Growing a good poinsettia crop is challenging enough without having to worry about production input costs. However, with fuel prices up once again, it is even more important to limit how much energy is consumed during the production process. There is growing demand for varieties with a short flowering response time, the time from the start of short days until plants are ready to sell. For example, a late-season poinsettia variety requires nine or 10 weeks at an average daily temperature of 68° F to finish plants, whereas a very early poinsettia variety requires only about seven weeks at 68° F. Thus, it is intuitive that the shorter the response time, the less energy required to produce the crop.
Cold poinsettia production is an appealing concept; plants are grown at “normal” temperatures (68-70° F) from transplant until first color and are then finished at cooler temperatures. Providing warm temperatures early in the crop (in August and September) is usually not expensive because of the naturally warm weather. However, heating costs escalate rapidly as outdoor temperatures become progressively cooler. For poinsettias, plants essentially stop growing at temperatures lower than 55° F.
Lower Temperature, Slower Growth
There is a strong temptation to lower the greenhouse heating setpoint as it gets colder outside. The tradeoff to lowering the thermostat is that plants develop more and more slowly as the average daily temperature decreases. Therefore, the lower the temperature, the longer it takes a poinsettia crop to become marketable. How much longer? Using data from Clemson University and the Paul Ecke Ranch, we have developed a finish crop schedule for different poinsettia flowering response categories. Table 1 provides the approximate date that plants will be ready for market when grown at different average temperatures beginning at the start of first color, which varies among response group. In other words, this schedule assumes that plants will be rooted and grown vegetatively under long days at 68-70° F, and then under natural short days at 68° F until first color.
Poinsettias with the shortest response time (very early cultivars such as Advent and Freedom Early) can be ready in early November if finished at an average daily temperature of 68° F. Lowering the average temperature by 8° F delays finish time by more than three weeks, so plants will not be ready for shipping until the last week of November. One advantage of growing these varieties is that they are easy to schedule for early-season sales. However, scheduling these varieties for market in late November requires that cool temperatures (62° F) can be provided beginning in early October, which may not be possible in the South. For many markets, a cool-temperature growing strategy will not work for mid- and late-season varieties because plants will not be ready for market until early December or later.
Projected Heating Costs
We calculated the projected greenhouse heating cost to grow poinsettias with different flowering response categories at average finish temperatures ranging from 60 to 68° F. We made calculations using the Virtual Grower software program developed by Jonathan Frantz and colleagues at USDA-ARS in Toledo, Ohio. The virtual greenhouses were located in Grand Rapids, Mich., and Newark, N.J., with identical greenhouse characteristics, fuel cost, heating system, etc., as described in Table 2.
The values in Tables 2 and 3 are the estimated heating costs per square foot of greenhouse space from Oct. 1 until the estimated market dates provided in Table 1. From Oct. 1 until the start of cool temperatures (at first color), the temperature setpoint was either a constant 68° F or a 12-hour day/night of 72/64° F. The remaining production periods (from first color until market dates) were at the temperatures indicated in Tables 2 and 3. Growers that use energy curtains could reduce this cost by 20 percent to 40 percent depending on the curtain material and system.
A grower that wanted to produce poinsettias ready for market around Nov. 22 to 24 could follow one of five different programs, assuming use of natural short days and the ability to deliver cool temperatures in early fall. As indicated in the shaded cells of Table 1, the grower could finish a 61?2-week flowering response variety at 60° F, a 7-week variety at 61° F, a 71?2-week variety at 62° F, an 8-week variety at 64° F or a 9-week variety at 68° F.
At a day/night finish temperature of 60/60° F, the estimated heating cost (beginning Oct. 1) to produce the 61?2-week variety is 371?2 cents per square foot (¢/ft2) in Grand Rapids, Mich., and 19.2¢/ft2 in Newark, N.J. To produce a mid-season (9-week) variety for a similar market date, a day/night of 68/68° F is required. In this case, the estimated greenhouse heating cost from Oct. 1 until finish is 54.2¢/ft2 in Grand Rapids and 33.2¢/ft2 in Newark. In other words, growing a mid-season variety for the same finish date costs approximately 44 percent more in energy for heating in Michigan and 73 percent more in New Jersey compared to producing a very early variety.
Day and Night Temperatures
Finally, because most of the energy used to heat a greenhouse occurs at night, providing a cooler night than day temperature can be a more economical way to deliver a desired temperature. Therefore, the same calculations as discussed above were made for a greenhouse with a +8° F DIF, meaning that the day was 8° F warmer than the night. Although a positive DIF promotes stem extension, less energy is consumed for heating while still achieving the same average daily temperature. For example, delivering a 68/60° F day/night (12-hour day, 12-hour night) saves 2 to 4¢/ft2 compared to a 64/64° F day/night, in both locations, during the finish poinsettia periods.
Growers are cautioned to experiment with cold-temperature poinsettia finishing in their own facilities before committing a significant percentage of their crop to this new technique. Conduct your own energy analysis by downloading Virtual Grower for free on your computer. Find it by typing “Virtual Grower” into an Internet search engine. For more information on cold-temperature poinsettia production, check out the February 2008 and July 2008 issues of GPN magazine, or visit the Paul Ecke Ranch’s website at www.ecke.com/html/tibs/energy_efficient_poinsettia.htm .