Just like a runner in the last 50 yards of a race, poinsettia growers also want to finish strong. Poinsettias with big, vibrant bracts, meeting their target height and flowering on time are the goals for every producer. There is no proverbial “silver bullet” to producing the best poinsettias, since there are several factors that will affect finished plant quality. This article is going to focus on some key points and considerations to ensure a high-quality poinsettia crop at the end of the season.
First, to finish on time, make sure poinsettias are being induced to flower. When trying to induce poinsettias to flower, light or heat during the night can cause problems. Since poinsettias flower under short days — with long nights — double-check for any light pollution that may interrupt flower induction. It only takes 2 µmol·m–2·d–1 or 10 foot candles for plants to sense “day” (and not “night”). In addition to light, warm temperatures (>75 °F) during darkness can result in heat delay. For early-season crops that require black cloth, this can be especially problematic. However, it can also happen for later-season crops without black cloth if plants are close to a heat source.
Once flowering is induced and bracts start to form, the goal is to finish crops with fully developed, vibrantly colored bracts, one of the hallmarks of a high-quality poinsettia. Warm temperatures, particularly at night, can cause bract coloration to appear dull or milky. Keeping night temperatures below 65 °F at night during short days can promote quality bract coloration. For the last few weeks of production, night temperatures can be dropped lower to between 55 and 60° F as long as crops are developing as desired. Do not drop the night temperatures if development is slow, as it will only exacerbate the problem. Avoid temperatures below 55° F, as that can promote root rot. Temperatures below 50° F can damage poinsettias.If bracts are not expanding as well as possible, whether due to cool growing temperatures or a potential over-application of growth retardants earlier in production, applying foliar sprays of benzyladenine (BA) and gibberellic acid (GA4+7) — sold as Fresco or Fascination — one to two weeks before cyathia shed pollen can help expand bracts. While applications made earlier, around one to three weeks after bract coloration has started, can also promote bract expansion, it can also cause unwanted stem elongation.
As the light intensity and temperatures decrease and humidity increases in the greenhouse during the fall, bract edge burn — the result of a calcium deficiency in bracts — becomes a potential problem. Try to avoid growing too cool, provide as much light as feasible, and vent excess humidity to promote transpiration and calcium uptake. However, preventative sprays of calcium chloride can help prevent bract edge burn from developing under conditions for its development.
While the goal should be to achieve all the growth regulation needed prior to the start of short days, some crops will require treatment during finishing. If plants are too short, applying the same BA+GA4+7 up to when bracts start to color-up can promote height. However, this can also delay coloration a bit, so only use this approach if a delay is acceptable. Otherwise, look to managing the average daily temperature (ADT) and difference between the day and night air temperatures (DIF) to promote height. A more common scenario is poinsettias exhibiting late stretch and requiring growth suppression as the canopy closes. Although daminozide (B-Nine, Dazide) and chlormequat chloride (Cycocel, Citadel) sprays are useful during long days for controlling vegetative poinsettias, use low-dose ancymidol (A-Rest, Abide) or paclobutrazol (Bonzi, Piccolo) drenches to control late stretch.
As long as poinsettias are appearing healthy and marketable, fertilizer can also be diminished during finishing to maintain healthy plants. Nitrogen concentrations can be cut around four weeks after short days. Since vegetative growth is complete by this point, cutting fertilizer by 25% to 50% will reduce the risk of damaging roots, and it can also help promote bract coloration. Nitrogen can be eliminated once cyathia are shedding pollen.
The take-home message
Each phase of poinsettia production — propagation, vegetative growth and flower development — has its own unique requirements to produce excellent finished crops. Hopefully those factors during flower induction highlighted in this article will help you and your poinsettia crop finish strong.