UMass Extension Greenhouse Crops and Floriculture Program

Growth Retardants for Poinsettias

A-Rest, B-Nine, Bonzi, Cycocel, and Sumagic are plant growth regulators (PGRs) labelled for use on poinsettias. The table accompanying this article, taken from the 2003-2004 edition of New England Floricultural Recommendations, provides the rates for use in New England and describes the specific conditions and methods for applying each PGR.

PGRs are very useful tools for controlling the height of poinsettias, but overeliance on PGRs is potentially costly in terms of material and labor. Also, because PGRs are treated as pesticides and have assigned re-entry intervals (12 hour REI - A-Rest, Cycocel, Sumagic; 24 hour REI - B-Nine, Bonzi), frequent use of PGRs may disrupt other work in some greenhouse operations.

Causes of Too Tall Poinsettias

Tall cuttings, early planting and pinching, too close spacing, and inadequate light are the major causes of poinsettias finishing too tall. Planting cuttings with long internodes start the plants at a disadvantage with some extra height even if the plants are to be pinched. Plants potted in July and early August often end up too tall because of the extra time for vegetative growth between planting, pinching, and the initiation of flower buds. Close spacing and inadequate light due to poor transmission by the greenhouse covering or cloudy weather are major causes of stretch late in the growing season. Proper spacing is a key. For example, six-inch pots containing one pinched plant should be spaced 13x14" (i.e., a minimum of 1.2 sq. ft./pot). All of the causes of excess height interact to affect final plant height. PGR use can be reduced or eliminated by the starting with good quality cuttings, use of a tighter schedule, and by providing the plants with as much light as possible by proper spacing and maximizing light transmission.

Cultivars

Most of the poinsettia cultivars introduced within the last 10 years require less PGR than earlier types. PGR use is most likely on cultivars such as 'Success', and 'Monet'. PGRs may not be needed on small and small-medium types such as 'Freedom'. Ultimately scheduling and the growing environment interact with cultivar to determine the need for PGR application.

Plant Growth Regulators for Poinsettias
PGR Rate and Method Purpose and comments
A-Rest 0.5-2 ppm drench Height control: Treat plants at pinch to 4 weeks after pinch, or 8-12 weeks before finish. Use late applications, when plants are 12-13" tall for a final target height of 15". May be applied after the start of short days without affecting bract size or quality. Cultivar sensitivity varies. Suggested trial rate 1ppm.
B-Nine 2000-3000 ppm spray Height control: Use 2000 ppm in the North. Apply 1-2 weeks after rooting or when new growth on shoots is 1½-2" long. Subsequent applications may be made at 7-10 day intervals. DO NOT apply after the start of short days.
Tank mix (B-Nine plus Cycocel) 800-1250 ppm B-Nine/
1000-2000 ppm
Cycocel spray
Height control: Spray tank mix of B-Nine SP and Cycocel. Do not apply after September 25 and make only one application. Late applications reduce bract size. This tank mix provides optimum retardation without marginal chlorosis. Mix 750-1250 ppm B-Nine and 1000-1250 ppm Cycocel. Only use rates as high as 2500 B-Nine and 1500 ppm Cycocel on stock plants in summer. DO NOT use higher rates at any time on the winter crop.
Bonzi 10-30 ppm (spray) or
0.25-3 ppm (drench)
Height control: Spray when new growth on shoots is 1½-2" long, or 2-3" long on slower varieties in cool climates. To avoid overdosing, use 1-3 sequential applications (7-14 days apart) at 50-100% of the lowest recommended rate. Do not apply sprays after the start of short days or bract size may be reduced. Drenches have less adverse effect on bract size than sprays. Drench when shoots are 2-3" long.
Cycocel 800-1500 ppm (spray) or
3000-4000 ppm (drench)
Height control, enhanced bract and leaf color: Apply when laterals are ½-3" long. On natural cycle poinsettias, apply no later than Oct. 15 (in the North) unless conditions are warm and sunny (then stop by Oct. 21). On natural cycle plants, make last application 6 weeks before flower maturity. Leaf chlorosis may occur if applied to dry plants or if temperatures exceed 75F. Apply at 3-14 day intervals as needed. Use lowest rate for frequent intervals; use 1000-1500 ppm for less frequent intervals, or early in growing season. Spray rates between 1500 and 3000 ppm often cause leaf yellowing. Drench when new growth or shoots are 1.5-2" long. Most effective during periods of rapid root growth (July to early Sept.). Do not drench after the critical cut-off dates listed above.
Sumagic 2.5-10 ppm spray Height control: Apply spray when breaks are 1½ long, about 10-14 days after pinching. Multiple applications, at lowest rates, may be necessary. Do not apply after the start of short days (Sept. 20-25 under natural days) or risk reduced bract size and plant marketability of 'Freedom'. Test to determine optimal drench rates.

Applying PGRs

PGRs must be applied uniformly in order to cause uniform inhibition to plant growth. Spraying is easiest but the least uniform way of applying PGRs to poinsettias. Sprays should be applied on the basis of volume of spray per square foot of bench (generally 1 gal./200 sq.ft.) to achieve uniform application. Never use a "spreader-sticker" unless directed to by the label. Drench applications are made on the basis of volume per pot (i.e., fl. oz./pot) so results are normally uniform. Actual volume depends on pot size. Of course, more labor is required to make drench applications compared to sprays.

Normally PGRs are applied during cloudy weather. The effectiveness of Cycocel and B-Nine is increased when conditions favor slow evaporation. Plants should be fully turgid when PGRs are applied (Cycocel injury is most common on water-stressed plants). Growth medium should be uniformly moist when drench applications are made. Additional specific precautions are discussed in the accompanying table.

Late Season PGR Applications

Growers can try to control late season stretch by applying a very dilute drench of A-Rest or Bonzi. These drench treatments can be applied after October 15 into early November without affecting bract diameter. Chemicals should not be sprayed because sprays may inhibit bract development. Drench solutions should be prepared to supply 1 or 2 ppm a.i. of A-Rest or Bonzi. The dosage rate is 4 fl. oz./6 or 6½" pot. The accompanying table has other suggestions for using low levels of A-Rest and Bonzi to control height.

Several Massachusetts growers have been making these treatments for a number of years and are pleased with the results. However, one might question if the "stretch" control gained offsets the time and labor involved in making the drench treatments. Therefore, it is probably worthwhile to leave at least 10 representative plants untreated to evaluate the effectiveness of the low concentration drench treatments.

Reference

  • Lopes, P. and L.Berg Stack. (eds.) 2003. 2003-2004 New England Greenhouse Floricultural Recommendations. A management guide for insects, diseases, weeds and growth regulators New England Floricuture, Inc.