Plant Growth Factors: Plant Hormones

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questionThought Questions

Explain the science behind the following gardening questions:

1. A couple of times a year, I sheer my shrubs into nice rounded shapes.  Now my shrubs have large woody stems with a lot of dead branches.  How do I correct this?

2. I put a stake next to a small tree trunk to keep it straight.  When I took it off a year later the trunk had a worse bend than before.  Why?


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Plant Hormones and Plant Growth Regulators

Another factor in plant growth is the influence of plant hormones.  Hormones are chemicals produced by plants that regulate the growth processes. 

Plant growth regulators are chemicals applied by a horticulturist to regulate plant growth.  In plant propagation, cuttings are dipped in a rooting hormone to stimulate root development.  In greenhouse production, many potted flowering plants (like poinsettias and Easter lilies) may be treated with plant growth regulators to keep them short.  Seedless grapes are treated with plant growth regulators to increase the size of the fruit.  In special situations, turf may be treated to slow growth and mitigate the need for mowing.  Because plant growth regulators are effective in parts per million or parts per billion, they have little application in home gardening.

Plant Hormones

Different hormones affect different plant processes.  Understanding how hormones work allows horticulturists to manipulate plants for specific purposes. 

  • Auxins produced in the terminal buds suppress the growth of side buds and stimulate root growth.  They also affect cell elongation (tropism), apical dominance, and fruit drop or retention.

Influence of hormones on tree shape
Figure 1. Auxins produced in the rapidly growing terminal buds suppress growth of side buds, giving a young tree a more upright form.  As growth rates slow with age, reduction in apical dominance gives the maturing tree a more rounded crown.


  • Gibberellins affect:

  • The rate of cell division
  • Flowering
  • Increase in size of leaves and fruits
  • Seed and bud dormancy
  • Induction of growth at lower temperatures (used to green up lawns 2 to 3 weeks earlier)

  • Cytokinins promote cell division, and influence cell differentiation and aging of leaves.

  • Abscisic acid is considered the “stress” hormone.  It inhibits the effects of other hormones to reduce growth during times of plant stress.

Hormone Influence on Pruning

Understanding hormones is key to proper pruning.  Auxin produced in the terminal buds suppresses growth of side buds and stimulates root growth. Gibberellins produced in the root growing tips stimulate shoot growth.  Pruning a newly planted tree removes the auxin, slowing root regeneration. 

Trees balance canopy and root growth
Figure 2. A tree balances canopy growth with root growth with the levels of auxins and gibberellins.



Heading cuts (removal of a branch tip) releases the apical dominance caused by auxins from the terminal bud.  This allows side shoots to develop and the branch becomes bushier.  On the other hand, thinning cuts remove a branch back to the branch union (crotch).  This type of cut opens the plant to more light.  Most pruning should be limited to thinning cuts.  For details on pruning, refer to CMG Pruning fact sheets.

heading and thnning cuts
Figure 3. Left: A heading cut releases apical dominance and the branch becomes denser as the lateral buds begin to grow.  Right: A thinning cut removes a branch back at a branch union (crotch), opening the plant for better light penetration.  Thinning cuts promote an open growth habit by redirecting sugars to the terminal shoots.


Tropism

Auxins also play a key role in tropism (controlling the direction of plant growth).

Geotropism
Figure 4. Geotropism – Under the influence of gravity, auxins accumulate in the lower side of a horizontal stem, causing cells to enlarge faster, turning the stem upright.



Phototropism
Figure 5. Phototropism The auxin concentration on the shaded side stimulates cell elongation, turning the stem to the sun.



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Authors: David Whiting (CSU Extension) with Michael Roll and Larry Vickerman (former CSU Extension employees). Artwork by Scott Johnson and David Whiting.

  • CMG GardenNotes are available online at www.cmg.colostate.edu
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  • Copyright. 2003-11. Colorado Master Gardener Program, Colorado State University Extension. All Rights Reserved. CMG GardenNotes may be reproduced without change or additions, for nonprofit educational use.

Revised December 2011

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