Pillars of PGR success

There are a lot of questions that can surround plant growth regulator (PGR) applications. When you find yourself considering the “what, where and when” for PGR applications, a little bit of knowledge will allow you to determine “that, there and then.” From seed to sale, let’s cover the fundamental considerations.

Everything begins with genetics, and it is important to know your plants and know them well. While some crops can jump out of their pots, others are barely crawling across the finish line in time. Plant size and vigor can vary widely among floriculture species and cultivars. Within any given species in commercial production, you will likely find variation in size across different cultivars. A consequence of varying growth and vigor is different degrees of growth regulation are required. For some compact annual cultivars, no growth regulation may be needed. Alternatively, vigorous or tall varieties will need a more aggressive growth control strategy.  

The degree of desired growth control varies depending on the crop and stage of production. Most PGR applications can be classified as one of three types: slow, hold or stop.

Slow and hold applications have the same end goal: to temporarily diminish stem elongation. The difference is when they are applied. Hold applications are made at the end of the crop cycle for plants that still have more growing ahead of them, such as annual or perennial bedding plants. Once plants reach the desired size at the end of production, you want to apply PGRs to hold the plant size until plants are in consumers’ gardens or on their decks, after which time you want growth to resume. Similarly, slow applications are meant to temporarily slow down growth without stopping a crop dead in its tracks. These slow applications are made in the early and midway points of crop production.

Finally, stop applications are made to completely halt the growth of crops that are not expected (nor desired) to change size much for the consumer. This would be primarily for flowering potted plants destined for interiors and placed on counters and coffee tables for a few weeks.   

Application methods

Not all application methods are created equally, but that is a good thing. And there are enough options that growers can select the most appropriate method for them and their crops. Foliar sprays are the most common application method because we are already used to applying sprays containing insecticides or fungicides.

But just because sprays are common doesn’t mean they are easy to perfect. Applicator experience and environmental conditions during the application affect the efficacy and consistency of foliar sprays.

Alternatively, using PGR drenches can produce more consistent results, since a defined volume is being applied to every container and the application environment really doesn’t affect treatments. However, they are more labor intensive to apply.

Plug/liner dips and bulb soaks are very efficient application methods with respect to conserving labor and chemical use, but plant growth regulators’ active ingredients have different degrees of activity. For example, 5 ppm can be a good concentration for uniconazole sprays, whereas 5,000 ppm can be good for daminozide sprays. Ethephon, chlormequat chloride and daminozide are unique and unrelated to other compounds, and recommended concentrations are 250 to 1,000 ppm, 750 to 1,500 ppm and 1,250 to 5,000 ppm, respectively.

However, trying to remember the differences between the “triazoles” — ancymidol, flurprimidol, paclobutrazol and uniconazole — can be more challenging. One way to remember the approximate relative differences among these related chemicals is to think relative to the concentrations of the most widely used triazole (and PGR overall): paclobutrazol. Uniconazole is twice as strong as paclobutrazol, so concentrations can be cut in half. Alternatively, ancymidol is half as strong, so concentrations can be doubled.

Flurprimidol is a little different. As a foliar spray, flurprimidol has activity comparable to paclobutrazol, while greater root activity makes it like uniconazole when drenching. Please keep in mind these are approximate relationships.   

Other influences

Is your crystal ball working? Because mine isn’t. Plant growth is heavily influenced by light and temperature. Throughout the year, the average daily temperature (ADT), difference between the day and night temperature (DIF), daylength (photoperiod) and photosynthetic daily light integral all change. And, except for daylength, the variation is not as predictable as we’d like it to be.

Diligent and judicious monitoring of the greenhouse environmental conditions will let you know exactly what is happening in your greenhouse and allow you to better predict plant growth responses and, subsequently, PGR requirements.

Just like crop scheduling, doing your homework can pay off. Making decisions in haste or under duress can cause less-than-ideal results, and you want to minimize the mistakes during your busiest and most profitable time of year.