Poinsettias are well adapted to commercial production. Their normal bloom date is near Christmas, but control of photoperiod and temperature allow flowering for any date. Newer cultivars are shorter in response, with many blooming for Thanksgiving on natural day lengths if potted by September.
Photoperiod and Temperature:
This cannot be stated precisely, since both temperature and cultivar influence response to photoperiod. Poinsettias are quite sensitive to light so that exposures of low intensities (on the order of less than 2 footcandles) will nullify the effect of darkness. Dark periods of 11 hours and 45 minutes will cause initiation in most cultivars, but initiation is most rapid at 14 to 14.5 hour dark periods at temperatures of 60-70oF. Night temperatures of 62-64oF ensure most rapid development under photoperiods occurring in late September and early October in the northern hemisphere. Following 10 to 14 days of inductive photoperiods, raising the temperature to 67-68oF will favor rapid bract development. About 40-60 inductive cycles are needed for full development of the 22 cyathia and 27 bracts in a normal inflorescence. After March 10, natural dark periods are not long enough for initiation in most cultivars.
Time to Flower:
Cultivars differ in the time required to reach a salable stage following the start of inductive short days. This time is called the “response” and cultivars with similar response times are grouped into “response groups.” Response may be conditioned by environmental factors, such as light and temperature. Modern cultivars range from 8-week response to as long as 11-week. If 11-week response cultivars are wanted for Thanksgiving sales, they will require black-clothing to provide absolute darkness for 14-14.5 hours each night beginning 11 weeks before the sales date desired. This black clothing can be discontinued about October 15 if no outside artificial light interferes with the natural dark period.
Eight-week cultivars will finish on time for Thanksgiving sales with planting about mid-September on natural days (providing no lights interfere). However, these and the 9-week response cultivars will be too early for Christmas sales unless they are lighted artificially to prevent exposure to the inductive, or “critical”, night lengths in late September and early October. Incandescent 60-watt bulbs spaced 4 feet apart and placed 2 feet over the plants are used from 10 p.m. to 1 a.m. to provide the artificial “long day” conditions. The “lights out” date, or start of short days, is determined by counting back from the desired sales date the number of weeks in the response group to which a particular cultivar belongs.
Such control of photoperiod provides other benefits than ensuring fresh plants for sale. High temperatures in late September usually prevent early initiation of flowers in poinsettias. The climate is so favorable most years that many growers become complacent about providing climate controls in the early part of the forcing season. A cool spell in late September, however, can start flower initiation earlier than wanted. This can seriously affect quality, since once pollen sheds in an individual flower in the cluster, the bract under that flower almost completely stops enlarging, and the result can be very small bracts and poor quality plants.
If the temperature is kept 70oF or above in late September, then initiation will be delayed, since longer nights are required at higher temperatures. The cost of heating makes this a less attractive means of delaying initiation than lighting.
Timing of the pinch in branched plant production can influence plant quality and can only be done properly when the start of short days is known. This is also true for the timing of growth regulator sprays or for determining if they are required or not.
The shoots on poinsettia plants will almost exactly double their length as measured four weeks after the start of short days. An observation of shoot length at that time can tell a grower if plants will finish too tall. This is early enough to apply growth regulators without adverse effects that may result from late applications. The start of short days can only be determined, however, if photoperiods are controlled, although careful temperature records in the house at night could allow an estimate of the start of effective natural short days.When the pinch and application of growth regulators are timed properly, a much better display of the colored bracts is assured, with higher quality that can command a better price. Plants on controlled photoperiods produce smaller, tighter flower clusters.
Timing a pinch can also affect plant quality. Pinching too early can result in plants that are too tall, and too late can result in plants too short. Usually pinching is done about 2 weeks before the start of short days. Unless you light or blackcloth you cannot accurately tell when short days begin. Since temperature also affects the length of the critical dark period, short days can start anywhere from about the middle of September to October 10.
Many manuals and bulletins will say not to pinch after September 15, but most of those publications are based on conditions in the northeast where light is of lower intensity, days may be shorter earlier, and temperatures are lower. Many of the newer varieties can be pinched as late as October 10 with no loss of quality for a Christmas crop.
The type of pinch you make can influence the lateral breaks you get in low-light situations. It is a generally recommended to leave about six leaves below the pinch. When the breaks come out, it is a good idea to remove weak ones. We like to leave four well-spaced breaks on a 6-inch plant. you may want to leave five or six.
If you direct-stick cuttings, enough time must be allowed to get them well-rooted before pinching, up to as long as five weeks for some, less for the whites, which root easily.