Fig. 1. These angelonia were untreated (left) or treated with drenches containing 100 ppm (middle) or 200 ppm ethephon (right). Ethephon suppressed stem elongation and, while flowering was unaffected when 100 ppm was applied, increasing the concentration to 200 ppm delayed flowering.
Photo: Christopher J. Currey

When we hear “PGR,” we often think about anti-gibberellin plant growth retardants that suppress stem elongation. Or, you may think of other plant growth regulators, like auxins which enhance adventitious rooting or cytokinins that improve branching. With all of these examples, we are usually talking about growth-regulating compounds that generally have one main effect on plant growth (suppressing elongation, stimulating rooting, enhancing branching, etc.). However, there is one growth regulator that can elicit a range of responses, depending on when and how it is applied — ethephon.

Ethephon is a unique active ingredient in that there are several uses for this one PGR, including height control, flower and bud abortion, enhancing, branching, promoting and fruit ripening (Fig. 1). All of these effects result from the ethylene that is generated after ethephon is applied to your crops. It is so important to understand all the aspects of applying ethephon to greenhouse crops because of these distinctly different effects. By the end of this article, you will know how you can use ethephon to improve the quality of your crops while avoiding or minimizing any unwanted side effects from this active ingredient.

Ethephon’s role in the greenhouse

First, ethephon is commonly used to inhibit unwanted stem elongation on containerized crops. However, unlike all the other growth-retarding active ingredients, ethephon does not inhibit gibberellic acid synthesis. Rather, the ethylene generated from ethephon causes plants to reorganize how their cells develop and, as a result, they elongate less when exposed to ethylene.

Another one of the most popular uses of ethephon is to keep plants from flowering (Fig. 2). Premature flowering can promote pests and diseases from the spent flowers on plants or require extra effort to keep plants sanitary. Ethephon applications are popular for large containers with long crop times, such as hanging baskets. However, it is also useful for plants that may flower early, or freely, and are grown in smaller containers (4.5-inch containers, for example), such as the numerous specialty annuals propagated from cuttings. Interestingly, there is one crop where ethylene actually stimulates flowering: bromeliads. Remember that this example is the exception, and not the rule.

In addition to controlling stem elongation and affecting flowering, ethephon also can enhance the branching on plants. When ethylene concentrations rise from the ethephon application, apical dominance is diminished and axillary buds develop into branches even without a mechanical pinch. Sometimes ethephon can be applied in addition to a pinch to increase uniformity in branching. Interestingly, both ethylene and cytokinin promote branching, as branching is also enhanced when benzyladenine (a cytokinin) is applied.

Finally, the ethylene released from ethephon can promote fruit ripening. The majority of ornamental greenhouse crops are marketed when flowers are blooming. However, crops like ornamental peppers are marketed when fruits are ripe. Ethephon can be applied to ornamental peppers to stimulate and enhance ripening, increasing the uniformity and rate of color development on fruits.

Fig. 2. Ethephon is frequently used to keep crops from flowering early. These vegetatively propagated geraniums we re-treated with ethephon and you can see the buds are turning yellow and senescing.
Photo: Christopher J. Currey

How to use ethephon

With the potential to affect height, branching, flowering and fruit development, you need to know when and how to apply ethephon to your crops. Let’s go sequentially through a production scheme, from propagation to post-harvest.

Ethephon can be used during cutting propagation to abort unwanted flowers on cuttings. However, if too much is applied the ethephon can have a negative impact on root development. Don’t apply too soon after sticking cuttings, and apply a lower volume of solution to minimize or prevent excess solution from running down the stem or being absorbed by substrate.

After propagation, the next applications are usually made once seedlings or cuttings have established in containers (approximately five to seven days for packs, 10 days for 4.5-inch, two weeks for 6-inch, depending on several factors). This is when applications to suppress elongation, promote branching, and prevent flowering should commence. Early applications can start improving plant size and architecture without delaying the marketability of your crop due to delayed flowering. For most plants, the later the ethephon application to flowering plants, the greater the potential for extending crop times. However, there is variation among species and cultivars; flowering of some plants is less-sensitive and may not exhibit as much or any delay. For promoting ripening, wait until fruits have begun to fill out; treating them too young can cause fruits to abort instead of ripen.

Christopher is an assistant professor of horticulture in the Department of Horticulture at Iowa State University.