Technical Information Bulletin

SUBJECT: Chemical Growth Regulators

The use of growth regulating chemicals to produce poinsettias has become a routine procedure for growers in all regions of the world. The most common plant growth regulators (PGRs) used by growers prevent or control internode stretch. Without the use of PGRs, plants are likely to become too tall for sale or too weak to withstand the rigors of transportation. Through the use of good cultural procedures, which includes using PGRs, growers are able to produce good quality poinsettia crops under even the most difficult conditions.

The rate of growth and potential internode stretch of poinsettias is directly influenced by environmental conditions, such as temperature, relative humidity and light intensities that are provided for the crop. Grower influenced conditions such as irrigation practices, spacing and crop scheduling also play an important role in preventing unwanted stretch. Understanding how to use all these factors together to minimize potential stretch is a challenge to poinsettia growers every year. When crop growth escalates to the point of needing chemical intervention, growers are fortunate to have several PGR options available for use on poinsettias. 

Currently, the plant growth regulators most commonly used with poinsettias are A-Rest®, B-Nine®, Bonzi®, Cycocel®, and Sumagic®. These chemicals vary in their level of activity and their application requirements for poinsettias. In order to determine which PGR is best to apply, growers should evaluate (1) the stage of growth at which the application will be made and (2) the amount of growth control or level of activity desired. Using this information, the process of selecting an appropriate PGR is fairly simple.

TRANSPLANT TO PINCH

Use of PGRs is generally limited during this developmental stage. Most cuttings have been treated with a growth retardant during propagation and are less likely to stretch at this time. As the roots begin to establish into the new growing medium, active growth will begin. If cuttings already have adequate node count, any new growth is likely to be removed by pinching. Therefore, application of PGRs is not warranted since existing internode spacing will not change. The exceptions to this occur when plants are grown as non-pinched forms, if premature lateral branching develops before pinch, or if the plants must be grown to a greater node count before pinch can take place. In all these situations, use of Cycocel® as a spray at fairly low concentration (1000 ppm) is advisable only as needed.

PINCH TO FLOWER INITIATION

Use of growth retardant generally begins when lateral shoots reach 3/4 to 1" in length. At this stage of development, any of the five PGR chemicals can be used. For light to moderate control, spray foliage with B-Nine and Cycocel combined at 1250 ppm each, or use Cycocel by itself at 1500 ppm. If stronger control is needed, spray applications of Bonzi (generally 10 to 25 ppm depending upon cultivar and level of activity desired) or Sumagic (2.5-10ppm) may be used. The recent pinch makes it easy to get uniform coverage and good contact with stems and petioles for proper absorption of the Bonzi or Sumagic. Drench applications of PGRs provide longer lasting and more uniform effects than spray applications. For moderate control, Cycocel may be applied as a drench. For stronger control, A-Rest may be used.  Bonzi and Sumagic are not generally used as a drench at this stage of the crop as they may retard the growth too much.

FLOWER INITIATION THROUGH MID OCTOBER

Application options become more limited during this developmental phase. Spray applications that include B-Nine are not recommended except in regions of extreme heat, such as south Florida, the Gulf States or the tropics. Use of B-Nine at this stage of development can delay flowering and reduce the size of bracts. Applications of A-Rest, Bonzi or Sumagic are effective at lower concentrations, however they are not as advisable at this time due to a mild reduction in bract size at time of finish. Studies conducted in 1994 by the University of Florida and by the Paul Ecke Ranch showed bract size reduction was more significant at this stage of development than at earlier or later application dates. Although Sumagic was not included in these particular studies, its response is generally similar to the PGRs just mentioned. Applications of Cycocel as either a drench or a spray are acceptable at this stage under a wide range of conditions.

MID OCTOBER THROUGH EARLY NOVEMBER

Growers should target to have all growth retardant applications completed by this time. However, in the event of late stretch due to weather conditions or unusual growth patterns, it is possible to apply either A-Rest (1 to 3 ppm) or Bonzi (.5 - 2 ppm) without significant bract size reduction or flower delay. The rates shown should be maintained at the low end unless you are located in the extreme heat regions of the south. Apply these chemicals as a soil drench using a volume of solution that is proportional to the volume of soil in the container. For 6 to 6.5" pots, 4 ounces of the PGR solution should be applied to a uniformly moist media. For accuracy of application it is not advisable (or legal by some labels) to apply the PGR through irrigation systems. The most common method of application has been by "dipping" the chemical out of solution tanks or using chemical spray equipment with tips removed form the nozzle. Drench applications are extremely effective with hanging baskets where spray applications are difficult to achieve without drift onto adjacent plants and where growing conditions encourage rapid, stretched growth of the poinsettias. For 10" baskets, 15 ounces of the PGR solution should be applied. Growers using a pine bark based media will need to increase rates by about 25% as this type of bark reduces the activity of these chemicals.

©1999 Paul Ecke Ranch All Rights Reserved	.
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