamental PGRs provide some benefits comparable to those of turf PGRs, such as trimming reduction. PGRs also allow grounds care professionals to eliminate messy or dangerous fruit from trees, and suppress sucker and sprout growth. Certain ornamental PGRs can even improve root development, branching and overall vigor.
Are PGRs right for you?
Successfully integrating PGRs into your maintenance program requires you to carefully balance the cost of the application with the anticipated savings in maintenance. PGRs may not always be an economical choice, but they often are. Using PGRs to improve plant appearance or health, or to improve establishment during overseeding, may be more difficult to quantify in dollars and cents, and can be more of a judgment call based on agronomics and aesthetics.
The following tables are a general guide to PGRs available for turf and landscape ornamental use. Use them for preliminary planning only. They are not a substitute for label instructions, which you must read and follow whenever you use a PGR, as with any pesticide.
What do turf PGRs do?
Type I and Type II
Traditionally, turfgrass PGRs have been classified as Type I (mefluidide and maleic hydrazide) or Type II (flurprimidol, paclobutrazol and trinexapac ethyl). Type I PGRs inhibit cell division (mitosis). This reduces growth as well as preventing flower-/seedhead formation (assuming a timely application). Type II PGRs inhibit gibberellic acid production, thereby limiting growth that would occur through cell elongation. This cannot prevent seedhead formation, though a properly timed application can reduce visibility of the seedheads suppressing their growth.
Ethephon, gibberellic acid and indolebutyric acid fall outside the Type I/II classification scheme. Ethephon promotes generation of ethylene, which has a variety of effects on plants, including growth reduction. Gibberellic acid has the opposite effect of the Type II PGRs, and can improve color retention. Indolebutyric acid (present in one of the listed products) is known to be a root stimulant.
Some researchers use a different classification where PGRs are classified as Class A, B, C, D or E. In this scheme, trinexapac ethyl is a Class A PGR that stops the production of gibberellic acid late in the biosynthetic pathway, producing relatively specific effects. This contrasts with Class B PGRs (flurprimidol and paclobutrazol), which stop the production of GA early in the biosynthetic pathway. The distinction can be important because many forms of GA are produced in plants. However, GA1 is the form that contributes most to cell elongation. Stopping GA1 production, which occurs late in the biosynthetic pathway, defines Class A PGRs. Class B PGRs also stop GA1 production, but because they do so earlier in the pathway, they also stop production of other forms of GA that have a precursor in common with GA1. This may affect things such as turfgrass tolerances.
Class C PGRs (maleic hydrazide and mefluidide) are mitotic inhibitors that stop cell division. Class D PGRs include herbicides that are used as PGRs (not used on high maintenance turf). Ethephon could be classified as a Class E PGR.
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