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Poinsettia Height Control Producing the perfect-height poinsettia requires skill, knowledge, and perhaps even good luck. Each year is a new challenge, and we cannot always rely on the height control program that may have been successful the previous season. Growers should design a production program that is flexible enough to allow compensation for unpredictable circumstances. Such a program should consider providing plants with the best environment for growth and development and the possible use of plant growth retardants to check internode elongation. The final height of a poinsettia crop is determined by many factors, including: (1) the cultivar being grown (its vigor and rate of development) (see: Cutlivar Response Table); (2) the production schedule employed (propagation date, potting date, pinch date, and number of long days after pinch); (3) environmental conditions (temperatures, light levels, and plant spacing); and (4) the height control program employed (plant growth retardant applications). In many cases, paying proper attention to the first three factors can greatly reduce the need for plant growth retardant applications. The following discussion applies to production of pinched plants in 6 inch pots under growing conditions in North Carolina. Many poinsettia production schedules recommend 7 to 14 days of vegetative growth (long-day conditions) between pinching and start of short days to allow for adequate shoot length for high quality bract displays and sufficient height at bloom. This is a good rule of thumb for medium-vigor cultivars such as 'V-14 Glory', 'Lilo', and 'Red Sails'. However, very vigorous cultivars such as members of the 'Annette Hegg' series may only require 5 to 7 days for sufficient development prior to start of short days. On the other hand, slower developing cultivars such as 'Freedom' 'Celebrate 2', and 'Supjibi' should be scheduled to include 7 to 10 more long days between the pinch date and start of short days than the standard 7 to 14 days recommended for 'V- 14 Glory'. Adjust the scheduled pinch date and start of short days to allow for adequate yet not excessive vegetative growth. Conversely, if your schedule requires earlier planting and pinching then holding of plants prior to start of short days, be prepared to make multiple applications of growth retardant the prevent too much internode elongation. Supplying Long Days and Short Days Just writing when short days are to begin is not enough to make it happen. Insure that plants are indeed subjected to long day conditions when the schedule call for long days. Supply a four hour night break (10 to 20 footcandles minimum at canopy level) or extend the daylength to a total of at least 14 hours to assure long days. Lighting to assure long days should begin September 5 for 'Freedom'; September 15 is an adequate starting date for most other cultivars. October 5 is a conservative starting date for natural short days in the Southeast, especially during a hot autumn when night temperatures may hover at or above 70°F. Night temperatures above 70°F can delay floral initiation in many poinsettia varieties and should be avoided under short day treatments. If your schedule requires short day conditions prior to October 5, blackcloth should be used to restrict daylength to a maximum of 11 hours (give at least 13 consecutive hours of darkness each 24 hours). Controlling photoperiod (and night temperature) is essential in precise programming of poinsettia market date and final height. Spacing and light levels also drastically affect plant height (see: Spacing). The concept of DIF (difference between the day and night temperature) and how it affects plant development is well described in the poplar trade magazines. Briefly, negative DIF's (night temperatures > day temperatures) reduce internode elongation as compared to a zero DIF (night = day) and positive DIF's (day temperatures > night temperatures) increase internode elongation as compared to a zero DIF. During poinsettia season in the Southeast, it is often difficult to maintain negative DIF conditions without raising night temperatures to unacceptable levels. The best way for growers in the Southeast to use DIF as a poinsettia height control technique is to maintain a minimum night temperature of 64°F then drop the morning temperature (one hour prior to sunrise on into the morning) as close to 58 to 60°F as possible. Also, try to prevent excessively high daytime temperatures, and reduce the degree of positive DIF as much as possible. Do not use too great of a negative DIF (more than a 5°F differential) or plant quality can be reduced. Too great of a negative DIF can deplete the carbohydrate status of plants and is not recommended. The most effective timing for a negative DIF for height control is three weeks after pinching (long enough to allow new shoots to develop) up to three weeks prior to anthesis. Do not use a negative DIF the last three weeks prior to anthesis or cyathia will drop, and little height control is possible then anyway. One word of caution about dropping temperatures and raising them-the rate of plant development is dependent to a large degree upon the average daily temperature (average of day + night temperatures). If using DIF changes the average daily temperature, then growth rates will also be affected and should be accounted for in scheduling. Selecting a Growth Retardant Treatment There are many chemical growth retardant treatments available to poinsettia growers. The decision of which treatments are best should be based on treatment effectiveness, the cost of use, the ease of use, and the degree of flexibility afforded by the treatment. For example, Cycocel and A-Rest drenches are very effective for poinsettia height control, but they are relatively expensive. Rates of Cycocel above 1,500 ppm can also cause yellow spots on the leaves and under severe conditions interveinal chlorosis and leaf cupping. To prevent damage, a lower concentration of Cycocel should be applied with repeat applications being used. Bonzi and Sumagic sprays are relatively inexpensive, but require a uniform application technique for consistency in height control. B-Nine + Cycocel as a spray is very effective, but along with Bonzi and Sumagic, use is restricted to the early portion of the growing season. Excessive rates or late applications of plant growth regulators delay flowering and reduce bract diameter. Rates of application are listed in the Chemical Growth Retardant Table.
