Strategic Temperature Management for Poinsettias

July 14, 2008 - 10:40

Trials at Clemson University show great promise for successful cold production of many poinsettia varieties.

As fuel prices soar this summer, growers continue to look for options to reduce energy inputs. Poinsettias offer real potential energy savings if finished cooler than normal in October and November, but this can be risky. In 2007, we conducted experiments at Clemson University in South Carolina to identify how cool finishing temperatures affect poinsettia height, bract development and timing.

Because South Carolina can be relatively warm in September and October, we provided night-interruption lighting on our crop until Oct. 8 so that bract development would occur in November and December, which is when we can reliably maintain the greenhouses as low as 60° F day and 55° F night temperature. The crop was grown under normal temperatures — about 70° F — until first color, around Oct. 30. The plants were then placed in greenhouses with setpoints ranging from 60/55° F to 75/70° F (day/night temperature).

Cultivar Selection

The ideal cultivars for fuel-efficient poinsettia production are those that naturally finish by Nov. 1 to 15. The cooler finishing temperature will slow bract expansion allowing these crops to finish during the mid- to late-November market. Additionally, using cultivars with naturally large bracts ensures that adequate bract size will be achieved. Finally, moderate-vigor cultivars are desirable so that adequate height can be achieved during the cool finish.

In our experiment, ‘Early Freedom Red’, ‘Early Prestige Red’, ‘Autumn Red’, ‘Advent Red’ and ‘Freedom Red’ performed exceptionally well when finished cool. ‘Early Freedom Red’ was the first variety to show first color after 17 short days; ‘Autumn Red’, ‘Advent Red’ and ‘Freedom Red’ reached first color after 22 short days, while ‘Early Prestige Red’ showed first color after 27 short days.

Height Management

Height management with a cool-finish poinsettia crop can be tricky because the amount of stem elongation during the cold period is less than normal. Our strategy was to target the upper limit of the graphical tracking curve during the normal temperature period (prior to first color). The plants were pinched to 6 inches and provided short days four weeks after pinch. At first color, the crop averaged nearly 12 inches tall. The targeted final height was 14 inches. Our plan was to avoid growth regulators after the start of short days, so 1,000-ppm Cycocel was applied to several cultivars on the last long day. ‘Advent Red’ was particularly vigorous, so an additional Cycocel application was applied one week into short days.

Plant height increased as day or night temperature increased — contrary to the DIF theory, which states that stem elongation will decrease as night temperature increases. We can not explain these results, but our data suggest that warmer temperatures increase stem elongation regardless of when the temperature is warm.

Figure 1 displays the average final height for each of the 16 temperature treatments. Even at the coldest average daily temperatures (cooler than 62° F), the plants grew 2 inches after first color, although that height increase occurred over a longer period of time (seven to nine weeks), so the elongation rate was relatively slow. At higher temperatures (warmer than 65° F), the plants elongated 21?2-4 inches after first color over three and a half to five weeks.

Bract Size

Bract area was quantified by removing the two largest stems per plants, stripping the red bracts from each stem and running the bracts through a leaf-area meter, which calculates the area of each bract to provide a cumulative bract area per stem. Our results show that bract area per stem decreased as the average daily temperature decreased from 64° F to 59° F.

However, plant size was also reduced at the cooler temperatures, so these plants were still well-proportioned, attractive and marketable. The optimal temperature for bract area occurred between 65° F and 68° F. We expected that bract area would actually be largest at the warmest temperatures (68-72° F), but this was not the case. Bract number per stem was actually slightly lower at warmer temperatures, so perhaps this explains the smaller overall bract area at the warmest temperatures.

Nonetheless, the take-home message is that bract area at average daily temperatures above 60° F was quite good. Figure 3 demonstrates this by showing the final plant appearance of ‘Freedom Early Red’ — which displayed the best bract expansion across all temperatures — grown at four different temperatures ranging from 71° F down to 59° F. If provided enough time for the bracts to fully develop, all of these temperatures yielded a nice crop.

