Growth Regulators for Bedding Plants

Links to plant growth regulator labels

Chemical plant growth retardants (PGRs) are very useful tools for controlling the height of bedding plants. Several other growth regulators are available to stimulate branching or to increase postharvest quality. This fact sheet outlines what chemicals are available for bedding plants, how they are applied, and some of the factors which affect the success of their use. For more information see the most recent New England Floriculture Guide.

Causes of Too Tall Plants

Achieving desirable plant height involves considering why plants tend to be too tall before starting on a PGR program. A PGR may not be necessary if the problem can be corrected. There are number of reasons why bedding plants may get too tall. If the plants are started too soon or tall cultivars are chosen may lead to overgrown plants. Transplanting to large volume packs or other containers may contribute to too large plants. Low light intensity, too close spacing of certain plants, and running much higher day than night temperatures are probably the most common reasons for too tall bedding plants. Plants that are shade by old plastic, neighboring plants, or hanging baskets tend to stretch. Lack of ventilation on clear days and resulting heating will increase the difference between the day and night temperature and cause plants to grow taller. Some of the factors interact to encourage development of tall plants (e.g., low light and poor ventilation).

PGRs may be helpful in overcoming some of these problems, but too much reliance on PGRs is potentially costly in terms of material and labor. Also, because PGRs are treated as pesticides and have assigned re-entry intervals, frequent use of PGRs may disrupt other work in some greenhouse operations. Like pesticides, PGRs need to be used wisely and not as substitutes for good cultural practices.

Growth Retardants for Bedding Plants

Plant growth regulators are most effective when applied at the appropriate times to regulate plant growth or development. Growth retardants cannot shrink an overgrown plants. They must be applied before the plant is overgrown to prevent plant stretch. When planning PGRs in your production schedule, consider what you want to accomplish with the treatment.

Table 1 lists the five PGRs for bedding plants.

Table 1. Growth retardants for bedding plants
trade Name/Chemical Method "Activity" REI
A-Rest (ancymidol) Spray & drench Low-moderate 12
B-Nine (daminozide) Spray only Low 24
Bonzi (paclobutrazol) Spray & drench High 12
Cycocel (chlormequat chloride) Spray Low 12
Sumagic (uniconazole) Spray, drench, & pre-plant surface High 12

A foliar spray is the preferred method of applying PGRs to bedding plants in flats, but growth medium drench can be used on pots. Drench applications of B-Nine have no effect on plants and Cycocel drench is less effective than a foliar spray.

Sumagic can be applied as a pre-plant spray made to the surface of the growth medium. This may be a more efficient way of applying PGR. A-Rest and Bonzi can also be applied by subirrigation. The efficacy of applying PGRs by subirrigation needs more research before it can be widely recommended.

The term "activity" refers to the general sensitivity of plants to the PGR and the persistence of the growth inhibiting effect after application. The height of plants treated with low activity PGRs is not greatly affected by variations in the spray or drench volume. The persistence of the growth inhibiting effect after application is short (2-3 weeks). Plants are very sensitive to high activity PGRs. Small variations in spray or drench volume can have significant effects on height. The growth inhibiting effect after application, especially by drench, is very persistent. What this all means to a grower is that the risk of undesirable side effects is much greater with Bonzi and Sumagic than A-Rest, B-Nine, or Cycocel. More attention to the details of rate, application volume, timing, and possible reapplication is required to successfully use high activity PGRs.

PGR Rates to Use

Choosing a rate (ppm) to apply starts with a check of the label for the recommended rate for the plant you wish to treat or a crop production such as the New England Floricultural Recommendations. In the absence of a specific recommendation the grower must run a trial. A trial should be replicated and consist of a small number of plants rather than whole crop! The general rates commonly used for plugs (Table 2) and flats or other containers (Table 3) are the starting point for a trial. Its only common sense to test the lowest rates first, especially with Bonzi and Sumagic.

Table 2. PGR rates for bedding plant plugs
Chemical rates (ppm) Precautions
A-Rest 3-35 Spray
0.5-1 Drench		 Use 5 ppm as a trial rate as spray.
B-Nine 1500-2500 Spray  ---
Bonzi 1-20 Spray NO on vinca and begonia.
NO drench.
Cycocel 400-750 Spray Low rate in north.
Sumagic 0.5-10 Spray
0.5-10 Preplant soil surface spray		 Plugs are highly sensitive.

Applying PGRs
Normally PGRs are applied early in the growth of bedding plants and seedlings. Timing is related to stage of plant development and not weeks from seeding or transplanting. Timing can be fairly specific for some species and the product label or other reference should be checked. In general plugs are treated when they have developed their first set of true leaves. Young plants after transplanting are generally treated when they begin to grow or when they are 2" tall or 2" wide.

