Photo: Christopher J. Currey

Poinsettias are one of the most important floriculture crops grown. And, like nearly every other container-grown plant, poinsettias require growth regulation to produce appropriately sized plants for the containers they are grown in and to meet the target specifications. Using plant growth regulators (PGR) to suppress unwanted growth is common for poinsettia crops, but the same PGR strategies can’t be used throughout the entire crop. There are different “phases” for poinsettia growth control: 1) long days; 2) short days, no bract color; and 3) short days, with bract color. This article will provide insight into approaching PGR applications during each of these poinsettia production phases.

Long days and vegetative growth

The first phase in poinsettia production, the long days phase, is when plants are grown under non-inductive flowering conditions to keep them vegetative. This phase lasts from when cuttings are planted until short days are started to induce flowering. Ideally, this is also the time when most chemical growth control should occur. More specifically, for pinched crops, the period between pinching and the start of short days is the best opportunity to control plant size.

The earliest PGR application(s) made to a containerized poinsettia crop after planting is the “ethephon sandwich.” This strategy is primarily to encourage even branching on pinched poinsettia crops, since ethephon promotes axillary branch development. Foliar sprays containing 250 to 500 ppm ethephon are applied 3 to 7 days prior to pinching, then a second application of 250 to 500 ppm is made 3 to 7 days after pinching. While the ethephon does encourage lateral buds to “break,” ethephon does also have growth-retarding activity.

For growth control under long days, both foliar sprays and drenches can be used to suppress stem elongation. Foliar sprays are easy to apply, as it is a very familiar pesticide application technique. There are also a number of active ingredients which can be applied via foliar sprays, including chlormequat chloride (750 to 1500 ppm), daminozide (1000 to 2500 ppm), a tank mix of chlormequat and daminozide (previously mentioned rates), flurprimidol (8 to 30 ppm), paclobutrazol (10 to 25 ppm), and uniconazole (2.5 to 10 ppm) applied at a rate of 2 quarts of solution per 100 square feet of growing area.

While substrate drenches take more labor to apply to poinsettias, the duration of growth control can be longer compared to foliar sprays. Because of the longer duration of control, drenches should be applied earlier in long days, with 2 weeks after pinching being the most common time to drench with PGR solutions. Active ingredients including flurprimidol (0.10 to 0.25 ppm), paclobutrazol (0.50 to 0.75 ppm), and uniconazole (0.25 to 1.0 ppm) can all be applied. Be sure to adjust the volume of the PGR solution applied with the container size. For instance, apply 4 ounces of solution per 6-inch container or 8 ounces of solution per 8-inch container.

Short days, no bract color

Whether inducing poinsettias to flower under natural short days or black cloth, once short days begin, PGR strategies have to change. If poinsettias require growth control during short days and no bract coloration has started, foliar sprays of chlormequat chloride (750 to 1500 ppm) applied at 2 quarts per 100 square feet can be used. While there are other active ingredients previously mentioned (i.e. daminozide, paclobutrazol and uniconazole) that could still control growth, they can also negatively impact bract development, stunting their growth.

Short days, with color

In a perfect world, no growth regulation is required once poinsettia bracts start to color-up. But sometimes it is necessary. Some cultivars tend to exhibit late stretch at the end of the crop and will grow more than desired. Alternatively, closely spaced plants can reduce the transmission of red light, which increases the relative amount of far-red light and promotes stem elongation. Regardless of the cause, there are few options at this point. Once color has formed on the bracts, low-dose drenches of solutions containing ancymidol (1 to 3 ppm) or paclobutrazol (0.5 to 2 ppm) may be applied to help manage late-season stretching. As previously mentioned, the volume of solution applied to each container varies with container size.

Meeting target heights is important for any poinsettia crop. This article provides the framework for how to approach PGR strategies for all phases of poinsettia production. While recommended concentrations are provided here, these are approximate ranges for effective concentrations. As with any PGR recommendation, the cultivars being grown, geographic location of the facility, growing environment and plant culture all affect poinsettia growth, so conduct trials to determine which active ingredients and concentrations are most effective for you.

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