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Chemical growth retardant recommendations for use on poinsettias in North Carolina. |
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Chemical |
Rate and application method |
Cost of material* |
Remarks |
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A-Rest |
2 to 3 ppm drench solution (0.97 to 1.45 fl oz/gal of drench solution); apply 4 fl oz/6 inch pot |
6.1¢ to 9.1¢ per 6 inch pot |
A-Rest activity is reduced by pine bark in the substrate. Can be used as a late drench, up to November 10, with little effect on bract size. For larger pots, use a greater drench volume--15 fl oz per 10 inch basket (would cost 22.7¢ to 34.1¢ per 10 inch basket). |
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B-Nine |
2,000 to 3,000 ppm spray (0.31 to 0.47 oz/gal) |
0.3¢ to 0.4¢ per 6 inch pot** ($0.67 to $1.01/100ft2) |
Not effective in our studies. |
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B-Nine + Cycocel |
800 to 2,500 ppm B-Nine (0.13 to 0.39 oz/gal) + 1,000 to 1,500 ppm Cycocel (1.08 to 1.63 fl oz/gal) spray |
0.6¢ to 1.2¢ per 6 inch pot ($1.47 to $2.65/100ft2) |
Use the higher rates of this tank mix spray on stock plants and for finishing crops in very warm regions. Outside of very warm areas, growers should use the lower rates. Too late of an application (about October 1) can delay flowering and reduce bract size. |
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Bonzi |
10 to 30 ppm spray (0.32 to 0.96 fl oz/gal) |
0.2¢ to 0.7¢ per 6 inch pot ($0.55 to $1.64/100ft2) |
Do not apply after October 10 or delay in flowering and reduction in bract size may occur. |
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1 to 3 ppm drench solution (0.064 to 0.128 fl oz/gal of drench solution); apply 4 fl oz/6 inch pot |
0.3¢ to 1.0¢ per 6 inch pot |
Bonzi activity is reduced by pine bark in the substrate. Can be applied up to November 10 with little effect on bract size or timing. For larger pots, use a greater drench volume--15 fl oz per 10 inch basket (would cost 1.3¢ to 3.8¢ per 10 inch basket). |
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Cycocel |
800 to 1,500 ppm spray (0.87 to 1.63 fl oz/gal) |
0.4¢ to 0.8¢ per 6 inch pot ($0.96 to $1.81/100ft2) |
Do not apply after 21 October or delay in flowering and reduction in bract size may occur. |
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3,000 to 4,000 ppm drench solution (3.25 to 4.34 fl oz/gal of drench solution); apply 6 fl oz/6 inch pot |
33.8¢ to 45.1¢ per 6 inch pot |
Drench volume varies with pot size. Consult the label for appropriate volumes for different pot sizes. |
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Sumagic |
2.5 to 10 ppm spray (0.64 to 2.56 fl oz/gal) |
0.5¢ to 2¢ per 6 inch pot ($1.10 to $4.40/100ft2) |
Do not apply after October 10 or delay in flowering and reduction in bract size may occur. |
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*Based on the following costs: $64/qt of A-Rest, $342/5 lb B-Nine, $109/qt Bonzi, $71/qt Cycocel, and $110/ qt Sumagic. **Spray applications assume an 8 x 8 inch pot spacing at time of application and an application volume of 2 quarts per 100 square feet of bench area. Wider spacing would increase the per pot materials cost. |
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Research at the University of Florida has demonstrated that A-Rest and Bonzi drenches can be used on poinsettias up through early November with little effect on crop timing or bract diameter. Many growers across the Southeast have tried this technique and reported very good success with Bonzi drenches. Applying 4 fluid ounces on 6 inch pots and 15 fl oz on 10 inch hanging baskets (pots/baskets in between these sizes would receive between 4 and 15 fl oz) is recommended. It is recommended that the lower rate given in the Chemical Growth Retardant Table for less vigorous varieties, such as 'Freedom' and the higher concentration for varieties such as 'V-14 Glory'. Apply drenches to moist substrates; water one day and apply the drench the next day. Both A-Rest and Bonzi activity is reduced by pine bark in the substrate. Consider increasing the drench concentration up to 50% if your substrate contains pine bark. The good news with A-Rest and Bonzi drenches is the flexibility in terms of timing of applications. The not-so-good news is the labor involved in applying drenches. Application Timing and Technique Applying a growth retardant at too early of a developmental stage can result in stunted growth and can cause great delays in plant development. Too late of an application will be ineffective; these are plant growth retardants, not plant reducers. Once elongation is present, there is no way to shrink the internodes. For early applications, wait for newly developing shoots to reach about 2 inches in length prior to applying a growth retardant. This stage should allow for proper plant development yet is prior to excessive elongation. Sprays should be applied at a uniform 1/2 gallon of solution per 100 ft2 of bench area. This volume is sufficient to allow for complete coverage of the plant canopy. Inconsistent application methods can result in uneven growth among the plants on the bench. Apply drenches according to the label, following both concentrations and volumes given. If you do run into late season height control problems, consider using an A-Rest or Bonzi drench. Keep some untreated plants in the greenhouse and use them as comparisons to judge plant growth rate. Measure the height of treated and untreated plants weekly. This will allow you to detect effectiveness of your treatments as well as give you an indication of when plants begin to grow out of the growth retardant. (Link to: Chemical Growth Retardant Table) |
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