Timing

Timing is the primary limiting factor for cold poinsettia production. As average daily temperatures decrease from 70° F to 60° F, the time from first color to flower decreases from 48 to 27 days (seven to four weeks) (Figure 4). For varieties that naturally flower in early November, a one- to three-week delay may be perfect to hit your market.

While there are small differences in the rate of bract development among the five varieties displayed, the differences were not significant enough to warrant different tables for different varieties. Average daily temperatures below 60° F will cause larger delays than most growers will find acceptable, so the table does not show responses below 60° F.

We did not observe any difference in bract expansion related to day or night temperature. This suggests that it doesn’t matter when the temperatures are warm or cool, it is simply important to hit the targeted average daily temperature. This is good news, suggesting that greenhouse temperatures can be allowed to warm up during sunny days and cool down during the night, as long as the average daily temperature is achieved. Nevertheless, we still wouldn’t let the crop temperature drop below 55° F or rise above 80° F.

Additionally, it is likely safe to assume that plants are relatively good integrators of temperature over several days. This means that the average temperature achieved over several days is what really matters. In other words, if you are targeting 62° F but had two warmer days that averaged 63° F each day followed by a third day at 60° F, you would still have averaged a 62° F in the three-day period.

Colors

As expected, the bract color of the red varieties benefited from cooler temperatures. We also grew white, pink and speckled poinsettias varieties — ‘Freedom Early White’ and ‘Freedom Pink’, ‘Enduring White’ and ‘Enduring Pink’, ‘Shimmer Surprise’, ‘Gala White’ — last fall to see how the bract color held up under different temperatures.

We expected the pink and white to be more grayish or greenish in the cooler temperatures, but we did not observe this. In fact, the best pinks occurred in the mid- to low-60° F range, while the whites were not noticeably different in the 60° F range. So, we do not anticipate problems including the whole family of colors in a cool-finish poinsettia crop.

Caveats

During our experiments, the weather was mostly bright and sunny, so we used shade cloth throughout the experiment to provide light levels more characteristic of Northern climates. Nonetheless, sunny days are not the norm for poinsettia crops grown in some geographic regions. The sunny days allow canopy temperatures to rise above the air temperatures. The temperatures that are described in this article are air temperatures, not plant temperatures; therefore, growers located in cloudier locations should cautiously interpret our data because they may experience slower bract development than what we achieved.

Be careful with plant growth regulator (PGR) applications made prior to the colder finishing temperatures. If the plants enter the cold phase with some residual growth regulator activity, this can limit bract expansion or can cause the leaves to be too stiff for the bracts to properly unfold above the canopy.

Finally, be careful about overwatering during the cold phase. Production staff must be aware that the plants will dry down more slowly when cold, and the foliage will stay wet longer. Because poinsettias are highly susceptible to Pythium root rot and Botrytis blight, proper adjustments in irrigation practices are essential. Cold poinsettia production will likely be easier to implement in relatively dry greenhouse facilities (concrete floors and glass glazing) compared to wetter and more humid greenhouses (soil floors and plastic covering).

There is little doubt that, given the proper variety choices, poinsettias can be grown considerably colder than in the past. However, just like any new production idea, growers should start experimenting on a small scale to gain experience and learn how to implement this new process in their own facility before attempting it on a large scale.

Acknowledgements

The authors acknowledge the Ecke Ranch for their financial support of this project.

About The Author

Jim Faust is associate professor at Clemson University; Kelly Lewis is research specialist; Niharika Komaraboina is a graduate student; Vijay Rapaka was a post-doctoral student and currently is a researcher for Smithers-Oasis; Ryan Knauer was an undergraduate student and currently is a grower at Peace Tree Farms; and Turoop Losenge is a visiting professor. Faust can be reached at jfaust@clemson.edu.

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