Fig. 1. Note the size of the older internodes (middle and lower part of the stem); plant growth retardants are excellent at suppressing excessive early vegetative growth.
Photo: Christopher J. Currey

Poinsettia season is chugging along full-steam. Rooted cuttings for late crops are still being planted, while some early crops are already receiving black cloth treatment to induce flowering. While each poinsettia grower’s cultivars, schedule and container size may be unique to them, there is a universal challenge that nearly everybody will face — controlling excessive growth. Applying plant growth retardants (PGRs) are an effective and efficient method of controlling growth — but when should they be applied? This article is going to focus on the timing of poinsettia applications.

Applications for early growth and long days

Between the time rooted cuttings are planted (or when direct-stuck cuttings begin to root) until plants are pinched, there are few PGRs applied. The reason is, relatively little of the growth at this phase will contribute to the finished plant height, as the plant will be pinched and newer growth removed. The most common PGR application at this point is the “Florel Sandwich”; foliar sprays of solutions containing 250 to 500 ppm ethephon (Florel, Collate, Ethrel) are applied three to seven days prior to pinching, and again three to seven days after pinching. While this is primarily done to release apical dominance and stimulate branch development, ethephon also will suppress the height of the newly developing breaks or branches.

Once plants are pinched, but prior to flower initiation or the start of short days, is the best time to control growth, for several reasons. First, poinsettias begin to grow at their most rapid pace during this period, evident by the sigmoidal shape of poinsettia growth curves (some cultivars can be more vigorous during this time than others). Second, the growth during this time is vegetative (i.e. green leaves and stems), not reproductive (red bracts and cyathia). We want to suppress vegetative growth (Fig. 1), not flower development. During this time, there are a number of active ingredients that can be applied, including ancymidol (Abide, A-Rest), chlormequat chloride (Citadel, Cycocel), daminozide (B-Nine, Dazide), flurprimidol (Topflor), paclobutrazol (Bonzi, Paczol, Piccolo), and uniconazole (Concise, Sumagic), as well as the popular tank mix including both chlormequat chloride and daminozide. All of the aforementioned active ingredients can be applied as a foliar spray. Furthermore, save for chlormequat chloride and daminozide, the rest of these active ingredients can also be applied as drenches.

The pros and cons of different application methods (sprays vs. drenches) prior to flower induction is another subject that could take up several pages, but we won’t get into that here. Early drenches are an increasingly popular height control method for poinsettias. Applying drenches 10 to 14 days after pinching, when breaks are ¾ to 1 inch long, can provide good control when poinsettia growth is vigorous and the effects can last longer than sprays. Applying drenches too late can adversely affect bract development. Alternatively, sprays are easy to apply on an as-needed basis, since the effect is more immediate and the duration of growth control not quite as long compared to drenches. Because daminozide has been shown to affect bract development after flower initiation begins, it is best to stop applying this chemical around two weeks before the start of short days.

Fig. 2. While some plant growth retardants can be applied during the first several weeks of short days, avoid applications starting at first color until the last few weeks of production.
Photo: Christopher J. Currey

Application during short days

Once short days start, whether natural or from black cloth, PGR options are reduced. As already alluded to, some active ingredients and application methods can adversely affect flower development by inhibiting bract expansion. All PGRs except ethephon inhibit the synthesis of gibberellic acid (GA), the plant hormone that promotes cell elongation. Because the red bracts are the attraction with poinsettias we want to avoid suppressing their expansion. While daminozide, flurprimidol, and uniconazole should not be applied during flower induction, other active ingredients such as ancymidol, chlormequat chloride, and paclobutrazol are still able to be applied during the first three to four weeks after the start of flower initiation prior to first color on bracts. Once color starts to develop, avoid applying PGRs until bracts are more fully developed during the last few weeks of production (Fig. 2).

Finishing strong

At the end of production, when bracts are expanding and coloring up and plants are approaching their target final height, some poinsettia cultivars have the tendency to elongate excessively in the weeks just before they are ready to sell. This late stretch is undesirable because it can result in plants getting taller than your target height, exceeding product specifications. Late-season applications of paclobutrazol applied as a drench can help control this late stretching for poinsettia crops.

While there are nuances to when PGRs should be applied to poinsettias, we should take advantage of the fact that there are options to treat plants with PGRs throughout the entire production cycle. This will allow you to successfully integrate PGRs into the production for the cultivars and container sizes you are producing. While specific concentrations are going to depend on your schedule, cultivars, container size, growing environment, and cultural practices, look to labels for suggested concentrations to try.

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