Purdue extension
HO-248-W Applying Plant Growth Retardants for Height Control
3
Ethephon
Unlike other PGRs, ethephon (Florel
®
) does not
inhibit GA production. Plants take up ethephon
through the leaves where it is converted to ethylene
in plant cells. The increased ethylene causes cells to
limit elongation and increase in width instead.
Ethephon’s mode of action can offer benets other
than height suppression. The release of ethylene
reduces apical dominance, which can increase
axillary branching. However, if the application is made
close to owering, the ethylene can result in ower
abortion and delayed owering.
Application Methods
Just as there are a variety of AIs to choose from,
there are also several methods for applying PGRs. In
addition to spraying and drenching (the most common
methods of application) PGRs can also be applied
by “sprenches” (high-volume sprays where the
solution is applied to both the foliage and the growing
substrate); liner dips; and bulb, tuber, and rhizome
soaks or dips.
Sprays
Foliar sprays are the most common PGR application
method. Many growers are attracted to the perceived
ease of applying foliar sprays on their crops.
However, there are several factors to consider when
applying PGRs in this manner.
First, the volume of solution to apply determines
how much active ingredient is applied to the crop.
Without careful consideration of application volume,
spray volume can vary considerably among different
personnel and can decrease uniformity among and
between crops.
Applicators often spray PGR solutions to “runoff,”
or until leaves look saturated. While this may be
adequate, a better strategy is to apply a specic
volume of the chemical over a specied area. Two
quarts of spray solution per 100 square feet is a
commonly recommended rate (although less may
be required for smaller plants, and more required for
larger plants, depending on coverage).
When an excess volume of PGR spray and runoff
reaches the substrate, this can result in additional
height control through substrate activity, depending
on the AI used. Intentionally doing this can be
an application strategy, which is referred to as
sprenching.
Other factors also inuence the effectiveness of
PGR sprays. The application environment can
affect PGR performance. For example, plants
take up ancymidol, urprimidol, paclobutrazol,
and uniconazole relatively quickly, but plants
require more time to take up chlormequat chloride,
daminozide, and ethephon.
To improve the effectiveness of these slow-uptake
AIs, it makes sense to apply them in an environment
that allows for longer drying times. There are several
ways to do this.
First, apply these products early in the morning, in
the evening, or on cloudy days.
Second, in the greenhouse, increase humidity
and turn off horizontal airow fans to minimize air
movement.
Third, avoid overhead irrigation before foliar PGR
applications have dried — the irrigation water can
wash off the AI and reduce its efcacy.
Finally, add an adjuvant or spreader-sticker to the
PGR solution to decrease surface tension and
increase application uniformity — always refer to
product labels before mixing anything; adjuvants and
spreader-stickers are not recommended with all AIs.
Drenches
Drenching (applying solutions to the growing
substrate) is another popular PGR application
method (Figure 2). However, not all PGRs are
effective as drenches.
Figure 1. ‘Durango Tangerine’ marigold plants and ‘Merisnow’ geranium
plants five weeks after being sprayed with Citadel
®
or Cycocel
®
(both
chlormequat chloride) at 750, 1,000, or 1,500 ppm or left untreated (control).