Growth Stimulants, Retardants, and
Rooting Hormones
Growth regulators

Hormones

Organic chemicals that act and interact
to affect growth rate

Auxins – accelerate growth by
stimulating cell enlargement
Gibberellins

Stimulate growth in stem and leaf by cell
elongation

Stimulates premature flowering, growth
of young fruits and breaking of dormancy
Cytokinins

Stimulate cell division

Work along with auxins

Will not work without auxins present
Inhibitors

Abscisic Acid and Ethylene Gas

inhibits seed germination

Inhibits stem elongation

Hasten ripening of fruit – ethylene gas

Auxins and Gibberellins promote cell
enlargement

Cytokinins stimulate cell division
Growth Hormones

Organic chemicals produced by actively
growing plant tissue

Shoot tips and young leaves

Move throughout the plant and can be
found in most tissues

These chemicals react with one another in a very
complex system in the plant

In some cases a concentration of one hormone
stimulates growth and a different concentration
restricts growth
Apical Dominance

Dominance of the terminal bud

Apical dominance exists there

Terminal bud secretes chemicals that inhibit
or prevent the growth of axillary buds on the
same shoot
Apical Dominance

Axillary buds are found in the axil – angle between a
leaf and the stem.

Causes the plant to grow tall and not branch

Once the plant reaches flowering age and the
terminal bud becomes a flower, the chemicals are
no longer secreted
Apical Dominance

The plant then starts to send out side branches

This allows the plant to grow above competing
plants

Once height and access to sunlight are secured,
the plant spreads out over its competitors
Apical Dominance

Pinching off terminal buds removes the
hormone and causes branching to happen
faster

Common practice in greenhouses and
nurseries to produce bushier plants with
many flower buds
Stimulants

Allow plants to grow taller

Most common is Gibberellic Acid (GA)

Causes stems of plants to stretch out

Nodes are farther apart
Growth Stimulants

Natural growth stimulants have been found in
alfalfa

It causes stimulation of growth

It is the alcohol called Triacontanol

Stimulation is brought about by mulching plants
with alfalfa hay

Or watering plants with a “tea” made from
soaking alfalfa feed pellets in water

Amount needed is very small

Also sold commercially
Chemical Retardants

Chemicals are used to retard growth of plants causing them to
be shorter and more compact

Plants are more attractive and the plants themselves are
stronger

Used commercially

B-Nine is commonly used on Azaleas

Newest uses of Plant Growth Retardants (PGR) is the
application to lawns

After first mowing in the spring “Limit” is applied and
absorbed by the roots

Restricts growth for 6-8 weeks

“Embark” is absorbed by the leaves and also
restricts growth

PGR’s absorbed by the leaves move to the growing
point where it interrupts cell division, stem
elongation and seed head formation

Roots continue to grow

Applied after first or second mowing
Sumagic

New growth retardant

Reduces height of plants by inhibiting production of
GA

Chemical is taken up by leaves and moves through
the plant to the terminal bud

Used to control growth of shrubbery and hedges

PGR Atrimmic applied after pruning can last an entire
season

Reduces or eliminates the need to prune
Atrimmic

A systemic

Penetrates the plant, enters the plant sap, and
moves through the plant

Blocks plant hormones that stimulate growth

Applied as a foliar spray
Rooting Hormones

Important when propagating plants by cutting

Helps cuttings to develop more roots faster than without a
rooting hormone

Indoleacetic Acid (IAA) naturally occurring, causes roots to form
on plant stems
IBA – Indolebutyric Acid

Most widely used rooting hormone

Most effective rooting hormone
Rooting Hormones

Either mixed with talc and used as a powder or dissolved in water
and used as a wet dip

Liquid can be slightly more effective

All rooting hormones should contain a fungicide
Fungicide

Helps prevent cuttings from rotting
Dwarfing Rootstock

Used in fruit trees

Shorter trees enable you to pick fruit without using a
ladder

First research done to prevent apple trees from
growing very tall

Trees growing from certain types of roots didn’t grow
as tall

These trees also bore fruit at an earlier age

Complete series of rootstock known as malling
rootstock developed in England

Controls size and rate of growth or apple trees

Stock has also been developed for peach
and pear trees as well

Dwarf trees can be purchased
commercially
Chemical Blossom Set

Used on tomato blossoms early in the season

Causes earlier development of fruit

Causes seedless tomatoes to set on first
blossoms and results in tomatoes ready to eat as
much as 10 days earlier than normal
Plant Biostimulants

Natural products

Organic

Work to stimulate soil microbial activity and improve soil cation
exchange capacity, stimulate plant growth and promote
disease resistance
Humic Acid

Product of rotting organic matte is an example of
a soil microbial stimulant
Root growth biostimulants

Improve water and nutrient uptake

Increase number of fibrous roots

High levels found in kelp plants
Biostimulants

Greatly reduce the need for fertilizers,
especially N

Saves money, reduces pollution
Methanol

A form of alcohol

Speeds plant growth

Research in California showed an increase in plant
yield of 36-100%

Works by blocking photrespiration

Plants use water for growth rather than transpiring it
into the air
Methanol

Works best on plants frown in full summer sun

Water use reduced by as much as 50% in some
plants
Allelopathy

Production of a chemical compound in one plant
that slows or stops the growth of another plant

Natural herbicide

Black Walnut, Millet