Some things in life are inevitable. In addition to death and taxes, we can also count on the need to control containerized crop growth. It is time to start planning plant growth regulator (PGR) applications for annuals and perennials. When it comes to PGRs, there are a number of factors that will affect success. These include the type of plant material you are treating, how much control is needed, how you will apply PGRs, which active ingredient you are using and what your growing environment is like. In this article, I’d like to take time to review these factors and how you can plan for success.
1. Know your genetics
Not all plants are created equal (Fig. 1). Some species are vigorous and jump out of the pot if not controlled, whereas it is a challenge to get other species sized-up by the sale date. Plant size and vigor can vary widely among species and cultivars. For example, poinsettia and potted chrysanthemum cultivars may be classified as short, medium or tall. Annual bedding plants such as petunia can be compact and suited for mixed containers or larger, more vigorous and appropriate for landscape plantings. A consequence of varying growth and vigor is different degrees of growth regulation required. For some compact annual cultivars no growth regulation may be needed. Alternatively, for vigorous or tall varieties, an aggressive growth control strategy may be required.
2. Know your goal
When regulating the size of greenhouse crops, the goals for each application are not the same. For instance, compare early and late PGR drenches on poinsettia. Early drenches are intended to prevent excessive stretch toward the beginning of poinsettia production. Growers do not want to stop or halt poinsettia growth altogether with early drenches; they only want to minimize or slow down stem elongation. Alternatively, late-season PGR drenches on poinsettia are intended to stop late stretch and stem elongation altogether. Most PGR applications can be classified as one of three types: 1) slow; 2) hold; or 3) 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 cycle for plants that still have 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 the consumer’s garden, 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. “Slow” applications are made early and midway through 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 primarily be for flowering potted plants, which are destined to go into homes and be placed on countertops and coffee tables for a few weeks.
3. Know your application method
Not all application methods are equal and producers should select the most appropriate method for them. Foliar spray applications are the most common method employed because growers are already used to applying sprays in the greenhouse. Applicator experience and environmental conditions during application affect efficacy and consistency of foliar sprays. Alternatively, using PGR drenches can produce consistent results because a specific volume is applied to every container and the application environment does not have as great an effect on treatments. However, drenches may take more labor to apply depending on how they are delivered. Plug/liner dips and bulb soaks are application methods that are efficient for treating a large number of plants easily, but these applications are more specialized than sprays and drenches and may not be options for your crops.
4. Know your chemicals
It is worth remembering that all PGR active ingredients do not have the same degree of activity. Some of the differences in active ingredients may be inferred by recommended concentrations. For example, 5 ppm is a reasonable concentration for uniconazole and 5,000 ppm is common for daminozide; the recommended concentrations clearly indicate differences in activity. While chlormequat chloride, daminozide, and ethephon are completely unique and unrelated to other compounds, ancymidol, flurprimidol, paclobutrazol, and uniconazole are grouped together due similarities in chemistry. One very simple way that I remember the relative differences among these chemicals is concentration relative to paclobutrazol concentrations: uniconazole is twice as strong (so concentrations would be cut in half) and ancymidol is half as strong (concentrations would be double). Flurprimidol is a little different. For foliar sprays, flurprimidol has activity comparable to paclobutrazol (Fig. 2), whereas as a drench it is more active and may be similar to uniconazole. Remember these are very rough rules of thumb to keep the activity of similar PGRs straight.
Select active ingredients that are effective on your species. If a few different active ingredients are effective, select the chemical that works best. For growers new to PGRs, more forgiving chemicals such as daminozide and chlormequat chloride may be useful, while the triazoles (flurprimidol, paclobutrazol and uniconazole) may be well-suited for applicators with experience.
5. Know your environment
Nobody can know what the future holds. That being said, diligent greenhouse growers who are constantly measuring and monitoring the air temperature and light levels in the greenhouse can more accurately understand the plant’s growing environment. Plant growth and development are strongly influenced by light and temperature. Are you seeing your light intensity and air temperature during the day increase? This may be resulting in a positive difference between the day and night temperature (DIF) and, therefore, promoting greater stem elongation. Alternatively, as the late summer turns into fall and winter, we can frequently measure a reduction in DIF and light intensity which would not have a promoting effect on plant growth. However, I have experienced some Novembers that are as warm and bright as September, as well as some Octobers that will make you think it is already winter. Only diligent monitoring will help you know what your greenhouse environment has been and how it is changing. That is going to help you adjust your growth regulation as needed.
Planning for success
Doing your homework early on can pay off. Making decisions in haste can cause less-than-ideal results, and you want to minimize mistakes during your busiest and most profitable time of year.