Plant Growth Regulators

Krishna Nemali, Ph.D.

Plant growth regulators (PGRs) are

• naturally occurring or synthetic compounds• inhibit, retard or promote growth of plant or specific organs• do not include substances for plant nutrition/trace elements/

amendments• treated as pesticides

Uses of PGRs

• Chemical growth retardants (CGRs) control plant height (there are non-chemical methods too!)

• Stimulate lateral branching• Promote flower initiation or early flowering• Fruit ripening• Rooting cuttings• Substitute for cold storage

Chemical Product

Ancymidol A-Rest (height control of plugs)

Chlormequat Cycocel (height control of poinsettias/geraniums)

Daminozide B-nine (height control, do not apply as drench)

Ethephon Florel (flower bud abortion/promote vegetative growth)

Paclobutrazol Bonzi (very popular, growth control)

Uniconizole Sumagic (for perennials)

Benzyladenine Configure (promote branching/flower set)

Gibberellin (GA3) ProGibb (promote growth/over application of growth retardants)

BA +GA(4+7) Fascination (avoid leaf yellowing/drop, overcome effects of growth retardants)

Indole Acetic/Butyric Acid Hormo-Root/RooTone (inducing roots)

Common PGRs used in the ornamental industry

Considerations for applying PGRs

Timing:

• Apply after sufficient plant growth has happened or rapid growth has started

• Earlier applications may stop growth completely• Remember PGRs can’t reduce already existing growth• Timing based on physiological stage and not on chronological

stage

Considerations for applying PGRs

Target:

• Drenches use larger volume but lower concentration• Foliar sprays should be uniform• If dipping, use consistent soaking time, 10 sec• Some PGRs are effective if applied to roots (ex: Bonzi)

Considerations for applying PGRs

Dosage:

• Read label• Dosage = concentration x volume applied• Dip (B-nine) and drench (Bonzi) methods can be fairly accurate)• Spray can be uneven, half gal per 100 ft2; base spray application

on areas and not plants

Considerations for applying PGRs

Environment:

• Drenching may not be suitable in some substrates with bark media

• Morning applications when evaporation is low are best• Plants should not be stressed at application• Plants should not be watered too soon after application

Crop growth retardants (CGRs)

• Blow over• Watering becomes difficult• Increased shipping costs• Leggy low quality plants• Damage during shipping• Customer preference

How to control crop growth?

• Pruning• Genetics• Environmental control• Mechanical stress• Chemical control

Micro Tina Tomatoes

Environmental control (modify fertility)

0.12 0.65 1.18 1.71 2.24 2.77 dS/m

0.12 0.65 1.18 1.71 2.24 2.77dS/m

Environmental control (mild drought stress)

Environmental control (negative DIF)

• DIF stands for difference in night and day temperature

• Negative DIF indicates that night temperature is greater than day temperature (night 60, day 55 deg F)

• Negative DIF for 3-4 h during the morning can reduce plant height

• This is possible in northern states during winter months by allowing outside air in the greenhouse

Mechanical Stress

• Thigmamorphogenesis: growth and developmental responses of plants to rubbing, twisting, or physical contact

• Using mechanical stress, we can control plant height, improve plant strength, stress tolerance, insect resistance (Latimer, 1998)

Chemical control• Effective and inexpensive

• They work by influencing Gibberellin biosynthesis pathway/ release of ethylene

• Provide additional benefits:• green foliage• resistant to mechanical damage• drought/ pest resistance