Heat Delay in Poinsettias By Rebecca Schnelle and Jim Barrett

Heat delay refers to high temperatures that cause poinsettia crops to develop color and reach a saleable stage of development later than scheduled. In the worst situations, crops can be two to three weeks delayed, but even a week's delay can be important at the peak of the shipping season. While heat delay is a yearly problem for growers in warmer climates, it can be a significant problem in much of the country during a warmer fall, such as fall 2007 in the Midwest and much of the East.

In the past few years, we have studied heat delay and re-evaluated several of the general perceptions about this problem. The work has included many varieties and investigated when poinsettias are most sensitive to high temperatures. Part of this worked involved microscopic examination of poinsettia meristems to more accurately identify when the plants were responding to high temperatures.

How High Temperatures Affect Color Development

Figure 1 shows the development of 'Prestige Red' plants at three different average daily temperatures (ADT). At 70¡ F, the meristems initiated flower development 14 days after start of short days. First color, visible bud and flowering occurred at 30, 35 and 54 days, respectively. At a 75¡ F, average temperature development was similar to 70¡ F. However, at an average of 80¡ F, development was different: Initiation occurred six days later and visible bud eight days later than with the 70¡ F temperature treatment. This led to an 11-day delay in time of flowering.

Notice the effect of the high temperatures on development of first color in this example. The normal development pattern for 'Prestige Red' and most commercial varieties is to develop first color a few days before visible bud as observed at 70¡ F and 75¡ F. However, at 80¡ F, bract color development was delayed more than bud development, so visible buds were showing on the plants before they started developing color. This pattern of development often occurs in heat-delay situations, and some varieties exhibit this pattern more than others. For growers, first color is the first indication of whether a crop is on schedule. This abnormal delay in color development can make a crop seem more delayed than it actually is. Visible bud is most often a better indicator of when the crop will finish.

What Causes Heat Delay?

It has been a common belief that heat delay is caused by high night temperatures and that the plants are not responding to day temperatures. Figure 2 shows results typical of several experiments where 'Red Velvet' was grown at different day and night temperatures. Treatments 1, 2 and 3 have an ADT of 75¡ F, and meristem initiation occurred in 18 to 19 days. For the treatments where ADT was 80¡ F (treatments 4, 5 and 6), meristem initiation was delayed to 23 to 25 days.

Compare treatments 3 and 4. Treatment 3 has the higher night temperature, but treatment 4 is the one with delayed initiation, which indicates that not just night temperature causes heat delay. Treatment 2 has a higher day temperature than treatment 5, but 5 is the one that was delayed. This means the plants are not responding only to day temperatures. The conclusion from these experiments is that both day and night temperatures affect the poinsettias, and ADT is the important factor to consider.

Results of Treatments

'Autumn Red' is an early flowering variety and under moderate temperatures finished Nov. 4 in the study shown in Figure 3, treatment 1. Meristem initiation occurred on Sept. 24 in this treatment. The other plants in this study were exposed to high temperatures at different times with the objective of determining when development was most sensitive to high temperatures.

In previous work, we determined that one week of high temperatures had little effect on initiation, so the treatments here are two weeks or longer. Treatment 2 in Figure 3 was in high temperatures for two weeks that ended Sept. 25, several days before initiation occurred. This treatment did not cause heat delay. Treatments 3 and 4 caused a similar amount of heat delay, and the high temperatures were occurring while the meristems should have been initiating. In treatment 5, the stress started Sept. 25 — after initiation had occurred — and did not cause heat delay. For treatment 6, the two weeks of high temperatures were not consecutive and delayed development as much as treatments 3 and 4 where stress was continuous for two weeks.

Notice that treatments 3, 7 and 8 all started on Sept. 11, and plants were in the high temperatures for two, three and four weeks, respectively. Two weeks of stress was enough to cause significant delay in development and increased time at high temperatures caused additional delay in initiation and flowering.

Additional treatments that are not shown in Figure 5 exposed the plants to two weeks of high temperatures later in development. High temperatures that occurred in the two-week period prior to visible bud had no effect on development. However, high temperature exposure after visible bud caused the plants to develop faster and finish earlier than control plants in treatment 1.

These results (Figure 3) indicate that heat delay is caused by high temperatures during a four-week sensitive period that starts about a week before normal meristem initiation. The high temperature exposure during this period does not have to be continuous, but it must start before the plants initiate to produce heat delay.

How Heat Delay Impacts Production

Another important conclusion from our work with multiple varieties is that all poinsettia varieties exhibit heat delay when grown under natural days in Florida. It was thought that later-flowering varieties were more sensitive and delayed more than varieties that normally finish early, but that is not the case. However, heat delay in later varieties may have more impact on production. A midseason variety that is scheduled to normally finish Nov. 28 but is delayed to Dec. 8 has a much greater potential economic impact than a delayed early variety. A delay in an early variety still means it finishes during a strong market period.

To judge the potential for heat delay, growers should monitor greenhouse temperatures to determine whether ADT is regularly above 75¡ F during the sensitive stage of development. If heat delay has occurred and crops will finish late, the effect can be mitigated by controlling temperatures after initiation. For a crop where initiation was delayed, it is important to keep temperatures up (75¡ F maximum ADT) during floral development to speed up the process.


Poinsettia Development

This is an illustration of development in a poinsettia crop. The time from planting to start of short days is the vegetative period, and the plant growing point (meristem) is forming leaves and internodes. Short days (either natural days or controlled with lights and/or black cloth) start the floral initiation stage, which is a transition to the formation of cyathia and bracts. The end of this stage is when the meristem has actually initiated or started forming flower parts.

Determining meristem initiation is a research procedure that requires considerable practice and experience with a dissecting microscope. During the floral development stage, the markers that can be seen are first color and visible bud. During the vegetative stage, temperatures affect the number of leaves formed and plant size. During the floral development stage, temperatures affect how fast the flower parts develop and finish timing: Lower temperatures cause poinsettias to finish later, and warmer temperatures cause them to finish earlier. High temperatures that cause a delay in flowering (heat delay) occur during the floral initiation stage before meristem initiation.

Note the difference between short days and initiation. It is common, in commercial recommendations, to confuse the two terms and refer to start of short days as initiation. For the best understanding of how the environment affects poinsettia crop development, it is important to keep the terms separate.

Rebecca Schnelle and Jim Barrett

Jim Barrett is professor of floriculture at the Univeristy of Florida in Gainesville, Fla. He can be reached at jbarrett@ufll.edu. Rebecca Schnelle is an assistant extension professor at the University of Kentucky. She can be reached at rebecca.schnelle@uky.edu.

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