Table 3. PGR rates for bedding plant flats, pots and baskets
Chemical Rates & method (ppm) Precautions
A-Rest 6-66, spray
1-2, drench		 15 ppm for trial sprays.
B-Nine 2500-5000 Spray only.
Bonzi 5-90, spray
0.5-1, drench		 15 ppm for trial sprays in north. NO on vinca and begonia.
Cycocel 800-1500 spray 1250 ppm for trial. Higher rates may be needed.
Sumagic 1-50, spray
0.5-20, preplant surface
0.1-2, drench		 5-15 ppm for trial rate sprays. NO on begonias.

PGRs must be applied uniformly in order to cause uniform inhibition to plant growth. For bedding plants in flats PGRs are applied by foliar spray application; drench application can be used for bedding plants in pots and hanging baskets. Sprays should be applied on the basis of volume of spray per area bench) to achieve uniform application (Table 4). Never use a "spreader-sticker" unless directed to by the label. Drench applications are made on the basis of an exact volume per pot to get a uniform effect. Actual drench volume depends on pot size (Table 5). 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 sprays is increased when conditions favor slow evaporation. Plants to be sprayed should also be fully turgid when PGRs are applied (Cycocel injury is most common on water-stressed plants).

Table 4. PGR spray coverage rates.
Chemical Quarts/100 sq. ft.
A-Rest 2
B-Nine 2
Bonzi 1-3 (For small plants in small containers, use 1-2 qts per 100 sq.ft.)
       (For large plants with well developed canopies, use 3 qts per 100 sq.ft.)
Cycocel 2-3
Sumagic 2

When drench applications are made the growth medium all pots to be treated should be at the same moisture level to get uniform results. Also, response to drench application is affected by the condition of the roots. A poor root system may limit the uptake of chemical. Extra care needs to be taken when drenching large containers like hanging baskets with more than one plant. Drench solution should applied so each plant is treated equally and the resulting growth inhibition will be the same.

Table 5. Drench volume for PGRs.
  Drench volume (fl. oz./pot) >
Pot size A-Rest Bonzi Sumagic Cycocel
4-inch 2 2 --- 3
6-inch 4 4 4 6
8-inch 10 10 --- 8

Other Bedding Plant Growth Regulators

There are several products with the same active ingredient as A-Rest, B-Nine, Bonzi and Sumagic. Read labels as there may be slight variations in rates and recommendations.

  • Ancymidol. (Abide, A-Rest)
  • Daminozide. (B-Nine, Dazide)
  • Paclobutrazol. (Bonzi, Downsize, Paczol, Piccolo, Piccolo 10XC)
  • Uniconazole-p. (Sumagic, Concise)

Flurprimidol. (Topflor, Topflor granular) This PGR reduces internode elongation through the inhibition of gibberellic acid (GA).

Other materials are available to treat bedding plants for reasons other than height control.

Ethephon.  Florel. This PGR is an ethylene-generating compound used to stimulate branching. Florel can substitute for hand pinching ivy geranium, cutting geranium, lantana, verbena, and vinca vine. Apply 500 ppm at the normal time of pinching and at least 6-8 weeks before bloom.

Fascination. This PGR increases the postharvest quality of plants. It is anti-ethylene compound combining gibberellic acid and benzyladenine.  It is labeled for preventing lower leaf yellowing on Easter lilies and other potted lilies.

Ethylbloc. This is another antiethylene compound which can prevent flower drop and leaf yellowing on bedding plants. The unique feature of this chemical is that it is applied as a gas to plants in an enclosed environment (e.g., tight greenhouse, special growth room, back of a truck). Quite a number of plants are listed on the Ethylbloc label including begonia, fuchsia, geranium, impatiens, salvia, and snapdragons. These species are quite sensitive to ethylene and often drop flowers or develop yellowed leaves during shipping.

Details about Ethylbloc and Fascination were discussed by Susan Han of Plant and Soil Sciences in issues of Floral Notes (May-June 2000 and January-February 2001, respectively).

References

  • Han, S.S. 2001. Fascination, a new product for preventing leaf yellowing in potted lilies. Floral Notes. 13(4):2. (January-February).
  • Han, S.S. 2000. Understanding ethylene and Ethylbloc. Floral Notes. 12(6):10-11. (May-June 2000).
  • L.B. Stack (eds.). . New England Floricultural Recommendations 2013-2014. New England Floriculture, Inc.
  • McAvoy, R.J. 2001. PGRs for spring crops. Yankee Grower. 3(1):3-5.

Websites where you can download PGR labels in PDF format using Adobe Reader.

Douglas Cox, Stockbridge School of Agriculture
University of Massachusetts, Amherst

Revised 2012